Sunday, December 26, 2010

Combined cycle power plant


 combined cycle power plant is more efficient than a conventional power plant because it uses a higher proportion of the energy that the fuel produces when it burns.
In a combined cycle power plant (CCPP), or combined cycle gas turbine (CCGT) plant, a gas turbine generates electricity and the waste heat is used to make steam to generate additional electricity via a steam turbine; this last step enhances the efficiency of electricity generation. Most new gas power plants in North America and Europe are of these types.
The Integrated Gasification Combined Cycle, or IGCC, is a process that turns coal into gas known as synthesis gas (syngas) before it is burnt. The impurities in the syngas, like CO2, can also be removed before it is burnt.
The syngas is often used to power a gas turbine generator for electricity whose waste heat is passed to a steam turbine system. If CO2 is removed before burning (pre-combustion) it can be stored in deep geological formations..

Saturday, December 25, 2010

Steam Turbine

Steam turbines are devices which convert the energy stored in steam into rotational mechanical energy. These machines are widely used for the generation of electricity in a number of different cycles, such as:
  • Rankine cycle
  • Reheat cycle
  • Regenerative cycle
  • Combined cycle
The steam turbine may consists of several stages. Each stage can be described by analyzing the expansion of steam from a higher pressure to a lower pressure. The steam may be wet, dry saturated or superheated.

Consider the steam turbine shown in the cycle above. The output power of the turbine at steady flow condition is:
P = m (h1-h2)
 
where m is the mass flow of the steam through the turbine and h1 and h2 are specific enthalpy of the steam at inlet respective outlet of the turbine.

The efficiency of the steam turbines are often described by the isentropic efficiency for expansion process. The presence of water droplets in the steam will reduce the efficiency of the turbine and cause physical erosion of the blades. Therefore the dryness fraction of the steam at the outlet of the turbine should not be less than 0.9.

Heat Engine

Heat engine is defined as a device that converts heat energy into mechanical energy or more exactly a system which operates continuously and only heatand work may pass across its boundaries. 
The operation of a heat engine can best be represented by a thermodynamic cycle. Some examples are: Otto, Diesel, Brayton, Stirling and Rankine cycles.

Forward Heat Engine


LTER= Low Temperature Energy Reservoir
HTER= High Temperature Energy Reservoir 

A forward heat engine has a positive work output such as Rankine or Brayton cycle. Applying the first law of thermodynamics to the cycle gives:
Q1 - Q2 - W = 0 

The second law of thermodynamics states that the thermal efficiency of the cycle, , has an upper limit (the thermal efficiency of the Carnot cycle), i.e.

It can be shown that:
Q1 > W

which means that it is impossible to convert the whole heat input to work and
Q2 > 0 

which means that a minimum of heat supply to the cold reservoir is necessary.

Reverse Heat Engine



LTER= Low Temperature Energy Reservoir
HTER= High Temperature Energy Reservoir 

A reverse heat engine has a positive work input such as heat pump and refrigerator. Applying the first law of thermodynamics to the cycle gives:

- Q1 + Q2 + W = 0 
In case of a reverse heat engine the second law of thermodynamics is as follows: It is impossible to transfer heat from a cooler body to a hotter body without any work input i.e.

W > 0 

which means that the coefficient of performance for a heat pump is greater than unity.

Friday, December 24, 2010

Fundamentals of Thermodynamics

  • Introduction, laws of thermodynamics, notation. 
  •  Work and processes. 
  •  First and second laws. 
  •  Gibbsian equations, chemical potential. 
  •  Mathematical methods, Legendre transforms. 
  •  Partial derivative game. 
  •  Process evaluation. Residual functions. 
  •  Residual functions and example problems. 
  •  Introduction to statistical mechanics and quantum mechanics. 
  •  Quantum mechanics I. 
  •  Quantum mechanics II. 
  • Check out this link for a description of the rotational energy of a molecule. 
  •  Statistics and ensembles. 
  •  Ensembles and partition functions. 
  •  Semi-classical partition function. 
  •  Properties of ideal gases. 
  •  Properties of ideal gases, examples. 
  •  Chemical equilibria: ideal and real fluids. 
  •  Pair potentials and nonideal behavior. Van der Waals partition functions for mixture, local compositions, activity coefficient models. 
  •  Conformal solution theory. 
  •  Conformal solution theory, pure fluids and mixtures. 
  •  Ideal solutions and partial molar quantities. 
  •  Partial molar properties and fugacity. 
  •  Local composition models. 
  • Applet for plotting radial distribution functions. 
  • Molecular simulation code written as a Java Applet. 
  •  Mixture thermodynamics calculations. 
  •  Local compositions. 

Thursday, December 23, 2010

Relation between PVT

Gas Laws:
Relationship between Pressure (P), Volume (V), Temperature (T) and quantity; Moles (n)









Boyle’s Law (PV = constant)

This is an inverse relationship:  As volume decreases, pressure increases.






Charles’ Law: (V/T = constant)

This is an inverse relationship:  As volume decreases, pressure increases.
Charles’ Law: (V/T = constant)

This is a direct relationship.  As Temperature decreases, Volume decreases.







Avogadro’s Law: (V/n = constant)
This is a direct relationship.  As the number of moles decreases, the volume decreases






Summary:

Combined Gas Law:
PV/nT = constant (T in Kelvins)
P1V1/n1T1 = P2V2/n2T2
Ideal gas Law:
PV = nRT
R = .0821L atm/mol K
Gay-Lussac’s/Avogadro’s Law of Combining Volumes

