Variable Speed Compressor Technologies
Presented By:
John Dolan, P.E.President - Thermosystems, Inc.
BS in Mechanical Engineering from UIUC
29 years HVAC Equipment Application Experience
www.thermohvac.com
March 14, 2017
Page 1
Variable Speed Compressor Technologies
• Why to Apply
― Energy Savings
― Capacity Control
• Available Technologies & Application
― Motors and Bearings
― Centrifugal Compressors
― Screw Compressor – Positive Displacement
― Scroll Compressor – Positive Displacement
• When to Apply
• When Not to Apply
• Questions
2
Positive Displacement Compressor
3
Basic Refrigeration Cycle
4
LIFT (ΔP)
Water Cooled Flooded Chiller Refrigeration Cycle
5
Basic Direct Expansion (DX) Refrigeration Cycle
6
Refrigeration Cycle
12
34
Reducing Compressor Lift (𝑃𝑐𝑜𝑛𝑑-𝑃𝑒𝑣𝑎𝑝 = ΔP)
8
EXV
EXV
Variable Speed Compressor Technologies
• Why to Apply?
• Energy Savings (Reduce Lift)
• Lower Compressor RPM = Less Work = Less Energy $
• Fan/Pump Laws - RPM ≈ Energy³
• Take Advantage of Lower than Design Condenser Temps
• How Often is Outside Ambient at Design Temp (DB or WB)?
• O'Hare Weather Bin Data – 6 hours/yr ≥ 95°F
• Match Compressor Operation to Actual Condenser Conditions
(Lower Temperatures ≈ Lower Pressures)
• Capacity Control
• How Often is Building or Space at Full Load? (<1%)
• Proportional Control with VFD vs. Staged Control (On/Off)
• Modulate Flow of Refrigerant to Match Load (Screw and Scroll)
• Chiller - More Stable Control of Leaving Water Temperature
• No Return Water Control or LWT Set Point Reset
• DX Air Handler - More Stable Control of Discharge Air
Temperature (DB & WB)9
Variable Speed Compressor Technologies
• Why to Apply?
• Higher Efficiency Motors with Variable Speed Operation
• AC Induction Motor with Variable Frequency Drive
• Permanent Magnet Synchronous Motor (PMSM)
• Motor Speed Operating Range with PMSM
• No longer limited to 1800 & 3600 RPM (AC Induction
Motor Speeds)
• Direct Drive Operation
• Gears to Increase Impeller Speed no Longer Required
• No Transmission Losses (Gears)
• Magnetic Bearing Technology
• Eliminate Oil and Oil Components (Oil Sump, Oil
Pump, Oil Heaters, Oil Separators, Safeties)
• Improve Heat Transfer & Reduce Compressor Wear
• Sustainable Efficiencies for Life of Chiller
Quick Electric Motor Review
• AC Induction Motor
― Most Common Motor in HVAC
― Fixed Speeds Based on 60Hz (1800 & 3600 RPM)
― Max Motor Speed 3600 RPM
― Slip
• Permanent Magnet Synchronous Motor (PMSM, ECM)
― More Efficient than AC Induction Motor - Especially at Part-Load
― Electronically Commutated (ECM)
― Synchronous Motor – Zero Slip
― VFD for Starting Required (Even in Constant Speed Applications)
• VFD Integral to Motor in Some Sizes
― Smaller Size than Comparable Hp AC Induction Motor
― Higher Speeds Allow for Matching Speed to Application
11
Motor Efficiencies (AC Induction vs. PMSM)
NEMA Premium EffAC Induction Motor
PMSM (ECM)
Oil Effects on Heat Transfer
Conclusions and Recommendations:
The heat transfer ratio drops steadily with oil
concentration and reaches a value of 0.65
[from 1.0 normalized] at an oil concentration of
10%.
