compressor design

1. ME 340A - Introduction to Refrigeration and Air
Conditioning
Lecture 23:
Compressor Analysis II
Dr. Abhijit A Sathe
IIT Kanpur
E-mail: asathe@iitk.ac.in

2. WHAT WE STUDIED LAST TIME
 Review of compressor types
 Definitions of COP/EER and SEER
 Compressor efficiencies – Volumetric, isentropic
 Importance of oil in refrigeration system
 Oil separators, suction line accumulators
 Capacity control
 Recent advances

3. COMPRESSORS IN HVAC
 The compressor is the heart in a refrigeration
system
» Only major component that has moving parts
 Compressors have a cooling capacity and a
COP/EER
» Compressors are often rated by cooling capacity
and COP/EER based on assumed system state
points (e.g. 1 ton compressor with EER of 12)
 The task of the compressor in a VC system is to
provide continuous mass flow rate of refrigerant
from a low pressure level to a high pressure level
Miniature compressor by
Aspen (200 W)

4. CENTRIFUGAL COMPRESSORS
• Dynamic, continuous-flow machines
• Achieve compression by a rotating impeller
• Generally a multi-stage for increasing pressure ratios

5. CENTRIFUGAL COMPRESSORS
• Fluid enters impeller in axial direction and discharged radially
• Fluid forced through impeller by rapidly rotating blades
• Fluid then flows through a diffuser which increases pressure
further
• Maximum ΔP depends on impeller RPM and diameter

6. CENTRIFUGAL COMPRESSORS
Major components
• Casing
• Rotor assembly
(impeller, shaft,
bearings, etc.)
• Seals

7. • Compressor rating is expressed in three parameters
 Mass flow rate, kg/s
 Power input, W
 Cooling capacity, W or ton
• A function of evaporating and condensing temperatures for fixed
compressor inlet superheat and condenser subcooling
• System independent rating, but refrigerant specific
• Capacity data facilitates compressor selection
P
h
s
2
4
2
3
4 1
Pcond
Pevap
T
Tcond
Tevap
1
TH
TL
subcooling
Tsc
3
superheat: Tsh
COMPRESSOR PERFORMANCE DATA

8. 0 5 10 30 35 40
Flow
rate
(kg/hr)
15 20 25
Evaporating temp (C)
• Mass flow rate as a function of evaporating temp (Tevap)
• Fixed condensing temp and constant suction superheat
Mass flow rate
600
500
400
300
200
100
0
Tcond = 45 C
COMPRESSOR PERFORMANCE DATA

9. 150
100
50
0
30 35 40 60 65 70
Flow
rate
(kg/hr)
45 50 55
Condensing temp (C)
• Mass flow rate as a function of condensing temp (Tcond)
• Fixed evaporating temp and constant suction superheat
Mass flow rate
250
200
Tevap = 5 C
COMPRESSOR PERFORMANCE DATA

10. 0 5 10 30 35 40
Cooling
capacity
(TR)
15 20 25
Evaporating temp (C)
• Cooling capacity as a function of evaporating temp (Tevap)
• Fixed condensing temp and constant suction superheat
Cooling capacity
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Tcond = 45 C
COMPRESSOR PERFORMANCE DATA

11. 1.5
1
0.5
0
30 35 40 60 65 70
Cooling
capacity
(TR)
45 50 55
Condensing temp (C)
• Cooling capacity as a function of condensing temp (Tcond)
• Fixed evaporating temp and constant suction superheat
Cooling capacity
2.5
2
Tevap = 5 C
COMPRESSOR PERFORMANCE DATA

12. 500
450
400
350
300
250
200
550
700
650
600
0 5 10 30 35 40
Compressor
power
(W)
15 20 25
Evaporating temp (C)
• Power consumption as a function of evaporating temp (Tevap)
• Fixed condensing temp and constant suction superheat
Power consumption
Tcond = 45 C
COMPRESSOR PERFORMANCE DATA

13. 30 35 40 60 65 70
Compressor
power
(W)
45 50 55
Condensing temp (C)
• Power consumption as a function of condensing temp (Tcond)
• Fixed evaporating temp and constant suction superheat
Power consumption
700
600
500
400
300
200
100
0
Teva p = 5 C
COMPRESSOR PERFORMANCE DATA

14. 30 35 40 60 65 70
Compressor
power
(W)
45 50 55
Condensing temp (C)
• Power consumption as a function of condensing temp (Tcond)
• Fixed evaporating temp and constant suction superheat
Power consumption
700
600
500
400
300
200
100
0
Teva p = 5 C
COMPRESSOR PERFORMANCE DATA

15. COMPRESSOR PERFORMANCE TABLE

16. MODELING OF COMPRESSOR
• Most compressor maps are developed from data fitting of
compressor curves
• ARI curve fit polynomial
T 2
T2
 c T 2
 c T 3
 c T  c T  c T3
6 cond 7 evap 8 cond evap 9 evap cond 10 cond
 c T 2
 c T T
Y fit  c1  c2Tevap  c3Tcond 4 evap 5 evap cond
Yfit  compressor power, mass flow rate, or cooling capacity
c1  c10  empirical coefficients
• fits are for specified superheat and subcooling
• unique to compressor and refrigerant
• doesn’t extrapolate well outside of data range used for curve fit

17. MODELING OF COMPRESSOR
• Superheat and subcooling correction


1


1,map

1 F 1,new
m˙new
h2s h 
 1 new
h2s h1map


m˙
new
m˙
map
W˙c,new
Ẇc,map m˙map
F =
Subscript “map” =
Subscript “new” =
correction factor (0.75 is recommended)
denotes superheat conditions associated with
the compressor map at specified evaporating
and condensing temperatures
denotes new superheat conditions at specified
evaporating and condensing temperatures

18. DESIGN IS SELECTION
To summarize, compressor design primarily involves selection
• Type of compressor
• Cooling capacity
• Budget constraints
• Compatibility with other system components
• Refer to manufacturer design curves to match system
parameters
• Use curve fit to interpolate system parameters such as power,
flow rate, etc for use in simulation model