Equal volumes of any gases at the same temperature and pressure contain the same number of moles of
gas.
The coefficients of a balanced equation can be used to calculate relative volumes.
Standard Molar Volume: At standard temperature and pressure (STP = 1atm and 273.15K) 1 mole of any
ideal gas has a volume of 22.4L
Variations on the ideal gas law equation:
PV = mRT/M (m = sample mass, M = molar mass of the gas)
d = MP/RT (d = density of the gas in g/L)
Examples:
1.  Calculate:
a.  The new pressure in a closed container if a 5.0L volume of gas at 2.5atm has its volume increased to
7.5L.
b.  The new volume of gas (at constant T and P) if 2.0mol of He in a 3.0L container has another 3.0mol of
He placed into the container.
Answers:
a.  (5.0L)(2.5atm) = (7.5L)(P2)
P2 = 1.7atm
b. 3.0L/2.0mol = V2/5.0mol
V2 = 7.5L
2.  When a rigid hollow sphere containing 680 L of helium gas is heated from 300.K to 600.K, the
pressure of the gas increases to 18atm.  How many moles of helium does the sphere contain?
Answer:
n = PV/RT = (18atm)(680L)/(.0821)(600.K)
n = 248.48 = 250moles
3.  A child has a lung capacity of 2.2 L.  How many grams of air do her lungs hold at a pressure of 1.0
atm and a normal body temperature of 37
o
C?  Assume a “formula mass” of 29g/mol for air.
Answer:
m = MPV/RT = (29g/mol)(1.0atm)(2.2L)/(.0821Latm/molK)(310.15K) = 2.5g
4.  A gas with a volume of 300.mL at 150.
o
C is heated until its volume is 600.mL.  What is the new
temperature of the gas if the pressure is unaltered?
Answer:
300mL/423.15K = 600mL/T2
T2 = 846K = 573
o
C5.  Calculate the number of liters occupied, at STP.
 a.  0.350 mol O2
 b.  63.5g He
Answers:
a. 0.350mol (22.4L/mol) = 7.84L
b. (63.5g)(1mol/4.003g)(22.4L/1mol) = 355L
6.  Determine the molar mass of a gas for which a 2.5g sample of that gas occupies a volume of 3.0L at
STP.
Answer:
M =(2.5g)(.0821)(273.15K)/(3.0L)(1atm) =18.7g/mol
or
(2.5g)/(3.0L/22.4L/mol) = 18.7g/mol
7.  Find the density of fluorine gas (g/L) at 700torr and 50
o
C.
Answer:
d = MP/RT
= (38.00g/mol)(700/760) / (.0821)(50+273.15) = 1.32g/L
8.  For the equation
Ag2S(s)
 + H2(g) → Ag(s)
  +  H2S(g)
How many Liters of H2S can be produced from 15.0g of Ag2S and 1.00L of H2(g)
 if the reaction occurs at
STP?
Answer:
Ag2S(s)
 + H2(g) → 2Ag(s)
  +  H2S(g)
mol Ag2S = 15.0g (1mol/247.8g) = .0605mol
mol H2 = (1.00L)(1mol / 22.4L) = .0446mol
Hydrogen gas limits
 0446mol H2 (1mol H2S / 1mol H2) = .0446mol H2S.
V = .0446mol (22.4L/mol) = 1.0L
(Note that the last two steps aren’t really necessary because of the 1:1 mole ratio between H2S and H2.)

Wednesday, December 22, 2010

Diesel Cycle

Diesel Cycle


The Diesel cycle is an ideal air standard cycle which consists of four processes:
  • 1 to 2: Isentropic compression
  • 2 to 3: Reversible constant pressure heating
  • 3 to 4: Isentropic expansion
  • 4 to 1: Reversible constant volume cooling
By defining the compression ratio, r, as:

and cut-off ratio, , as:

The thermal efficiency of an Diesel cycle with a perfect gas as working fluid is:

where,
n=  = a constant depending on specific heat capacity

Tuesday, December 21, 2010

Heat balance

An application of the first law of thermodynamics to a process in which any work terms are negligible.
For a closed system, one that always consists of the same material, the first law is Q + W = ΔE, where Q is the heat supplied to the system, W is the work done on the system, and ΔE is the increase in energy of the material forming the system. It is convenient to treat ΔE as the sum of changes in mechanical energy, such as kinetic energy and potential energy in a gravitational field, and of internal energy ΔU that depends on changes in the thermodynamic state of the material. Because the rates at which any changes occur are usually of interest, heat balances are often written in terms of heat flow rates (heat per unit time), sometimes denoted by a dot over the symbol, , so that for a process with negligible work, kinetic energy and potential energy terms, , the rate of change of internal energy with time.
Often it is more convenient to apply the first law or a heat balance to an open system, a fixed region or control volume across the boundaries of which materials may travel and inside which they may accumulate, such as a building, an aircraft engine, or a section of a chemical process plant. Then the first law is expressed by the equation below, where is the rate of doing shaft work
on the system; is the mass flow rate of any stream entering or leaving the control volume; h is the enthalpy per unit mass; c is the velocity; gz is the gravitational potential for each stream at the point of crossing the boundary of the control volume; and E is the energy of all material inside the control volume. When conditions inside the control volume do not change with time, although they need not be spatially uniform, dE/dt = 0, and the balance equation is known as the steady-flow energy equation.
Enthalpy is a thermodynamic property defined by h = u + pv, where u is the specific internal energy (enthalpy per unit mass), p the pressure, and v the specific volume. It is used, along with shaft work, because the derivation of the first-law equation for a control volume from the more fundamental equation for a closed system involves work terms pv that are not available for use outside the control volume. Changes in enthalpy occur because of changes in temperature, pressure, physical state (for example, from liquid to vapor), and changes in chemical state

Monday, December 20, 2010

Differnace Between Thermal and internal energy.


According to many sources, internal energy is the kinetic and potential energies of molecules of a substance – combined. Some sources also say that thermal energy is that definition.
However, I don’t think that thermal energy is kinetic and potential energy combined. I know that heat transfer occurs between a region with higher temperature and a region with lower temperature, but it isn’t necessarily the case with tworegions of different internal energies. So that seems to imply to me that temperature is a measurement of thermal energy – so in that sense thermal energy is merely the average kinetic energy of molecules and as such, doesn’t include the potential energy. Is that right?

Saturday, December 18, 2010

EFFECT OF THERMAL HISTORY ON CRYSTALLIZATION

Experimental 
Two  samples  of LLDPE with  different structural parameters  studied  in  this
paper are  listed in Table  1.
Specimens were prepared by cutting the granular resins into slices of mass ap-
proximately  5.5  mg.  A  Perkin-Elmer  model  DSC-2C  differential  scanning
calorimeter  was  used  to  measure  the  enthalpies  of  fusion  and  crystallization.
Three reference materials, indium, phenyl ether and o-terphenyl were used for in-
strument calibration. The  temperature range  of scanning was 213  K  (-60~  to
443 K  (170~  high purity nitrogen was used as purging gas. Unless indicated the
scanning rates  (both heating and cooling) were  10 deg.min -1.
1.  For  the  observation  of  structural  differences between  these  two LLDPE
resins,  identical  thermal  treatment  were  applied  to  all  specimens  in  order  to
eliminate the  thermal history caused by processing and storage conditions. Each
specimen was  heated up  to 443 K  and held at  this  temperature  for  10 min, then
cooled down  to 213  K  and held isothermally for  10 minutes. The DSC measure-
ment was made during reheating to 443 K.
2.  To observe  the  effect of heating rate on  the melting behaviour, specimens
were isothermally conditioned at 443 K for 10 min cooled to 213 K, then reheated
up  to  443  K  at  four  separate  heating  rates  (20,  10,  5  and  1 deg.min-t).  A DSC
measurement was made during reheating.
3. To observe  the effect of cooling rate on  the crystallization behaviour,  the
specimens  were  isothermally conditioned at  443  K  for  10 min  then  cooled  to
213  K  at  three cooling rates  (20,  10 and  1 deg.min-~). Finally, the  samples were
reheated up to 443 K and DSC measurement recorded during this reheating cycle.
4.  To  observe  the  annealing  effect  on  the  melting  behaviour,  the  melt
specimens were  cooled  to  four given  annealing  temperatures of 398  K,  393  K,
383 K and 373 K, and held at these temperatures for  10 and  120 minutes respec-
tively. The annealed specimens were cooled to 213 K  and reheated up  to 443  K
finally, DSC measurements being made for the final heating process.

