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ASHRAE Refrigeration Seminar
Joe SanchezBITZER US, Inc
2/11/2014
AGENDA
Compressor Capacity Control Methods
� Blocked Suction
� Variable Speed Drives
� Integrated
� External Frequency Inverter
CO2 Systems
� Introduction
� Subcritical System
� Transcritical Systems
Introduction
Why Do We Need Capacity Regulation?
• For other operating conditions refrigeration system is oversized
Refrigeration systems must be designed for maximum operating conditions
• Rapid changes in pressure in cooling system
• Unstable process temps
• Short cycling / flooding of compressors
• High oil carry over rate (Oil logging?)
Load & ambient variations
• “Fine" adjustment reduces pressure fluctuations
• Increasing the evaporation pressure possible
• Better efficiency of the refrigeration system
• (to: +1K (+1.8R) � COP/EER: + 2.5%)
Part-load operation isrequired
Introduction
Suction pressure control
SST
Time
w/ Capacity Control
∆to
∆to
w/o Capacity Control
• Ideally Matches Part-Load Capacity
• Reduces TXV Hunting
• Raises Average Suction Pressure
• Efficiency Improvement
Control Principle
Cylinder Suction Chamber
Blocked Suction Capacity Regulation
Capacity Regulation
Application
Example: 4-cylinder compressor
� Virtually - stepless: 100% .. 50%
� Virtually - stepless: 50% .. 10%
De-energized
Energized
Intermittent
Capaciity Control
Application
Advantages of Blocked Suction Unloading
/ High adaptation to system cooling demands down to 10%
/ Reduction compressor number of starts
� 50% .. 10% operation (4-cylinder)
� 33% .. 10% operation (6-cylinder)
/ Rapid reaction to system changes
/ Reduction of pressure fluctuations:
� Possibility to rise set-point pressure keeping the same product
quality, e.g. in the cold cabinet
/ Improvement of overall system efficiency,
also between 50% - 0% load
Frequency Inverter
Frequency Inverter
Frequency Inverter
Typcially
Requires 0-10V or 4-20mA
signal
Typcially
Requires 0-10V or 4-20mA
signal
� V / f ⇒⇒⇒⇒ constant
� Asynchronous motors are designed for a defined ratio of V/f
� V/f = 480V/60Hz = 8
� V/f = 400V/50Hz = 8
� V/f = 240V/30Hz = 8
Lower frequency⇒ lower resistance of the stator
In case the ratio V/F is not constant:
⇒ Increasing operating current
⇒Magnetic saturation of iron
⇒Consequence: temperature of motor winding increases
VARIABLE SPEED DRIVE (VSD)
VARIABLE SPEED DRIVE (VSD)
APPLICATION RANGE WITH VSD
Lower frequencies cause: lower effect of the suction gas cooling, torque demand becomes more asymmetric, lower efficiency of the motor, harmonic content increases, rising motor temperatures
� Inverter solid flanged on motor cover
� Suction gas cooled
� no additional fans
� no additional service
� Completely parameterized inverter
� Simple commissioning
� Simple control
� 0-10V or 4-20mA
� Failure mode output
� Unable to bypass
Compressor with Integrated VFD
With Specical Design
/ Full torque even above 60 Hz net frequency through 230/60 motor
(230V/ 60 Hz -> 333V/ 87 Hz)
/ Through operation up to 87 Hz the compressor cooling capacity can
be increased by more then 40% compared to fixed speed at 60 Hz
/ Optimum capacity adaptation due to high capacity control range of
more than 3:1 which means step-less speed control between 25 Hz
and 87 Hz
Compressor with Integrated VFD
Benefits� Increased system efficiency, especially at part-load,
� Extended compressor life (fewer start/stop cycles),
� Integrated soft-start (lower inrush current),
� Reduced risk of liquid slugging,
� Over-speeding possible (obtain up to 50% more
capacity than at full load),
� Perfect size to meet sub-cooler load
Compressor with Integrated VFD
Compressors and Frequency Inverters
� Critical Frequency and Speed Ranges
�dependent on operating conditions resonances may occur in certain
frequency (speed) ranges
� Compressor & Pipe lines
� Coupling (observe moment of inertia & natural frequency)
� Belt drive (possibly idle pulley required)
�Examination by tests under real conditions
� critical frequency ranges must be "jumped" by adequate
programming of the inverter
CO2 Systems
COMPRESSORS FOR CO2 APPLICATIONS
IMPRESSIONS
COMPRESSORS FOR CO2 APPLICATIONS
IMPRESSIONS
CO2 Review
CO2 – A Special Refrigerant with Unique
Properties and Specific Requirements
Pro‘s
� Long tradition in refrigeration
� Low Global Warming Potential (GWP =1)
� Chemically inactive, non-flammable
� Not toxic in the classical sense
� Very high volumetric refrigerating capacity
� Subcritical: 6 to 8 times higher than for R22, R404A or NH3
� Transcritical: 4 to 5 times higher than for R22, R404A
� Low refrigerant mass flow
� … and it’s in beer � so it must be good
The ideal refrigerant???
