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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.