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Compressor Basics

As with pumps, dh=vdP. However, v is no longer a

constant, making calculation of h2-h1 more

complicated.

For a polytropic process, Pvn=C

For an isentropic process, n=k (where k=cp/cv), andfor an isothermal process, n=1.

( )1

2 2

2 1

1 1

(1)n

Ch h vdP dPp

= =

Compressor Efficiency The adiabatic compressor efficiency:

Total compressor efficiency:

Typical efficiencies are 90% for the motor drive at

peak load, 90% for the mechanical efficiency, and76% to 97% for the adiabatic efficiency.

Typically, as the compressor size increases, so doesthe adiabatic efficiency.

( )( )

2 1

2 1

(2)isentropic sa

actual actual

h hW

W h h

= =

(3)compressor a motor drive mechanical =

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Exit temperature

A maximum recommended fluid temperature is given based on

compressor and fluid type.

Air compressors typically shouldnt have an air exit temperature

greater than 300-375F to prevent carbonizing, combustion of oil

vapor, or weakening of parts over time.

Air can be modeled as a perfect gas where

and

These can be substituted into Equ. (1), and using Equ. (2) or (3)

as well, the exit air temperature T2 can be found as a function of

pressure ratio r, n, k, and efficiency. Once T2 is known, the compressor work can be found using

/Pv RT M = p vRc c

M =

( )2 1 2 1actual pW h h mc T T = =

Decreasing Compressor Work Two possible methods

Minimize irreversibilities due to friction, turbulence, and non-quasi-

equilibrium compression

Make the specific volume of the gas as low as possible by cooling

the compressor since

One cooling possibility is to use multi-stage compression with

intercooling.

2

2 1

1

compressorW h h vdP= =

From Cengel, Thermodynamics: An Engineering Approach, 4th ed.

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Compressors

Five most common types

Reciprocating

Uses a piston-cylinder and valves

Most common type of compressor

Screw

Lobes of two rotating screws trap and compress gas

Centrifugal

Uses centrifugal force to compress gas

Common in large refrigeration systems (200 to 10000 kW ofrefrigeration capacity)

Vane

Uses a roller to compress gas

Used in most domestic refrigeration and ac systems

Scroll

Two inter-fitting spiral-shaped scrolls compress the gas

Used in 1-15 ton (3.5 to 53 kW) range ac applications

Compressor pressure ranges

From Burmeister, Elements of Thermal-Fluid Design.

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Terminology

Open-type compressor Crankshaft extends through housing to connect with the

motor

Seals are used to limit leakage

Hermetically sealed Motor and compressor are combined in the same housing

Used for small domestic air conditioning systems

Semi-hermetic Cylinder heads are removable for serviceability. Good for c

systems larger than domestic.

Condensing unit Motor, compressor, and condenser are combined in one unit

and sold together

Reciprocating Compressors

Gas in the clearance volume must expand to V1 before thepressure is low enough to open the suction valves and drawmore gas in.

Stoecker and Jones, Refrigeration and Air Conditioning, 2nded, Mc-Graw Hill.

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Reciprocating Compressors, cont.

Actual volumetric efficiency

Clearance volumetric efficiency

The clearance volumetric efficiency tells us what percent of

the clearance volume is used to bring new gas in.

Percent clearance

( )( )

3

3

mvolume flow rate entering compressors

100mdisplacement rate of compressor

s

va x =

3 1

3

volume of gas drawn into cylinder100 100

useable volume of cylinderva

c

V Vx x

V V

= =

3

100c

c

Vm x

V V=

Reciprocating Compressors, cont.

After some algebra

where

vsuc=specific volume of vapor entering compressor

vdis=specific volume of vapor after isentropic

compression

1100 1vc

c

Vm

V

=

1 uc

c dis

V v

V v=

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Reciprocating Compressors, cont.

To find mass flow rate (kg/s)

The displacement rate is a volumetric flow rate; vsucconverts that to a mass flow rate

As the suction pressure (and evaporating

temperature) drops, what happens to the mass flow

rate? What does this tell us about running a room air

conditioner during the winter?

vc 100displacement rate xsuc

mv

=

Reciprocating Compressors for refrigeration

Stoecker and Jones, Refrigeration and Air Conditioning, 2nded, Mc-Graw Hill.

