1
VariableVariableVariableVariable----Speed CompressorsSpeed CompressorsSpeed CompressorsSpeed Compressors
centrifugal
scrollreciprocating
helical-rotary
Methods of Compressor Unloading
� Reciprocating
� Scroll
� Helical-Rotary
� Centrifugal
Cylinder Unloaders
Cycle On and Off
Slide Valve
Inlet Vanes
VARIABLE SPEED DRIVES
2
Basic Refrigeration System
compressor
condenser
evaporator
expansion device
dischargeline
suctionline
liquidline
A
B
C
D
Split DX SystemSplit DX System
cooling coil(evaporator)cooling coil(evaporator)
supply fansupply fan
air-cooled condenserair-cooled condenser
compressorscompressorsrefrigerant pipingrefrigerant piping
2-stage centrifugal chiller
Refrigeration Cycle
condenserevaporator
expansion devices
economizer
2-stagecompressor
3
Affinity Laws2012 ASHRAE Handbook HVAC Systems and Equipment
1. “Flow (capacity) varies with rotating speed”
2. “Head varies as the square of the rotating speed”
3. “Brake horsepower varies as the cube of the rotating
speed”
“The (third) affinity law assumes that the system curve is
known and that head varies as the square of the flow.”
Many systems don’t follow this assumption
VSDs and Their Effect on System Components
• Cooling tower fans –
Load/power curve nearly cubic
sumpsump
outdoorair
louvers
fill
propeller fan
0%
20%
40%
60%
80%
100%
0% 50% 100%
% Load
Speed^3
85°F Setpoint
Perc
ent
pow
er
4
Chilled and Condenser Water Pumps
• Head/flow curve not “squared”
− Control setpoint
− Minimum Flow requirements
− Cooling tower static “lift”
water flow, %
100
20 40 60 80 1000
80
60
40
20
0
pum
p h
ead dynamic
pressure
fixed pressure/lift
Chilled and Condenser Water Pumps
• Power/flow curve not “cubic”
−DP curve not squared
−Minimum flow rates
water flow, %
100
20 40 60 80 1000
80
60
40
20
0
pum
p p
ow
er
VSDs and Their Effect on System Components
Variable speed chillers:
Load/power curve not cubic
• Lift depends on system
operating conditions
• Power is dependent on
load and lift
• The relationship of power
changes for different compressor
types
coolingtower
water-cooledchiller
coolingload
P
P
44°F
condenser
evaporator
88.2°F
Lift
5
Centrifugal Compressor
volute
diffuserpassages
radial impeller passages
blades impeller
Impeller Dynamics
diameter
Vr ∝ refrigerant flow rate
Vt ∝ rotational speed × diameter
rotational
speed
VrR
Vt
refrigerant flow rate
Compressor Unloading
VrR
Vt
Vr
Vt
full
loadpart load
R
6
Vr
Vt
Surge
Vr < static pressureR
pre
ssure
ris
e
flow rate – capacity (cfm-tons)
best
efficiency
decreasing
efficiency
Fixed-Speed Compressor Map
inlet guide vanes
fully open
minimum
guide vane position
surge
boundary
pre
ssu
re r
ise
design
selection
flow rate - capacity (cfm - tons)
Fixed-Speed Compressor Map
chiller capacity (%)
ch
ille
r p
erf
orm
an
ce (
kW
/to
n) 1.0
0.0
0 50 100
85°F
75°F
65°F
55°F
7
pre
ssu
re r
ise
inlet guide vanes
fully open
minimum
guide vane position
surge
boundarybest efficiency
a best efficiency zone is
created through the
operating map
60 Hz
50 Hz
min.