CO2 – A Special Refrigerant with Unique
Properties and Specific Requirements
Con‘s
� Critical temperature at 31°C (87.8°F)
/ requires trans-critical operation for single stage and
compounded 2-stage applications (HP of >2000 PSI)
/ unfavourable thermodynamic properties for systems with
higher discharge pressures / gas cooler outlet temperatures
� Extremely high discharge pressures
/ safety aspects (regulations) and component design
� Limited low temperature range (triple point -56.6°C / -70°F)
� Lower practical limit in air than with HFCs (3.5 to 6 times less)
/ CO2 is odourless − for closed rooms this may require special
safety and detection systems
CHALLANGES & LEGAL ASPECTS
ON HFC REFRIGERANTS (2)
European F-Gas Regulation
/ Nov 2012: European Commission Proposed Revisions With FGas Phase Down And Some Specific Bans
/ June 2013: European Committee For Environment, Public Health And Food Safety (ENVI) Came Up With A Stricter Phase Down And Several Application Specific Bans
/ December 2013: EU reaches informal compromise
/ January 30th, 2014: European Committee adopts compromise
/ March 2014: European Parliament will vote
CHALLANGES & LEGAL ASPECTS
ON HFC REFRIGERANTS (2)
So What?
/ Europe pushing to cut f-gas emissions by 2/3 by 2030
/ [Pending] Bans in commercial refrigeration as of 2022:
� Hermetically sealed commercial refigeration with GWP > 150
� Centralized system for commercial use with capacity over 40kW with GWP > 150
� Exemption for cascade systems
− Primary circuit may use GWP < 1500 (R134a)
COMPRESSORS FOR CO2 APPLICATIONS
IS IT A SUCCESS?
COMPRESSORS FOR CO2 APPLICATIONS
IS IT A SUCCESS?
COMPRESSORS FOR CO2 APPLICATIONS
IS IT A SUCCESS?
Comparison of Properties & Performance –
Low Temp CO2 Cascade vs. R22
0
100
200
300
400
500
600
700
800
900
1000
R22
Reference
Cooling
Capacity
COP Suction
Pressure
Discharge
Pressure
Vapour
Density
(LP)
Co
mp
ariso
n C
O2 v
s. R
22
[%
]
R22 / to -35°C, tc -10°C
CO2 / to -35°C, tc -10°C
Compressor
displacement counter-
proportional to relative
cooling capacities
CO2 vs. R22
DL
SLSL
Suction liquid
line / line /
return line supply line
DX system R404A 100%
Secondary system
with brine250%
DX system with CO2 35%
Comparison of
Compressor sizes R22(R404A)CO2
CO2 Basics
DL
SLSL
Supercritical CO2 Video (Home Experiment)
CO2 Video (Danfoss)
PPM EFFECTS ON HEALTH
380 Average value in the atmosphere
< 800 EN13779: Good indoor air quality
5000
(0.5 Vol-%)
Maximum Workspace Concentration (MAK)
Threshold Limit Value, 8 hours, weighted average
10,000 Short Time Exposure Limit (Germany)
60 min, 3 times per shift
20,000 50% increase in breathing rate! Can affect the respiration function& cause
excitation followed by depression of the central nervous system
30,000 100% increase in breathing rate after short term exposure
50,000 Immediate Danger to Life or Health (IDLH)
“Escape” after exposure time of 30 min without irreversible health effects
100,000 Lowest lethal concentration
Few minutes exposure produces unconsciousness
200,000 Death accidents have been reported
300,000 Quick results in unconsciousness & convulsions
INFLUENCES ON THE HUMAN BODY (2)
Minimal burst pressure requirements of CO2 compressors:
/ According to EN 12693
● Safety factor of 3
● Type approved relief valves to the atmosphere
● According to EN 378-2 maximum flow section of the relief valvesmust be available at MOP x 1.1
/ According to UL● Safety factor of 5
or
● Fatigue Test
● First cycle at safety factor of 3
● 249,999 cycles at max working pressure
● Hold for 1 min safety factor of 1.5