Results  and  discussion
The  results  of the effects of thermal history are  shown  in Fig.  1.
Comparing  the peak  shape of the  two LLDPE samples as received  in Fig.  1,  it
can  be  seen  that more  comonomer content  in LLDPE  leads  to a broader peak  and
lower peak  temperature.  The  amount of comonomer  in Dowlex  2045  is  less  than
that in  Stamylex  1048,  i.e.  the degree of linearity for Dowlex 2045  is higher than
that  for Stamylex  1048.  This  is  in  accord with  the  structural parameters  listed  in
Table  1.  The  melting  temperatures  of  both  LLDPE  samples  measured  after
eliminating thermal history were a  little higher than those measured before. These
indicate  that  the  crystalline  integrity  of both  LLDPE  increased  during  the  new
thermal  history. The  changes  of peak  shape  illustrate  that  the  size distribution  of
crystailites  also  changed.  The  changes  of  fusion  enthalphy, AHf,  for  Stamylex
1048  were  slightly higher  than  those  for Dowlex  2045  before  and  after  elimina-
tion  of  the  thermal  history effect. These  reflected the  small difference  in  crystal-
linity between  these  two samples. The effect of heating rate on melting behaviour
is  shown  in  Fig.  2.  It  can  be  seen  that  the  lower  the  heating  rate,  the  better  the
peak  resolution, and  consequently  the clearer  the  shoulder peak. This may be  in-
terpreted  that  the  slower  heating  rates  enable  a  semicrystalline polymer  to  have
more  time  for crystal  growth prior  to  final melting. Curve  'a'  in Fig. 2  illustrates
 ~

Wednesday, December 15, 2010

Compensating for ideality factor and series resistance differences between thermal-sense diodes

Abstract: When using an external thermal diode to measure temperature, the accuracy of the temperature measurement depends on the characteristics of the external diode. Two critical parameters that affect measurement accuracy are ideality factor and series resistance. This application note explains the effects of these parameters on remote temperature-sensor measurements and discusses how to determine compensation factors for their effects.

The most common approach to measuring temperature with a "remote-diode" temperature sensor is to force two different currents through the diode¹, typically with a current ratio of about 10:1. The diode's voltage is measured at each current level and the temperature is calculated based on the equation:
Equation 1.
Where:
IH is the larger diode bias current.
IL is the smaller diode bias current.
VH is the diode voltage while IH is flowing.
VL is the diode voltage while IL is flowing.
n is the ideality factor of the diode (nominally 1, but varies with processing).
k is Boltzmann's constant (1.38 × 10-23joules/K).
T is the temperature in K.
q is the charge of an electron (1.60 × 10-19C)

If Equation 2.= 10, this can be simplified to:
VH - VL = 1.986 × 10-4 × πT

Ideality factor correction

Note that the accuracy of the temperature reading depends on the value of n. If the remote diode sensor is designed to produce correct readings with a diode that has a specific value of n, then changing to a diode with a different ideality factor will change the apparent measured temperature.

Correcting for differences in ideality factor is done as follows. Assume that a remote diode sensor designed for a nominal ideality factor, nNOMINAL, is used to measure the temperature of a diode with a different ideality factor, nACTUAL. The measured temperature, TMEASURED, can be corrected using:
Equation 3.
Where T is the temperature in K.

Most remote diode temperature sensors for CPUs are designed to produce accurate temperature data when used with an ideality factor of 1.008. Some newer CPU thermal-sense diodes have lower ideality factors. To use a CPU optimized for an ideality factor of 1.008 with a CPU that has an ideality factor of 1.0021, the data can be corrected (assuming no series resistance) as follows:
Equation 4.
For an actual temperature of 85°C (358.15K), the measured temperature will be 82.91°C (356.06K), an error of -2.09°. Note that the error is proportional to absolute temperature. At 125°C, the error increases to -2.32°.

Series resistance correction

Series resistance in one of the diodes contributes additional errors. For the nominal diode currents of 10µA and 100µA used in Maxim's remote temperature sensors, the change in the measured voltage will be:
RS(100µA - 10µA) = 90µA × RS
Since 1°C corresponds to 198.6µV, series resistance contributes a temperature offset of:
Equation 5.
Assume that the diode being measured has a series resistance of 3.86Ω. The series resistance contributes an offset of:
3.86Ω × 0.453°C/Ω = 1.75°C
If the diode has an ideality factor of 1.0021 and series resistance of 3.86Ω, the total offset can be calculated as follows. Combining the correction for series resistance with the correction for ideality factor, we have:
1.75°C - 2.09°C = -0.34°C
This is for a diode temperature of 85°C. Thus, in this case the effect of the series resistance and the ideality factor partially cancel each other.

Note that if the diode bias current is different, the effect of series resistance will change proportionally. For example, some remote temperature sensors have diode bias currents two or more times larger than those of Maxim's remote sensors. The resulting temperature errors can be on the order of two or more degrees larger than those observed with Maxim's sensors.

Some temperature sensors include automatic series resistance cancellation within their remote-diode sensing circuitry. When this function is enabled, these sensors bias the external diodes with three or four different current levels and use the resulting voltage measurements to eliminate the effect of series resistance from the temperature calculation. The MAX6654 and MAX6690 temperature sensors have a single remote channel with optional series resistance cancellation. Several multichannel remote sensors, including the MAX6602, MAX6689, MAX6697, MAX6698, and MAX6699, have series resistance cancellation on one of the remote channels. The MAX6581, with seven remote channels, includes series resistance cancellation on all remote channels.