Variable-Speed Compressor Map
40 Hz
flow rate - capacity (cfm - tons)
pre
ssu
re r
ise
inlet guide vanes
fully open
surge
boundarybest efficiency
flow rate - capacity (cfm - tons)
speed modulation;
ride wide-open
vane curve
speed and
guide vane
modulation
Variable-Speed Compressor Map
pre
ssu
re r
ise fully
open
surge
boundarybest efficiency
flow rate - capacity (cfm - tons)chiller capacity (%)
ch
ille
r p
erf
orm
an
ce (
kW
/to
n)
1.0
0.0
0 50 100
85°F
75°F
65°F
55°F
Variable-Speed Compressor Map
8
chiller capacity (%)
ch
ille
r p
erf
orm
an
ce (
kw
/to
n)
900-ton chiller selections at 44°/85°F
Design Point Performance
VS @ 0.585 kW/ton
FS @ 0.513 kW/ton (-12.4%)
85°F75°F
65°F
55°F
variable-speed
fixed-speed
1.0
0.0
0 100
Centrifugal Performance Comparison (Equivalent Price)
Fixed Speed versus Variable Speed
40
50
60
70
80
90
100
Fixed Speed / Variable Speed Comparison
leav
ing
co
nd
en
ser
wate
r te
mp
era
ture
(F
)
chiller load (% of design)
0%
+10% VS+20% VS
+30% VS
+40% VS
10% FS
0 100
Positive Displacement Compressors
ScrollScroll Rotary ScrewRotary Screw
• 10-40 tons
• Few moving parts
• 35-450 tons
• Few moving parts
9
Centrifugal capacity vs lift
60% 140%
lift
Rotary screw capacity vs lift
60% 140%
lift
flow rate - capacity (cfm - tons)
Flow versus Pressure Differential
flow rate - capacity (cfm - tons)
Rotary Screw Compressor
meshing
pointdischarge port
chiller capacity (%)
ch
ille
r p
erf
orm
an
ce (
kw
/to
n)
0.0
0
1.0
0.0
100
85°F
75°F
65°F
55°F
Part Load Efficiency with Slide Valve
10
chiller capacity (%)
ch
ille
r p
erf
orm
an
ce (
kw
/to
n)
0.0
0
1.0
0.0
100
85°F
75°F
65°F
55°F
Part Load Efficiency with Variable Speed
chiller capacity (%)
ch
ille
r p
erf
orm
an
ce (
kw
/to
n)
0.0
0
1.0
0.0
100
85°F
75°F
65°F
55°F
Slide Valve versus Variable Speed
@ equivalent price
variable-speed
slide valve
chiller capacity (%)
leav
ing
co
nd
en
ser
wate
r te
mp
. (º
F)
40
0
100
100
+10% VS+20% VS
+30% VS
+40% VS
0%10% SV
Slide Valve versus Variable Speed
11
chiller capacity (%)
ch
ille
r p
erf
orm
an
ce (
kW
/to
n)
1.0
0.0
0 100
85°F
75°F
65°F
55°F
CentrifugalCentrifugal Rotary ScrewRotary Screw
chiller capacity (%)
0.0
0
1.0
100
Variable Speed Centrifugal vs. Rotary
Summary of performance differences
The Physics of Operation are Different
• Centrifugal impellers operate on the principle
of dynamic compression
• Rotary compressors operate on the principle
of positive displacement
• Unloading Performance may varies greatly for
different modulation technologies
• VSDs on centrifugal chillers benefit from reduced lift
• VSDs on screw chillers benefit from reduced load or lift
• Analysis needs to account for
• Chiller comparison
• Same-price VSD or premium efficiency chillers
• Actual chiller performance
• Actual utility rates
Summary
12
Chiller Chiller Chiller Chiller
Table 6.8.1C Table 6.8.1C Table 6.8.1C Table 6.8.1C ---- ExcerptExcerptExcerptExcerpt
0.639 kW/ton0.450 IPLV
0.634 kW/ton0.596 IPLV
Centrifugals < 300 tons
0.590 kW/ton0.400 IPLV
0.570 kW/ton0.539 IPLV
600+ tons
0.600 kW/ton0.400 IPLV
