1. Chapter 17A: COMPRESSORS AND EXPANSION DEVICES
Agami Reddy (rev- May 2017)
Compressor types
Reciprocating compressors:
- volumetric efficiency
- compressor efficiency
Rotary compressors
Scroll
Screw
Centrifugal
Expansion devices: capillary, TXV
HCB 3- Chap 17A: Compressors and Exp Devices

2. HCB 3- Chap 17A: Compressors and Exp Devices
At the heart of the vapor compression air conditioner is the COMPRESSOR
Two general categories:
Positive displacement- increases
the pressure of the vapor by
reducing the volume of the
compression chamber
Centrifugal- increases
the pressure of the vapor via
exchange of angular momentum between the rotating element and the refrigerant vapor
Reciprocating
Rotary
Screw
Scroll
HCB 3- Chap 17A: Compressors and Exp Devices

3. HCB 3- Chap 17A: Compressors and Exp Devices
Fig Polytropic compression process p–v diagram used for chiller compressor work calculation.
HCB 3- Chap 17A: Compressors and Exp Devices

4. Reciprocating Compressors
Most popular
Sizes: ½ to 200 T
Familiar piston/
compressor
Moderate compression ratios 7:1
Speeds- up to 3600 rpm
Different types
- single acting
- single/multiple cylinder
Hermetic
Open
HCB 3- Chap 17A: Compressors and Exp Devices

5. HCB 3- Chap 17A: Compressors and Exp Devices
Expansion and compression
are polytropic:
pvn = cte
Distinguish between theoretical
and actual polytropic exponent
1-2 Intake or suction stroke
2-3 compression stroke
3-4 Exhaust or discharge stroke
Fig Reciprocating chiller:
piston-and-valve arrangement and
corresponding p–V diagram.
HCB 3- Chap 17A: Compressors and Exp Devices

6. Capacity loss due to high temperature and re-expansion
What happens to mr as the
suction pressure is decreased?
As psuction decreases,
also decreases
Eventually we will reach a point
when
Also, temperature of compressed gas much higher than the incoming gas
HCB 3- Chap 17A: Compressors and Exp Devices

7. Efficiency Measures (a) Volumetric efficiency
Measure of effectiveness of compressor as
a refrigerant “mover”, > 90%
(a) Volumetric efficiency
(17.2)
(b) Compressor efficiency
(same as isentropic effy but
with motor effy included)
Measure of thermodynamic efficacy
of the compression process: 75-80%
HCB 3- Chap 17A: Compressors and Exp Devices

8. HCB 3- Chap 17A: Compressors and Exp Devices
Rotary Compressors
Rolling piston
Fig Rotating vane:
2 vane and 4 vane
HCB 3- Chap 17A: Compressors and Exp Devices

9. HCB 3- Chap 17A: Compressors and Exp Devices
Primary applications:
Refrigerators
Room air conditioners
Automotive air conditioners
Sizes: 1/6 – 3 Tons
Characterized by CIRCULAR or ROTARY
motion to compress refrigerant
Compression ratios limited to 7:1 (otherwise too much leakage)
Good part load performance (down to 20%)
Inherently balanced
Relatively light and have long life
Relatively low starting torques and need inexpensive motors
Have high volumetric efficiencies
Low noise
Low vibration
HCB 3- Chap 17A: Compressors and Exp Devices

10. HCB 3- Chap 17A: Compressors and Exp Devices
Scroll compressors
Fig. 17.4
Operation of a scroll compressor at different angles of rotation
Scroll is the curve formed by end of a string as it is unwrapped from a cylinder
HCB 3- Chap 17A: Compressors and Exp Devices

11. First manufactured by Hitachi in 1983
First patented in 1905
First manufactured by Hitachi in 1983
US manufacturers: Copeland and Trane
Has potential to increase efficiencies by 10-15%
Large compression ratios: 20:1
Principal applications 1.5 – 10 Tons (can go up to 100 T)
Capacity control: variable discharge ports or speed control
Quiet operation, low vibration and wear
Main Features:
Fixed scroll
Orbiting scroll
No suction or discharge valves
Requires two revolutions to compress refrigerant
Needs precision machining
COP: 2.9 – 3.2
HCB 3- Chap 17A: Compressors and Exp Devices

12. HCB 3- Chap 17A: Compressors and Exp Devices
Screw Compressors
Developed in 1930s, appeared in 1960s
Capacities: 2- to 1300 Tons
Rotational speeds of male rotor ~ 3600 rpm
Intermediate between reciprocating and centrifugal
Good part load operation (down to 10% full load)
Fig Dual-screw compressor
HCB 3- Chap 17A: Compressors and Exp Devices

13. HCB 3- Chap 17A: Compressors and Exp Devices
Fig Typical screw compressor efficiency curves for R Vi is the compressor volume ratio which is varied by a slide valve that changes the size and shape of the discharge port.
HCB 3- Chap 17A: Compressors and Exp Devices

14. Centrifugal Compressors
200 kW to 10 MW
HCB 3- Chap 17A: Compressors and Exp Devices

15. Centrifugal Compressor
Dynamic compression based on speed and type of impeller
Can move large refrigerant volumes at moderate lift
Compression ratio per stage up to ~4:1
Large capacities (up to ~20,000 tons)
Multistage compression needed for ice-making and other higher lift applications
Capacity control by inlet guide vanes or speed control
Impeller schematic
Chiller Plant
Multistage compressor
HCB 3- Chap 17A: Compressors and Exp Devices
Slide 15