¹This diode is not a two-lead rectifier or signal diode like a 1N4001. Such diodes will not work with remote-diode temperature sensors. Instead, the diode is really a bipolar transistor connected as a diode. If the transistor is a discrete unit, its base and collector should be connected together. If the transistor is a substrate PNP, the collector will be grounded and the base and emitter serve as the cathode and anode. When "diode" is used in this document, it refers to the diode-connected transistors described above.

HEAT,WORK,AND TEMPERATURE

Heatis thermal energy in transit. It cannot be accumulated or stored.
When thermal energy flows from a hot body to a cold one (the reverse never occurs without the expenditure of work), it should be called heat. However, when flow ceases as a result of temperature equilibrium, the thermal energy gained by the cold body should not be referred to as heat.
Work(W) is also a quantity in transit. It is a force acting through a
distance [equivalent to lifting a weight a distance (l) vertically:W =Fl].
Static energyis the capacity for doing work.
Temperatureexpresses the thermal energy level of a body. The
molecules of a body vibrate at very high frequency, but with an amplitude that increases with the thermal energy level of the material. The structural configurations called solid, liquid, and gas are states of matter. The two temperature scales in common use today are defined in terms of the transition of one of the mostcommon substances (water) from solid to liquid (freezing point) and from liquid to vapor (boiling point) at one atmosphere pressure. Celsius (1741) took the freezing point of water as 100° and the boiling point as 0°, and divided the scale between into 100 equal parts. Later the scale

Tuesday, December 14, 2010

THE HYDROLOGIC CYCLE

As shown in above diagram shows the various pathways of (1) water to the oceans (rivers, glaciers, precipitation); (2) water into the atmosphere by evaporation (from falling rain, rivers & lakes, soil, the oceans, transpiration by plants); and (3) onto the landmasses (by rain, snow). Water movement/transport occurs through movement of clouds, by rivers, ocean circulation, groundwater flow, and evaporation.
The bulk of the water is contained in the oceans, which contain about 30000 times more water than atmosphere and continents combined, cover approximately 70% of the Earth's surface and are on average 3800 m deep. The remainder of the water is found in ice caps & glaciers (3%), groundwater (1%), and rivers and lakes (0.01%). The latter two reservoirs constitute the terrestrial fresh water supply. Thus, only a very small fraction of the overall water supply is suitable and available for human use. The water transfer between these reservoirs is accomplished by the processes of evaporation, transpiration, precipitation, and flow of water (following gravity).
Two minor reservoirs by size, the atmosphere (0.001%) and the biosphere (the totality of living tissue; 0.0001%) play a role that is not reflected by their size. As we shall see below, cycling of water through the atmosphere is an important factor for energy transfer in the atmosphere, and in Chapters 7 through 11 we will look at the important role that organisms play in driving the evolution of the climate and atmosphere evolution through the production and consumption of the greenhouse gases carbon dioxide and methane.
Every year about 30000 to 40000 cubic kilometers (a cube 30-35 km in size) of water move across the surface of the continents to the oceans, profoundly shaping the surface of the continents. Evaporation by the sun effects lifting of water into the atmosphere, and the counterforce to this process is gravity that forces rain to fall back on the earth and causes water move back to the oceans in streams (river systems), on the way eroding soils, cutting canyons, and transporting solids (silt, sand, clay) and dissolved salts to the oceans. The transfer of water vapor from the oceans to the atmosphere goes hand in hand with the transfer of tremendous amounts of thermal energy to the atmosphere and is very important for atmospheric circulation (see below). For this reason atmospheric circulation and winds can be considered part of the hydrologic cycle.
In cold climates water in the form of glaciers (glacial systems) moves downslope due to gravity, erodes bedrock by abrasive action, and transports sediment. Glaciers may either enter the sea directly, or melt at the end and add their water to the continental surface runoff.
Water may also seep into the porous portions of the earth's surface and is then called groundwater (groundwater systems). It may dissolve rocks in the subsurface and create karst systems (caves in limestone) and sinkholes.
Along the shores of oceans (shoreline systems) water that is agitated by waves (wind, atmospheric circulation) erodes and moves sediment and forms beaches, sandbars, lagoons, and related coastal features.
Winds (eolian system) can carry considerable amounts of sand and dust and can profoundly shape landscapes (sand seas of deserts) as well as human destiny (the highly fertile loess plains of southern China, without them it would be much more difficult to feed a population of 1 billion people).
Thus, the hydrologic cycle affects a broad range of landscape-shaping dynamic systems. We therefore consider river systems, glacial systems, groundwater systems, shoreline systems, and eolian systems as subsystems of the hydrologic system. In terms of overall importance for the shaping of the Earth surface, river systems are the primary force. Glaciers, although very powerful in modifying the land surface, were only abundant for brief intervals in Earth history.

Rain on the Cumberlands

Through the stricken air
Through the buttonwood balls suspended on twig strings
The rain-fog circles and swallows
Climbs the shallow plates of bark, the grooved trunks
And wind pellets go hurrying though the leaves
Down, down the rain
Down in plunging streaks of watered gray.
Rain in the beechwood trees
Rain upon the wanderer whose breath lies cold upon the mountainside
Caught up with broken horns within the nettled grass
With hooves relinquished on the breathing stones eatened with rain strokes
Rain has buried her seed and her dead

Sunday, December 12, 2010

Applied Mathematical Software and a Web-Based Interactive Handbook for Thermal Engineering: Problems and Solutions