0.576 kW/ton0.549 IPLV300 – less than 600 tons
0.639 kW/ton0.490IPLV
0.620 kW/ton0.540 IPLV
300+ tons 0.718 kW/ton0.540 IPLV
0.680 kW/ton0.580 IPLV
150 - less than 300 tons
0.790 kW/ton0.586 IPLV
0.775 kW/ton0.615 IPLV75 - less than 150 tons
0.800 kW/ton0.600 IPLV
0.780 kW/ton0.630 IPLVWater cool. pos. displ. >75 tons
9.562 EER12.75 IPLVAir cooled 150+ tons
9.562 EER12.5 IPLV
Air cooled < 150 tons
Path BPath A
90.1-2007 Addendum M
stationaryscroll
drivenscroll
dischargeport
intake
discharge
intakemotorshaft
Scroll Compressor Fundamentals
13
Single On/Off CompressorOperation Evaluation
Cap
acit
y
Mo
du
lati
on
Co
ilT
em
p
Sy
s A
pp
Co
ntr
ol
Co
mp
lexit
y
Co
st
On/OffOn/OffOn/OffOn/Off PoorPoorPoorPoor WideWideWideWide LowLowLowLow LowLowLowLow
Single On/Off CompressorUnit EER
5
10
15
20
25
30
100% 75% 50% 25%
On/Off
evaporator
condenser
compressor
hot gas bypass valve
expansion valve
distributor
suction line
liquid line
oil return line
Hot Gas By-Pass
ModulationOperation Evaluation
Hot Gas By-Pass Unloading
Cap
acit
y
Mo
du
lati
on
Co
ilT
em
p
Sy
s A
pp
Co
ntr
ol
Co
mp
lexit
y
Co
st
BestBestBestBest BestBestBestBest BestBestBestBest MediumMediumMediumMedium MediumMediumMediumMedium
14
Hot Gas By-Pass ModulationUnit EER
5
10
15
20
25
30
100% 75% 50% 25%
On/Off
HGBP
Higher
▲
Or
Lower ▼
Hot Gas By-Pass ModulationUnit EER
5
10
15
20
25
30
100% 75% 50% 25%
On/Off
HGBP
Ported / 2-Stage ScrollCompressor Evaluation
Cap
acit
y
Mo
du
lati
on
Co
ilT
em
p
Sy
s A
pp
Co
ntr
ol
Co
mp
lexit
y
Co
st
GoodGoodGoodGood GoodGoodGoodGood WideWideWideWide LowLowLowLow MediumMediumMediumMedium
15
Ported / 2 Stage ScrollUnit EER
5
10
15
20
25
30
100% 75% 50% 25%
On./Off
HGBP
Two Stage
Higher ▲
Or
Lower ▼
Ported / 2 Stage CompressorUnit EER
5
10
15
20
25
30
100% 75% 50% 25%
On./Off
HGBP
Two Stage
Digital ScrollCompressor Evaluation
Cap
acit
y
Mo
du
lati
on
Co
ilT
em
p
Sy
s A
pp
Co
ntr
ol
Co
mp
lexit
y
Co
st
BestBestBestBest BestBestBestBest MediumMediumMediumMedium HighHighHighHigh MediumMediumMediumMedium
Suction / Compression /
Discharge
HotGas
Release
RotationWith
No Work
16
Digital ScrollUnit EER
5
10
15
20
25
30
100% 75% 50% 25%
On./Off
HGBP
Dual Stage
Digital
Higher ▲
Or
Lower ▼
Digital ScrollUnit EER
5
10
15
20
25
30
100% 75% 50% 25%
On./Off
HGBP
Dual Stage
Digital
VSD ScrollCompressor Evaluation
Cap
acit
y
Mo
du
lati
on
Co
ilT
em
p
Sy
s A
pp
Co
ntr
ol
Co
mp
lexit
y
Co
st
GoodGoodGoodGood GoodGoodGoodGood WideWideWideWide HighHighHighHigh HighHighHighHigh
17
VSD ScrollUnit EER
5
10
15
20
25
30
100% 75% 50% 25%
On./Off
HGBP
Dual Stage
Digital
VSD
Higher ▲
Or
Lower ▼
VSD ScrollUnit EER
5
10
15
20
25
30
100% 75% 50% 25%
On./Off
HGBP
Dual Stage
Digital
VSD
Hz 20 40 60 80 100 120 140
RPM 1200 2400 3600 4800 6000 7200 8400
Inverter/Compressor Operating Range
cooling
heatin
g
18
Variable Refrigerant Flow System
Source: AHRI1230 Directory
October 20, 2014
19
Size EER SEER
3 Ton 13.0 20.7
4 Ton 14.1 20.2
5 Ton 13.0 20.0
Size EER IEER
12.5 Ton 12.5 20.0
15 Ton 12.7 20.1
17.5 Ton 12.3 19.3
IEE
R/E
ER
What is the value of VSD vs Std
Efficiency (IPAK1 and 2)?