PROTECTION AGAINST EXCESSIVE PRESSURE (4)
/ Prior to charging the system must be evacuated first
/ Never brake the vacuum with liquid CO2
● Spontaneous evaporation with severe cooling effect
● Brittleness and thermal shock of the materials
● Formation of solid CO2
/ Charge the plant with gaseous CO2
up to approx. 150 - 300 psi
CORRECT HANDLING OF THE REFRIGERANT
CO2 Subcritical Systems
The basic cycle: Single stage compression and expansion
Super-critical gascooling
DEFINITION OF CO2 PROCESS CYCLES
Sub-critical heat absorption
DEFINITION OF CO2 PROCESS CYCLES
/ Single stage system under sub- and trans-critical conditions:to = -26°F ; tc, tGCout = 58°F ; pc, pHP = 725 psi, 1130 psi, 1450 psi
/ High heat sink temperatures require trans-critical operation
/ In super-critical state with gas cooling, pressure and temperature are independent from each other
/ The limiting factor for an LT application in single stage configuration with highheat sink temperatures is the resulting discharge temperature
MT-stage
(R134a, R404A, etc)LT-stage
R744 - compressors
sub-critical operation
R744 evaporators
Q0 CO2
Q0 R134a=QC CO2
QC R134a
Cascade HX
Pel_CO2
Pel_R134a
• •
•
•
MT HFC / LT CO2 CASCADE - A HYBRID SOLUTION
MT HFC / LT CO2 CASCADE: log p, h - DIAGRAM
QC CO2=Q0 R134a
QC R134a= Q0_R134a+Pel_R134a
Q0CO2
Pel_R134a
Pel_CO2
IHX
IHX
• •
• •
•
INDEPENDENT HFC-CO2 CHILLERS FOR
MT AND LT
MT HFC-CO2
chiller
LT HFC-CO2
chiller
MT CO2
pump circulation
LT CO2
pump circulation
sim
plif
ied s
ketc
h
LT CO2 DXCascadeHeat Exchanger
sim
plif
ied s
ketc
h
ETX
LT evaporators
HX
MT evaporators
Alternative:
Secondary
system (also
in combination
with NH3 or HC)
MT compressors
LT compressors
MT R134a or R404A DX
MT HFC / LT CO2 CASCADE
LT evaporatorsMT evaporators
H(F)C-CO2
chiller LT CO2 DX
MT CO2
pump circulation
CO2 pumps
CO2 receiver
LT compressors
HX
sim
plif
ied s
ketc
hH(F)C CHILLER / CO2 PUMP CIRCULATION
+ CASCADE
PR valve
Strength Weakness
MT HFC /
LT CO2
Cascade
/ High efficiency
/ Reduced HFC charge and
risk of leakage
/ Cost neutral to conventional
DX system
/ Interaction of MT & LT systems
/ MT with higher (potential)
leakage rates
/ Different refrigerants
/ Specific CO2 knowledge required
H(F)C Chiller /
CO2 pump
circulation +
Cascade
/ Higher efficiency
/ Small ∆T for the MT
evaporators
/ Chiller unit with minimum
charge and leak potential
/ Natural refrigerants possible
/ Interaction of MT & LT systems
/ More complex system
/ High investment cost
/ Part load conditions challenging
(VSD of circulation pumps)
/ Specific CO2 knowledge required
COMPARISON OF SYSTEMS
CO2 offers unique properties in heat transfer:
/ High thermal conductivity, λ
/ High heat transfer coefficients, α
This offers the possibilities to:
/ Operate with 2 K higher evaporating temperatures vs. R404A (same surface on air side as with R404A)
/ Operate with small temperature differences between CO2 and cooling agent in gas coolers, cascade coolers and IHX
FEATURES & DESIGN ISSUES
FEATURES & DESIGN ISSUES
CO2 offers an abundance of pressure:
Sub- and trans-critical applications:
� Lower influence of pressure drops in temperature difference @ compressor inlet
� Same velocities as with R404A systems: 2 R lower pressure drop
CO2 Basics
DL
SLSL
CONTROL OF SUPERHEAT (2)
Comparison of evaporation enthalpy
Bear in mind:
High vapour density (700% compared to R134a @ -10 °C)
High evaporating enthalpy (125% of R134a @ -10 °C)
Consequence:
The gradient between velocity of vapour and liquid phase is lower
compared to HFC’s. More heat energy is required to evaporate
liquid CO2.