Currently, different handbooks used in science and technology are being transferred from “paper” carriers to Internet sites. At present, if you need the value of heat conductivity of brass at a certain temperature, it is easier to type the key words “heat conductivity,” “brass,” and “temperature” into the entry window of a search engine (www.yandex.ru, www.google.ru, http://search.msn.com, etc) rather than browse through a voluminous book. However, new problems (and solutions of them!) of a special kind are emerging, and they are considered in this study.
The reliability of the information contained in “paper” handbooks is to some extent ensured by the reputation of the corresponding publishing houses and their staff of scientific consultants, editors, and correctors. Internet sites are, as a rule, created by nonprofessional developers and their content is not subject to strict editing and careful proofreading procedures. Nevertheless, I have found a comparatively large number of misprints in handbooks published by authoritative publishing houses. For example, 0.02387 maybe printed instead of 0.02387 (i.e., the digit 2 denoting the number of repeated zeros is misused), or 72.93 instead of 27.39 (in this case, the number was probably typed by a German-speaking person, in whose language “twenty-seven” is “seven and twenty” (siebenundzwanzig) and “thirty-nine” is “nine and thirty” (neununddreissig)), etc. Such typos may remain unnoticed in the process of traditional (visual) proofreading of “paper” handbooks, and, as a result, the book appears with a list of misprints (Errata – see one example >>>>>>>). In brief, misprints remain misprints with all the ensuing consequences. Reportedly, Academician A.N. Krylov (1863-1945) applied while a student for a position in an engineering bureau. As a test, he was asked to go over a bridge-building project. The future academician started to examine the project and soon exclaimed, “It is not possible to erect such a bridge: it will collapse!” The answer was: “That’s true; it has been built and has collapsed. You are hired!” Perhaps the future expert in mechanical science found a typo in a reference table used in the bridge project similar to those described above?
The instrumental means considered below allow timely identification of such typos in reference data or, at least, reduction of negative consequences deriving from them.
Recently, tools for publishing documents on the Web (on the Internet or corporative networks) created using applied mathematical software have been multiplied greatly. In the case of Mathcad [1 – 4], the Mathcad Application Server [4 – 6] is the kit that is used for this purpose. However, one can publish on the web, apart from pure calculation (examples of such calculations for thermal power engineering are posted at www.vpu.ru/mas), hybrid forms consisting of tables, plots, formulas (the dominant of “paper” reference information), and calculations. The instruments built into these mathematical packets make it possible to carry out statistical processing of tabular data and display the requested information in an “intelligent” form. For example, having opened the page of a “paper” handbook [7] containing information about the heat conductivity of brass, one will see a table the side column of which contains a list of alloys, including brasses of different composition, and a heading with the values o temperature for which the values of heat conductivity are displayed in the table. If one visits the Internet site located at http://twt.mpei.ac.ru/MAS/Worksheets/Therm/Heat_Cond_metal_e.mcd, one will see the data shown in Fig. 1.
A visitor to this website may select the alloy he is interested in the list, enter the value of temperature (in different scales – Celsius, Kelvin, Fahrenheit, or Rankine), and obtain the required value of heat conductivity (also in various units). The system also displays plots showing dependence of heat conductivity on temperature, which makes it possible to study the required quantity as a function of temperature and see the current point on a curve. Moreover, a visitor to the website may set the power n of the polynomial approximating the tabular data an output its coefficients for subsequent use of the displayed dependence in other applications, for example Excel broadsheets. Figure 2 shows how the coefficients of the approximating polynomial of the third power are entered (copied from the web page shown in Fig. 1) into an Excel formula field (cell B3) to calculate the value of specific heat conductivity for the value of temperature contained in cell B2. The reader can easily understand that these coefficients may be copied as well into the fields of programs newly developed or edited in BASIC, Pascal, C, Fortran, etc.
Figure 1 displays two plots: a spline interpolation (upper curve) and approximation (lower curve). This is done intentionally to provide more information and the option of choice to a visitor to the site. In addition, this duality may be considered a significant characteristic of the website. The point here is that the interpolation procedure (when the curve exactly passes through the points not displayed in the first plot) makes it possible to clearly identify misprints that were contained in the original tables or were made while transferring  data from “paper” source to a computer by scanning or typing. At the same time, the approximation procedure (when the curve passes in the vicinity of the points that are displayed in the second plot) makes it possible to minimize the consequences of such misprints if they have not been identified by means of interpolation.
Reference information also includes different formulas needed for calculations. In this case, the Mathcad Application Server may also prove to be very useful for publishing formulas on the Web.
Figure 3 shows as an example a web page containing four formulas. These expressions describe a change in temperature in a spherical wall under the conditions of steady heat transfer and with the heat conductivity of the wall material not depending on temperature (a simplified problem). The formulas are “animated”, i.e., the visitor can “play” with the variables: change the initial data and see new values retrieved by the formulas and the corresponding point on the plot, which also changes depending on the initial data. “Animation of formulas serves two purposes. First, the visitor can immediately obtain a result that follows from the formulas without entering them into his computer or calculator. Second, this provides an extra option to check whether the formula on the “paper” contained an error or the formula was incorrectly typed into computer when creating respective Mathcad document posted on the Web using the technology of the Mathcad Application Server. The Mathcad package, which contains a mechanism for checking dimensions [8], significantly reduces the probability of such errors. The Web page may contain a hyperlink to a scanned “paper” page of the source, for example, a handbook or even an experiment log (see the penultimate line in Fig. 3).
The formulas contained in the reference Web pages may be transferred to other program environments either manually, in the visual mode, or automatically. The most recent versions of the package, Mathcad 12 [4] and Mathcad 13, support recording of files used by this mathematical package in HTML format (hyper text markup language). Therefore, it is now possible to view and edit Mathcad files on a PC without using Mathcad itself. Such a file posted on the Web for downloading (see the link in the lower part of Fig. 3) may be opened by any word processor. In this case, a reference formula presented in text format may be copied to any program environment, for example, Maple (fig. 4), where the formula will be converted into a graphical form more convenient for visual inspection.
The list of typos usually present in each handbook often contains as well information about typos in formulas. This is a consequence of the formulas being prepared for publication using equation editors, such as MS Equation, rather than mathematical packages that allow testing of their “operability.”
Implementation of reference in the references on the Web using instruments of mathematical packets and their tools for graphic visualization makes it possible to represent information about the functions of two or more arguments in an innovative form. The web page shown in Fig. 5 displays not only values of entropy s, specific volume v, and enthalpy h of water or steam in different units for the parameters specified by site the visitor (steam pressure and temperature), but also the respective isotherm and isobar on the respective thermodynamic surface with saturation lines for water and steam (see also www.wsp.ru, the website of the package WaterSteamPro [9], the functions of which were used for plotting the thermodynamic surface shown in Fig. 5).
I have developed templates of Mathcad documents for Web “animation” of various data: fully populated tables, scattered tables with shifted arguments or diffused ranges, etc. [4]. The main amount of labor is needed in this activity to digitize tables. I was assisted by the students attending the course on information processing given at the Institute of Thermal Power Engineering and Engineering Physics of Moscow Power Institute. “Outlines” of popular scientific and engineering handbooks, textbooks, training aids, and other sources of information are gradually being formed on the Internet [10, 11]. For example, Fig. 6 shows a web page of an “animated” electronic version of a textbook by V.Ya. Rotach, Theory of Automatic Control (http://twt.mpei.ru/MAS/Worksheets/Rotach/index.html).
The calculations presented in Rotach’s textbook are performed using the Mathcad environment. The reader can download the calculation files from the site (see the third row in Fig. 6). Alternatively, he can handle them interactively by changing the initial data and obtaining the result without installing on his computer additional software, an operation that can be either forbiddingly expensive or involve violation of license agreements.
The field of Internet handbooks based on mathematical programs makes it possible to easily implement requests related not only to separate points, i.e., fixed states of materials and coolants (see Figs. 1 – 5), but to entire processes. For example, Fig. 7 shows the page of a Web-based reference (www.vpu.ru/mas) displaying the process of isoentropic steam expansion, with the isotherm, isobar, and other curves that characterize this process being displayed on the h,s chart as well. This website also offers calculations and displays on the process charts of different expansion processes specific to actual steam-turbine engines, gas-turbine engines, combined cycle turbines, and “classical” cycles (Carno, Otto, Diesel, etc.). Different sets of variables (h and s, t and s, p and v, etc.) are supported.
Currently, some books are in the process of being digitized (see www.vpu.ru [12], www.thermal.ru [13], http://twt.mpei.ac.ru/GDHB [14], etc.). The Moscow Power Institute publishing house is now preparing for publication the fifth (additional) volume of handbook [7], containing a description of methods that can be used for creation of Web-based interactive tabled, plots, and formulas. Some chapters of this volume are already posted on the Internet at http://twt.mpei.ac.ru/TTHB.
REFERENCES
1.      V.F. Ochkov, V.F. Utenkov, and K.A. Orlov, “Thermal Engineering Calculations in the Mathcad Environment,” Teploenergetika, No. 2, 73-78 (2000) [Thermal Engineering 47 (2), 173-180 (2000)].
2.      V.F. Ochkov, A.P. Pil’shchikov, et al., “Analysis of Ion-Exchange Isotherms Using the Mathcad Software Package,” Teploenergetika, No. 7, 13-18 (2003) [Thermal Engineering 50 (7), 537-543 (2003)].
3.      V.F. Ochkov, A.P. Pil’shchikov, and Yu. V. Chudova, “Open Calculations in Thermal Power Engineering,” Energosberezheniye I Vodopodgotovka, No. 1, 21-24 (2002).
4.      V.F. Ochkov, Mathcad 12 for Students and Engineers (BKhV, St Petersburg, 2005) [in Russian].
5.      V.F. Ochkov, “Mathcad: from Plot to Formula, from Computer Calculation to Internet Calculation,” Exponenta Pro. Matematika v Prilozheniyakh, No.4, 84-85 (2003).
6.      V.F. Ochkov, “Mathematical Packages: from Natural Economy to Production of Computer Commodities via Internet,” ComputerPress, No. 5, 172-173 (2004).
7.      Theoretical Foundations of Thermal Engineering. Thermal Engineering Experiment. Handbook, Ed. by A.V. Klimenko and V.M. Zorin (Publishing House of the Moscow Power Institute, Moscow, 2001), 3rd revised and extended edition [in Russian].
8.      V.F. Ochkov, Physical and Economic Quantities in Mathcad and Maple (Finansy i Statistika, Moscow, 2002) [in Russian].
9.      A.A. Alexandrov, K.A. Orlov, and V.F. Ochkov, “Study of the Schemes for a Combined-Cycle Plant with Steam Injection into the Gas Path on the Basis of Development Applied Programs for the Properties of Working Fluids in Combine-Cycle Plants,” Novoe v Rossiiskoi Elektroenergetike, No. 4, 27-31 (2004).
10.  V.F. Ochkov, O.G. Osipov, and M.V. Volokitin, “New Approaches to Publication in Industry Standards and Other Regulatory Documents Containing Calculations in a Power Engineering Corporate Network,” Novoe v Rossiiskoi Elektroenergetike, No. 10, 21-25 (2005).
11.  V.F. Ochkov, “Thermal Engineering References in Internet,” Novoe v Rossiiskoi Elektroenergetike, No. 4, 48-58 (2005).
12.  A.S. Kopylov, V.M. Lavygin, and V.F. Ochkov, Water Trearment in Power Engineering (Publishing House of the Moscow Power Institute, Moscow, 2003) [in Russian].
13.  A. Solodov and V. Ochkov, Differential Models. An introduction with Mathcad {Springer-Verlag, 2004).
14.  V.F. Kasilov, Handbook on Gas Dynamics for Specialists in Thermal Power Engineering (Publishing House of the Moscow Power Institute, Moscow, 2000) [in Russian].