From ASHRAE Research Project 751-RP,
“Experimental Determination of the Effect of Oil on Heat Transfer
with Refrigerants HCFC-123 and HFC-134a”,
35% heat transfer reduction with 10%
oil concentration in refrigerant
Oil Effects on Chiller Efficiency
Source: The News, 04/15/04, by Jack Sine
Direct Drive PMSM Oil-Free Compressor Design Eliminates
the Performance Degradation Due to Oil Contamination of the
Refrigerant
Traditional Centrifugal Design (Oil Based)
3600 RPM AC
Induction Motor
Thrust Bearings
(Oil)
Impeller Gear Set
(Oil)
Radial
Bearings
(Oil)
Traditional Centrifugal Compressor
(Geared Impeller & Traditional Oiled Bearings)
Traditional Centrifugal Design (Oil Based)
Magnetic Bearing Centrifugal Compressor
High Speed
Permanent Magnet
Synchronous Motor
Axial
Thrust
Bearing
Impeller +
Inlet Vanes Front
Radial
Bearing
Rear
Radial
Bearing
Magnetic Bearing Compressor
Magnitude™ WME Compressor Rotating Group
Magnetic Bearings
and Sensors
Permanent Magnet
Synchronous Motor
Suction Gas
Discharge
Inlet Guide
Vanes
Single Stage
Impeller
VFD in
External Panel
Water Cooled Centrifugal Chiller Efficiencies
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50 60 70 80 90 100 110
Chiller percent load
kW
/ t
on
10% energy
reduction
Fixed Speed Motor
AC Induction w/ VFD
High Speed VFD w/
Magnetic Bearings
30% energy
reduction
Note: Based on 500 ton Chiller with Same Condenser and Evaporator
Operating Cost Comparison – Centrifugal Chillers
0
100
200
300
400
500
600
700
800
900
0 10 20 30 40 50 60 70 80 90 100
Chiller Percent Load
Op
erat
ing
Ho
urs
0
0.2
0.4
0.6
0.8
1
1.2
kW /
to
n
Fixed Speed ~ $243,287/year
Traditional VFD ~ $164,934/year
High Speed VFD w/ Mag Bearing ~ $131,709/year
Cooling Load Profile
10% energy
reduction
Note: Based on 500 ton Chiller with Same Condenser and Evaporator and $0.10/ per kWh
©2016 Daikin Applied
CONSTANT SPEED & VOLUME SCREW COMPRESSOR
Pd
Ps
Over Compression
or Lost Work
Pc’’
• P
fixed
fixe
d
Discharge
to
Condenser P cond
V displacement
A
B
©2016 Daikin Applied
VARIABLE SPEED SCREW COMPRESSOR
VFD on Compressor Motor
Varies the Volume of
Refrigerant and Discharge
Pressure is Controlled by
the Slide Valves
Vd
PcA
•
Point APoint B
Discharge to
Condenser
variable
vari
able
B
Scroll Compressor Technologies
23
Digital ScrollConstant Speed
Variable Capacity
Load/unload 20sec time step
Variable Speed PMSM or
AC Induction Motor w/VFDVariable Speed Compression
and Capacity Control
Fixed Speed ScrollOn/Off
Staged Control
Variable Speed vs. Variable Capacity
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
25
%
35
%
45
%
55
%
65
%
75
%
85
%
95
%
% C
om
pre
sso
r Po
wer
% Cooling Capacity
Digital Scroll
RebelVariable
Speed
24
Digital scrolls are Off/On for Compression with
Constant Speed AC Motor
Scroll Compressor Staging (Variable and Constant Speed)
25
NONINV NONNONINVNON7%
100%
System Capacity
Capacity Control2 Condensing Units
4 Compressors
Co
mp
ress
or
Op
erat
ion
Airside Compressor Cycling Effects
26
When to Apply
• Significant Hours with Lower than Design Condenser Temperatures
― Take Advantage of Lower ECWT or Ambients
― Reduce Lift Wherever Possible
• Lower Condenser Temps (Water)
• Higher Evaporator Temps (Chilled Water Temp or Discharge Air)
• Significant Hours with Lower than Design Load
― Watch Minimum Loading with Constant Speed Machines
• Excessive Cycling on Large Hp Compressors
• Generator Back-Up
― VFD Compressors will have Lower Inrush at Start-up
• Variable Chilled Water Flow Applications
― Better Control of LWT
― Less Cycling of Compressors (Air Cooled Chillers)
27
When to Apply
• Meeting or Exceeding ASHRAE 90.1 & IECC Efficiencies
― Water Cooled Compliance (Full Load kW/ton and IPLV)
― Air Cooled Compliance (EER and IEER)
― Path A (Constant Speed) vs. Path B (Variable Speed)
― Must Meet Both Full Load kW/ton and IPLV
• Utility Rebates for Higher Efficiency Equipment
― More Rebate $ for Better IPLV or IEER
― Rebate May Offset or Pay for VFD
• Humid Areas
― Stable Leaving Water Control or Leaving Air Control
― More Latent Cooling & Stable Coil Discharge Temperatures
― Minimize or Eliminate Condensate Re-Evaporation when
Compressor Shuts Off
• Sound Sensitive Applications
― Variable Speed Equipment is Quieter than Constant Speed
28
When Not to Apply
• Centrifugal Chiller Applications with Minimal Condenser Relief
― Humid Areas with Constant Loads (High WB)
• Very Large Tonnage Plants
― 60,000 ton Plant with 20+ Chillers
• Large Airside Applications with Multiple Scroll Compressors
― 100+ tons with 6 or 8 stages
― Compressor that is Off is Always More Efficient than Operating Compressor
• Airside DX Applications in Dry Climates (Low Latent Loads)
― Sensible Load Applications
― Discharge Air Reset or Leaving Water Reset
• Heat Recovery Chiller Applications – Fixed Lift
• Any Others? Page 29
Page 30
Questions