We recommend providing a suction accumulator!
Proper superheat control is of utmost importance !
CONTROL OF SUPERHEAT (3)
In most of the cases the observation of the minimum operating
temperatures is more challenging than the observation of the
maximum limits.
/ Oil temperatures above 70 °F
/ Discharge temperature > tc + 70 R for sub-critical operation
/ Avoid high amounts of CO2 solved in the lubricant
/ Pressure fluctuations: Degassing & oil foaming
/ Maximum discharge temperature 320°F (280 °F)*
* Measured on the surface of the discharge line
OPERATING TEMPERATURES (1)
48
0
5
10
15
20
25
30
35
1 2 3 4 5 6 7 8 9 10 11 12
Time in min
Eva
po
ratin
gp
ressu
rein
ba
r
Rapid changes in suction pressureInstable control of superheat=> Poor efficiency of the evaporator(s)
Suction pressurewithout FI
Suction pressure withFI
CAPACITY REGULATION
/ Capacity regulation (CR) is essential for CO2 applications
/ Minimize differences of capacity per step
/ Full load = “easy” & Part load = “challenging“
Frequency Inverter
CAPACITY CONTROL
CO2 Transcritical Systems
The basic cycle: Single stage compression and expansion
Super-critical gascooling
DEFINITION OF CO2 PROCESS CYCLES (1)
Sub-critical heat absorption
Single stage compression, two stage expansion and flash gas bypass
Reference: Carrier
DEFINITION OF CO2 PROCESS CYCLES (2)
Evaporator
HP Regulation Valve
Liquid
Receiver
Gas
Cooler
Compressor
(HX)
Cascade system with “independent” refrigerant circuits� “Exchange” of thermal energy but no “exchange” of refrigerant & lubricant� HT-stage: Single stage compr., two stage exp. & flash gas bypass� LT-stage: Single stage compr. & single stage exp.
DEFINITION OF CO2 PROCESS CYCLES (3)
First Transcritical Supermarket Rack
Trans-critical Compressors
LT Cascade Compressors
Booster system with common refrigerant circuit:� “Exchange” of heat energy, refrigerant mass flows & lubricant� Externally compounded two stage system, two stage expansion� Flash gas bypass
DEFINITION OF CO2 PROCESS CYCLES (4)
CO2 TRANS-CRITICAL PROCESS:
OPTIMUM HIGH PRESSURE
For a given gas cooler outlet temperature COP is a function
of high side pressure. Why?
Temperature and pressure are independent from each other
Optimum HP
Reference: Sintef-NTNU
95 °F
Exercise: Estimate the optimum
discharge pressure for
tGC = 95 °F, t0 = 14°F
14 °F
CO2 TRANS-CRITICAL PROCESS:
OPTIMUM HIGH PRESSURE
95 °F
Published Algorithms for a calculation:pHP [bar] = 1 + 2.44 x tGC [°C]
pHP [bar] = (2.778 – 0.0157 x t0 [°C]) x tGC [°C] + 0.381 x t0 [°C] – 9.34
14 °F
CO2 TRANS-CRITICAL PROCESS:
OPTIMUM HIGH PRESSURE
88 bar
(1275 psi)
sim
plif
ied s
ketc
h
Gas Cooler / Condenser
ETXETX
HX
CascadeHeat Exchanger
HP Control
MT Evaporators
Heat Reclaim
PR
Receiver
ETX
LT CO2 DXMT CO2 DX
LT Evaporators
MT CO2 DX / LT CO2 CASCADE SYSTEM –
“1st GENERATION”
LT CO2 DX
MT CO2 DXGas Cooler / Condenser
ETXETX
HX
HP Control
MT Evaporators
Heat Reclaim
PR
Receiver
LT Evaporators
MT Compr.