Saturday, December 11, 2010

THERMAL ENGINEERING IN POWER SYSTEMS

Volume 22 in the WIT Press Developments in Heat Transfer series looks at the research and development in thermal engineering for power systems that are of significant importance to many scientists who work in power-related industries and laboratories. To be competitive in today's market, the editors say, power systems need to reduce operating costs, increase capacity, and deal with many other tough issues. Among the topics the book covers are: relevance of heat transfer and heat exchangers for development of sustainable energy systems; advanced technologies for clean and efficient energy conversion in power systems; virtual engineering and the design of power systems; and innovative gas turbine cooling techniques.

Monday, December 6, 2010

Heat Exchangers

Process Heat Exchangers

TEi has incorporated the knowledge and experience from recent acquisitions of EFCO, which included previous Westinghouse and Marley feedwater heater products, to provide our customers with a wide array of quality process heat exchangers. We supply high and low pressure feedwater heaters, oil coolers, component coolers and service water coolers to the petrochemical, petroleum refining, process chemical, electric generation, synthetic fuel processing, natural gas processing, solar and gasification markets.
The combination of TEi’s experience, shop capabilities (Joplin, MO and Sapulpa, OK), financial strength, and field service support, offers the process industry a reliable supplier that is capable of providing exchangers which can meet the most demanding operating service conditions. In order to more effectively service the process industry, TEi has opened a Houston, TX Process Heat Exchanger Division sales office, staffed with experienced employees.

TEi is a member of HTRI and HEI and builds to TEMA standards. TEi is registered with ASME as an ASME Sec VIII Div.1 and 2 Code manufacturer. TEi also holds certifications for ASME “U”, “S”, and “P” Code Stamps, NBIC “R” Stamp, China Safety License and ISO 9001 Certification

Sunday, December 5, 2010

Thermal power plant or Steam power plant

A generating station which converts heat energy of coal combustion in to electrical energy is known as Thermal power plant or Steam power plant. Some of its advantages and disadvantages are given below.