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Reciprocating Compressors for Refrigeration

Most refrigeration

systems operate on

the left side of the

power curve.

During startup, the

power requirement

may pass the peak

and demand more

motor power.

Stoecker and Jones, Refrigeration and Air Conditioning, 2nded, Mc-Graw Hill.

Reciprocating Compressors, cont.

Adiabatic compression efficiency (use this to find the actual

enthalpy at the compressor exit)

Losses are due mainly to friction of rubbing surfaces and

pressure drop across valves

Watch your exit conditions. If the exit temperature is too hot, the

oil will break down and reduce the life of your valves. Themaximum recommended oil temperature varies with the oil type.

This can be a problem especially with ammonia, which tends to

have high discharge temperatures. Ammonia compressors often

are equipped with external water cooling.

100isentropic

a

actual

wx

w =

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Reciprocating Compressor Variation

The rate at which a compressed vapor is produced

by a given compressor can be varied in two ways.

Compressor speed can be varied (pistons RPM increases or

decreases)

Compressor can be unloaded (see Burmeister pg 165 for

details). Examples of how this is done include

Holding open inlet valves to no vapor is compressed

Varying the clearance volume by using actuators to open

some extra piston chambers

Rotary Screw Compressors

Good efficiency (60-80%) for pressure ratios above approx. 2.5.

Stoecker and Jones, Refrigeration and Air Conditioning, 2nded, Mc-Graw Hill.

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Vane Compressors

No suction valve needed. Minimum gas pulsation

Stoecker and Jones, Refrigeration and Air Conditioning, 2nded, Mc-Graw Hill.

Vane Compressors, cont. May be single-vane (roller) type or multiple-vane

For the roller type, displacement rate D

Where A=cylinder diameter, B=rollerdiameter,L=cylinder length (all in m), and the rotativespeed is in revolutions per second.

( ) ( )2 2 3 /4

D A B L rotative speed m s

=

Stoecker and Jones, Refrigeration and Air Conditioning, 2nded, Mc-Graw Hill.

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Dynamic Compressors -- Centrifugal

Commonly used for large systems, including chillers

Gas enters a spinning impeller and is thrown to the

outside of the impeller through centrifugal force

Impeller provides the gas with a high velocity (kinetic

energy) which is converted to pressure (internal

energy); remember Bernoullis Law!

70-80% isentropic efficiencies

Axial compressors are a somewhat less common

form of dynamic compressors

Centrifugal Compressor Performance

Stoecker and Jones, Refrigeration and Air Conditioning, 2nded, Mc-Graw Hill.

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Surging To the left of the surge envelope line, the compressor capacity

decreases significantly due to flow separation on the blades andthe characteristics of the backward-curved blades used. If used

for refrigeration, this can cause problems.

At point C, capacity drops, but the evaporator heat load continues

to boil off refrigerant. This increases the evaporator pressure

(decreasing the pressure ratio), and moves us back to point A.

Then the process continues, which is called surging.

Surging causes noise, load fluctuations on the motor and

compressor, and reliability problems over the long term.

Stoecker and Jones, Refrigeration and Air Conditioning, 2nded, Mc-Graw Hill.

Scroll Compressors

Need close machining

tolerances

Low noise, high efficiency

Incompatible with solid

contaminants and poor

performance at low

suction pressures

From McQuiston, Parker, and Spitler, Heating, Ventilating, and Air Conditioning Analysis and Design

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Assignment

Put together an EES code for a reciprocating compressor with70% isentropic (adiabatic) efficiency, a displacement rate of 100L/s, and five percent clearance. .

Use P1=130 kPa and P2=800 kPa and R134a.

Use 5C superheating for the gas entering the compressor (thesuction gas). This means that T inlet=Tsaturation+5

On EES, use a format similar to the one shown below to find yoursuction temperature T1. Then use T1 and P1 to find h1 and vsuc.

T1=Tsat+5

Tsat=Temperature(R134a, x=0.5, P=P1)

Here we could have use any quality from 0 to 1 in the Tsat callstatement; as long as we have a liquid/vapor mixture, theresulting temperature will be the saturation temperature.

Solve for the exit enthalpy, compressor power, and refrigerantmass flow rate.

Turn in your EES code along with the solution.