16. HCB 3- Chap 17A: Compressors and Exp Devices
Centrifugal vs Reciprocating Performance
1) Effect of evaporator
temperature for
centrifugal chillers:
Capacity reduces
from 240 to 100 Tons
when evaporator temp
reduces by 100 F
2) Centrifugal compressor will maintain a near constant evaporator temp. when cooling load changes
Fig Comparison of centrifugal and reciprocating compressors: effect of evaporator temperature on refrigeration capacity
HCB 3- Chap 17A: Compressors and Exp Devices

17. HCB 3- Chap 17A: Compressors and Exp Devices
Centrifugal vs Reciprocating- Effect of Condensing Temperature
Fig (a and b) Comparison of centrifugal and reciprocating compressors: effect of condensing temperature on refrigeration capacity and brake horsepower
The condenser temperature of centrifugal compressors has a much larger effect on cooling capacity. This underlines importance of having a cooling tower
(air cooled condensers adequate for reciprocating compressors)
Power consumed by centrifugal compressor decreases with condenser temperature, but that of reciprocating compressor actually increases
HCB 3- Chap 17A: Compressors and Exp Devices

18. HCB 3- Chap 17A: Compressors and Exp Devices
Fig Comparison of centrifugal and reciprocating compressors: effect of speed on (a) refrigeration capacity and (b) brake horsepower. The evaporator and condensing temperatures are held constant
HCB 3- Chap 17A: Compressors and Exp Devices

19. HCB 3- Chap 17A: Compressors and Exp Devices
Table 17.1 Comparison of Different Chiller Compressor Types
Chiller Type
Capacity Range (kW)
Operating Cost
Full-Load COP
Part-Load Efficiency
Part-Load Limit
Compression Ratios Up to
Recip
2-600
High
Very good
30%
10:1
Rotary vane
0.5-10
Low
3.0 – 4.0
20%
7:1
Scroll
5-200
Medium
Good
50%
Twin-screw 5%
15:1
Centrifugal
400-10,000
Poor
10%
3:1
Numbers shown are approximate
HCB 3- Chap 17A: Compressors and Exp Devices

20. HCB 3- Chap 17A: Compressors and Exp Devices
Ways to control capacity
Cylinder loading
Moveable slot at compressor
entrance
Inlet guide vanes to compressor
Variable speed motors
Hot gas bypasses
HCB 3- Chap 17A: Compressors and Exp Devices

21. HCB 3- Chap 17A: Compressors and Exp Devices
Expansion Devices
Expansion devices serve two purposes in A/C:
- Reduce pressure of liquid refrigerant
- Regulate the flow of refrigerant to the evaporator
Expansion devices found in A/C systems:
- Capillary tubes (small units)
- Short-tube orifices (Large centrifugal chillers)
- Valves (medium sized- widely used)
(i) Thermostatic expansion valves (TXV)
(ii) Electronic expansion valves (EEV)
(iii) Float valves
HCB 3- Chap 17A: Compressors and Exp Devices

22. HCB 3- Chap 17A: Compressors and Exp Devices
Capillary Tubes
- Used exclusively on small refrigeration systems
- Used on older residential A/C
- Valve replaced by long thin tube
- Tube allows flow of liquid more readily than vapor
- Operates best with one set of conditions
- Simple, low cost, no moving parts
- Consists of a small diameter (0.02 – 0.2 inch) line
that connects outlet of the condenser to inlet of evaporator
Length may vary from 3 to 5 feet
Disadvantage; small tube subject to clogging
HCB 3- Chap 17A: Compressors and Exp Devices

23. HCB 3- Chap 17A: Compressors and Exp Devices
Thermostatic Expansion Valve
Device to control the mass flow rate of refrigerant liquid
entering the evaporator
Its function is to assure that refrigerant is superheated vapor
before entering compressor
Misnomer: control not by refrigerant temperature but by magnitude of
refrigerant vapor superheat leaving evaporator
Fig Sketch slowing the location, operation, and control of a thermostatic expansion valve.
HCB 3- Chap 17A: Compressors and Exp Devices

24. HCB 3- Chap 17A: Compressors and Exp Devices
Major components
Filler bulb: Filled with same or similar refrigerant to that used in system
Capillary tube: used to transmit pressure (superheat) signal back to diaphragm
Diaphragm: Focus point of force inside valve
Spring: keeps valve closed until pressure above diaphragm overcomes the spring
force plus evaporator pressure
HCB 3- Chap 17A: Compressors and Exp Devices

25. HCB 3- Chap 17A: Compressors and Exp Devices
Outcomes
Understanding the importance of compressors in VC systems
Knowledge of the different generic categories of compressor types and understanding differences between them in capacity, efficiency, part-load
Understanding how a reciprocating chiller operates and knowledge of relevant performance measures
Understanding how rotary, screw and scroll compressors operate
Understanding of how centrifugal compressors operate
Knowledge of different ways of capacity control
Knowledge of interaction of cooling capacity and evaporator and condenser temperatures for reciprocating and centrifugal chillers
Understanding of the purpose of expansion devices
Understanding the operating principles of capillary and TXV
HCB 3- Chap 17A: Compressors and Exp Devices