Advantages

The fuel used is quite cheap.
Less initial cost as compared to other generating plants.
It can beinstalled at any place iirespective of the existence of coal. The coal can be transported to the site of the plant by rail or road.
It require less space as compared to Hydro power plants.
Cost of generation is less than that of diesel power plants.
Disadvantages

It pollutes the atmosphere due to production of large amount of smoke and fumes.
It is costlier in running cost as compared to Hydro electric plants.

Saturday, December 4, 2010

OFFICE SAFETY

Staff who work exclusively in offices should be aware that they have the RIGHT TO KNOW any laboratory hazards in the surrounding area and they should feel free to discuss any questions or concerns with any member of the Department Safety Committee.
It is the responsibility of each employee to perform his or her job in a safe manner. Safety is as important in the office as it is in the laboratory.
Offices should be inspected by the occupants for earthquake hazards. Tall bookshelves and cabinets (including lateral file cabinets) must be anchored to the wall or made secure by other approved means (contact John Souza, 2-3314). There should be no overhead storage that could create a falling hazard.
Extension cords are not to be used. Approved multi-plug strips may be used as long as they have an internal breaker and are not run in series with other cords (daisy-chained). All cords should be inspected for wear, frayed cords are to be replaced.
Use of space heaters has been specifically prohibited by the State Fire Marshal. Problems with room heat should be reported to Physical Plant, 2-1032.
Furniture arrangement in offices should permit a quick exit in an emergency. Quantities of paper or other combustibles must be kept at a minimum.
EH&S (2-3073) has many brochures available regarding display terminals (VDT) and other office machinery.

Saturday, November 27, 2010

Thermodynamics process of a perfect gas

Adiabatic process



Polytropic process


Constant Volume Process


Throttling Process




Friday, November 19, 2010

Free Expansion ( Unresisted Expansion ) Process

               The free expansion process is an irreversible non-flow process. A free expansion process occurs when a fluid is allowed to expand suddenly into a vacuum chamber through an orifice of large dimensions.
               Consider two chamber A and B separated by a partition. Since there is no expansion of the boundary of the system, because it is rigid, therefore no work is done. Thus, for a free expansion,
                                    Q1-2 = 0; W1-2 = 0 and dU = 0
                The following points may be noted regarding the free expansion of a gas:
1. Since the system is perfectly insulated so that no heat transfer takes place therefore the expansion of gas may be called as an adiabatic expansion.
2. Since the free expansion of the gas from the equilibrium state 1 to the equilibrium state 2 takes place therefore the intermediate state will not be in equilibrium states.
etc...

Thursday, November 18, 2010

Rate of Heat Transfer

                    Heat transfer during a polytropic process
                                        Q1-2 = (segma) - n / (segma) - 1 * W1-2
 where W1-2 is the work done during polytropic process.
                     If dQ is the small quantity of heat transfer during small change of pressure and volume, then
                                                 dQ = (segma) - n / (segma) - 1 * pdv
Rate of heat transfer per unit volume,
                                                dQ / dv = (segma) - n / (segma) - 1 * p

and rate of heat transfer per second,
                                     dQ / dt = dQ / dv * dv / dt = (segma) - n / (segma) -1 * p * dv / dt
where dv / dt is the swept volume of the piston per second.

Saturday, November 13, 2010

Constant Tempreture Process (Isothermal Process)

             A process, in which the tempreture of the working substance remains constant during its expansion or compression, is called constant tempreture process or isothermal process. This will happen when the working substance remains in a perfect thermal contact with the surroundings, so th
at the heat "sucked in' or 'squeezed out' is compensated exactly for the work done by the gas or on the gas respectively. It is thus obvious that in an isothermal process:
    1. there is no change in tempreture.
    2. there is no change in internal energy, and
    3. there is no change in enthalpy.
          Now consider m kg of a certain gas being heated at constant tempreture from an intial state 1 to final state 2.
    Let                 p1v1 and T1 = Pressure, volume and tempreture at the intial state 1, and
                          p2v2 and T2 = Pressure, volume and tempreture at the final state 2.



Tuesday, November 2, 2010

Hyperbolic Process

           A process, in which the gas is heated or expanded in such a way that the product of its pressure and volume (i.e *v ) remains constant, is called a hyperbolic process.
          It may be noted that the hyperbolic process is governed by Boyle,s law i.e p v = constant. If we plot a graph for pressure and volume, during the process as shown in fig we shall get a rectangular hyperbola. Hence, this process is terned as hyperbolic process. It is merely a theoretical case, and has a little importance from the subject point of view. Its practical application is isothermal process, which is discussed below.

Friday, October 29, 2010

General Laws for Expansion and Compression

     The general law of expansion or compression of a perfect gas is pVn = Constant. It gives the relationship between pressure and volume of a given quantity of gas. The value of n depends upon the nature of gas., and condition under which the changes take place. The value of n may be between zero and infinity. But the following values of n are important from the subject point of view.

1. when n = 0. This means pV0 = constant, i.e.p = constant. In other words, for the expansion or constant of a perfect gas at constant pressure, n = 0.

2. when n = 1 ; then pv = constant, i.e the expansion or compression is isothermal or hyperbolic.
3. when n lies between 1 and n, the expension or compression is polytropic, i.e. pVn = Constant.
4. when n = & the expension or compression is adiabatic
5. when n = infinity the expansion or compression is at constant volume, i.e. v - Constant.

Monday, October 25, 2010

Importance of Critical Steel Ratio in Calculating Thermal Reinforcement

The fulfillment of critical steel ratio means that in construction joints or planes of weakness of concrete structure, steel reinforcement will not yield and concrete fails in tension first. This is important in ensuring formation of more cracks by failure of concrete in tension, otherwise failure in steel reinforcement would produce a few wide cracks which is undesirable. 

   What is the difference in arranging pumps in series and in parallel… solved example extracted from the topic Pumping Station of steel structures based on civil engineering… identical pumps with similar functions, if the pumps arranged in series, the total head is increased without a change to maximum discharge...

  • Functions of Cap Block, Drive Cap and Pile Cushion
       What are the functions of cap block, drive cap and pile cushion in driven piles… Solved example based on the topic piles and foundation Marine Piles of Reinforced Concrete Design of civil engineering subject… Cap block is installed between the hammer end and the drive cap to control the hammer blow in order to protect both the hammer and the pile from damage.




  • Relation of Bearing Pressure
       What is the relation of bearing pressure on soil nail head to the ratio La/Lb, where La is the length of soil nail before the potential slip circle…sample problem based on the topic Soils & Rocks from the Soil Mechanics of Civil Engineering subject… unstable soil mass before the potential circular slip is resisted by two components: soil nail head bearing pressure and friction of soil nail in the unstable soil mass...