IEE
R/E
ER
8
9
10
11
12
13
14
15
16
17
18
20 25 30 40 50 55 60 70 75 90 105 120 130 150
eFlex/Dig
Scroll IEER
Std Eff.
IEER
eFlex/Dig
Scroll EER
Std Eff.
EER
BLACK BLACK BLACK BLACK ▲▲▲▲ –––– Digital Scroll
SIZE (tons)
Variable Speed WSHPCompressor Technology
20
Model LoadRated
Airflow
(CFM)
WLHP
Cooling
Capacity
(BTUH)
WLHP
EER
WLHP
Heating
Capacity
(BTUH)
WLHP
COP
GWHP
Cooling
Capacity
(BTUH)
GWHP
EER
GWHP
Heating
Capacity
(BTUH)
GWHP
COP
GLHP
Cooling
Capacity
(BTUH)
GLHP
EER
GLHP
Heating
Capacity
(BTUH)
GLHP
COP
VSVE
024
Full 930 24,600 18.4 30,300 6.1 27,800 33.2 24,400 5.2 25,900 22.3 18,400 4.2
Part 625 13,100 22.6 15,900 7.6 15,000 48.5 12,100 6.1 14,300 35.3 9,900 4.8
VSVE
033
Full 1,200 32,900 15.5 40,400 5.5 36,600 24.7 32,400 4.8 34,300 18.1 24,500 3.9
Part 720 17,100 20.8 21,500 7.0 19,400 40.8 16,800 5.6 18,500 31.6 14,100 4.6
VSVE
042
Full 1,650 44,100 18.3 54,700 6.0 50,100 32.5 43,600 5.1 46,200 22.0 31,900 4.0
Part 1,200 25,500 24.2 29,400 7.5 28,700 51.3 22,700 5.8 26,900 40.5 17,800 4.6
VSVE
050
Full 1,890 51,200 15.7 68,800 5.3 57,800 26.2 54,600 4.6 54,100 18.8 41,400 3.8
Part 1,200 29,900 22.1 36,800 6.9 33,500 42.5 28,500 5.5 32,800 34.0 23,400 4.6
VSVE
060
Full 2100 61,800 15.6 81,200 5.5 70,400 25.4 65,400 4.8 64,600 18.3 50,000 4.0
Part 1323 35,900 21.3 46,300 7.3 41,300 41.4 36,200 5.9 39,600 32.9 30,400 5.3
Variable Speed WSHP Unit Efficiency
• ASHRAE 90.1 minimum requirement
• Water Loop (13 EER / 4.3 COP)
• Ground Water (18 EER / 3.7 COP)
• Ground Loop (14.1 EER / 3.2 COP)
• Part Load Efficiency
• AHRI allows additional points for
part load in lieu of IEER or IPLV
What is the Next Refrigerant?
600 GWP
150 GWP
Considering GWP
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