  • Acetylene Gas Cylinders used for Gas Welding
       Why should acetylene gas cylinders used for gas welding be erected in upright position… Solved Example based on the topic Steel Structural Analysis of structure from the subject Civil Engineering… Acetylene gas is commonly used for gas welding because of its simplicity of production and transportation and its ability to achieve high temperature in combustion...




  • Major Problems in Using Pumping
       What are the major problems in using pumping for concreting works… solved example extracted from the topic Composite Steel-Concrete Construction of steel structures based on civil engineering… The main problems associated with pumping are the effect of segregation and bleeding…



  • Saturday, October 23, 2010

    SAFETY PRECAUTIONS AND OPERATING INSTRUCTIONS

    When engineering personnel work outside of the engineering department, the responsibility for training and enforcing safety precautions rests with the head of the  department  controlling  the  operation.  For  example, weapons and ammunition handling requires special instructions by the weapons officer. The   engineer   officer   has   the   following responsibility  for  safety  in  the  engineering  department: l l l Be  sure  safety  precautions  are  posted  in  a conspicuous  and  accessible  places. Be sure all persons in the department and others who  may  be  concerned  with  engineering  matters observe safety precautions. Drill personnel in the safety procedures that apply to their work. Each   division   officer   has   the   following responsibilities for safety in his division: l l l Instruct  subordinates  in  all  safety  precautions that  apply. Require  subordinates  to  observe  all  safety precautions that apply. Post   safety   precautions   and   warnings   in conspicuous  places.  This  includes  posting warnings on dangerous equipment and in areas of the ship where there are particular hazards. Each  member  of  the  engineering  department  has the  following  responsibilities: l l l l l Report  unsafe  conditions  and  correct  the conditions  where  possible. Warn  others  of  unsafe  conditions. Use  approved  protective  clothing  and  equipment where it is called for. Report injury or ill health to supervisors. Use caution in emergency conditions or other dangerous  situations. WARMING-UP SCHEDULES Warming-up schedules for propulsion machinery and boilers are chronological checklists of the key steps used to light boiler fires and warm up the ship’s main engineering plant.

    Saturday, October 16, 2010

    Types of Air compresser

    Centrifugal
    in service. Where capacity or horsepower rather than numbers is considered as a measure, the centrifugal,
    without a doubt, heads the compressor field. During the past 30 years, the centrifugal compressor,
    because of its smaller relative size and weight compared to the reciprocating machine, became
    much more popular for use in process plants, which were growing in size. The centrifugal compressor
    does not exhibit the inertially induced shaking forces of the reciprocator and therefore does not need
    the same massive foundation. Initially, the efficiency of the centrifugal was not as good as that of a
    well maintained reciprocating compressor. However, the centrifugal established its hold on the market
    in an era of cheap energy when power cost was rarely, if ever, evaluated.
    The smaller compressor design was able to penetrate the general-process plant market, which had
    historically belonged to the reciprocating compressor. As the compressor grew in popularity, developments
    were begun to improve reliability, performance, and efficiency. With the increase in energy
    cost in the mid-1970s, efficiency improvements became a high priority. Initially, most development
    had concentrated on making the machine reliable, a goal that was reasonably well achieved. Run
    time between overhauls currently is three years or more, with six-year run times not unusual. As
    plant size increased, the pressure to maintain or improve reliability was very high because of the
    large economic impact of a nonscheduled shutdown.
    Centrifugal compressors are dynamic types with rotating impellers that impart velocity and pressure
    to air (Fig. 61.6). Their design is simple and straightforward, consisting of one or more highspeed
    impellers with cooling sections. The only lubrication required is in the drive system, which is
    sealed off from the air system.
    Integral gear-type centrifugal air compressors are generally used in central plant air applications
    requiring volumes ranging from 1000-30,000 cfm and discharge pressures from 100-125 psig.
    Centrifugal air compressors are normally specified on the basis of required air-flow volume.
    However, there are several ways to calculate volume and serious problems can result unless both
    user and manufacturer use the same method. At the very least, the user can have problems comparing
    bids from competing manufacturers. At worst, he may choose the wrong compressor.
    These problems can be avoided by specifying capacity in terms of actual inlet conditions and by
    understanding how compressor capacity is affected by variable ambient conditions such as inlet
    pressure, temperature, and relative humidity. Factors such as cooling water temperature and motor
    load must be considered before a compressor and its drive motor can be sized.
    A multistage arrangement for integral gear-type compressors is shown in Fig. 61.7. The flow path
    is straight through the compressor, moving through each impeller and cooler in turn. This type of
    centrifugal compressor is probably the most common of any found in process service, with applications
    ranging from air to gas.
    compressors are second only to reciprocating compressors in numbers of machines
    Sliding-vane
    61.8). As the rotor turns, the vanes slide out against the stator or housing. Air compression occurs
    when the volume of the space between the sliding vanes is reduced as the rotor turns. Single- and
    multistage versions are available.
    compressors consist of a vane-type rotor mounted eccentrically in a housing (Fig.

    • Oxidation
    • Condensation
    • Viscosity
    • Outgassing in the inlet
    • Foaming
    • Separation performance
    • Chemical reaction
    Some problems can be solved with specially selected oil grades. Another solution is synthetic oils,
    but cost is a problem, particularly with silicone oils. Alternatives need to be reviewed to match
    service life of the lubricant with lubrication requirements in the compressor.
    One consideration for flooded compressors is the recovery of liquid. In conventional arrangements,
    the lubricating oil is separated at the compressor outlet, cooled, filtered, and returned to the compressor.
    This is fine for air service, where oil in the stream is not a major problem, but when oilfree
    air is needed, the separation problem becomes more complex. Because the machine is flooded
    and the discharge temperature is not high, separation is much easier relative to compressors that send
    small amounts of fluid at high temperature down stream. Usually part of the lubricant is in a vaporized
    form and is difficult to condense except where it is not wanted. To achieve quality oil-free air, such
    as that suitable for a desiccant-type dryer, separators that operate at the tertiary level should be
    considered. Here, the operator must be dedicated to separator maintenance, because these units require
    more than casual attention. Separation by refrigeration is not as critical if direct expansion chillers
    are used. In these applications, the oil moves through the tubes with the refrigerant and comes back
    to the compressor with no problem, if the temperature is not too low for the lubricant.
    Advantages of helical screw compressors include smooth and pulse-free air output, compact size,
    high output volume, low vibration levels, and long life.