LT Compressors
• More efficient than “cascade” due to:
- reduced LT discharge pressure
• Oil distribution more challenging
MT / LT CO2 BOOSTER- SYSTEM WITH FGB –
“2nd GENERATION”
MT / LT CO2 BOOSTER- SYSTEM WITH EXTERNAL
PEAK LOAD UNIT – “3rd GENERATION”
• High efficiency due to:
- External cooling
- Refrigerants used:
1/COP < 1/COP CO2
@ high sink temperatures
sim
plif
ied s
ketc
h
Gas Cooler / Condenser
ETXETX
HX
HP Control
Heat Reclaim
PR
ECO Receiver
LT CO2 DX
MT CO2 DX
LT Receiver
MT Evaporators
ECO
• High efficiency due to:
- ECO operation of MT compressors
- Double expansion for LT
LT Evaporators
EXTERNALLY COMPOUNDED MT / LT CO2
SYSTEM – “3rd GENERATION”
MT / LT CO2 BOOSTER- SYSTEM WITH
EXTERNAL FG UNIT – “3rd GENERATION”
• High efficiency due to:
- External flash gas condensing
- Refrigerants used: 1/COP < 1/COP CO2
@ high sink temperatures
Strength Weakness
MT CO2 DX /
LT CO2
Cascade
/ Competitive efficiency in
moderate climates
/ Single fluid, environmentally
benign � low TEWI
/ High Tech, differentiation from
competition, environm. image
/ Interaction of MT & LT systems
/ High investment cost
/ Efficiency losses � cascade HX
/ Specific CO2 knowledge required
MT/LT CO2
Booster
System with
FGB
/ Higher efficiency than above
solution � no cascade HX
/ Other topics like above
/ Complex oil distribution system
/ Topics like interaction, investment
cost & knowledge like above
MT / LT CO2
Booster
System with
External FG
Unit
/ High efficiency
/ FG condensing unit with
minimum charge and leak
potential
/ Commissioning / standstill
/ Natural refrigerants possible
/ Reliability of HX
/ Issues concerning FG circulation
/ Topics like interaction, investment
cost & knowledge like above
COMPARISON OF SYSTEMS - OVERVIEW
CO2 attributes and system efficiency
/ Advantage of energy efficiency dependent from the ambient
temperature
� A customers statement about the energy efficiency of a
CO2 booster system vs. R404A system
Rela
tive e
nerg
y s
avin
g
Ambient temperature
32 36 39 43 46 50 54 57 61 64 68 72 75 79 82 86 90 93 97°C
°F
-26 -21 -15 -9 -4 2 7 13 18 24 29 35
-100000.0
-80000.0
-60000.0
-40000.0
-20000.0
0.0
0
500
1000
1500
2000
2500
3000
-25 -15 -5 5 15 25 35 45 55 65 75 85 95 105
Ho
urs
pe
r ye
ar
Ambient Temperature [°F]
Annual Weather Data Profile
Atlanta
Denver
Chicago
Los Angeles
Miami
Seattle
France: Strasbourg
Data Source: ASHRAE Inc., Weather Data Viewer Version 4.0
Ambient temperature [°C]
CO2 attributes and system efficiency
� Local weather data profile decisive for seasonal energy efficiency (SEER)
Potential for subcr. operation
-26 -21 -15 -9 -4 2 7 13 18 24 29 35
-100000.0
-80000.0
-60000.0
-40000.0
-20000.0
0.0
0
500
1000
1500
2000
2500
3000
-25 -15 -5 5 15 25 35 45 55 65 75 85 95 105
Ho
urs
pe
r ye
ar
Ambient Temperature [°F]
Annual Weather Data Profile
Denver
Chicago
Seattle
France: Strasbourg
Data Source: ASHRAE Inc., Weather Data Viewer Version 4.0
Ambient temperature [°C]
CO2 attributes and system efficiency
� Local weather data profile decisive for seasonal energy efficiency (SEER)
Potential for subcr. operation
FLASH GAS HX
Minimum load conditions:
/ Reduced mass flow
/ Increased surface / mass flow ratio
/ Lower velocities
/ Reduced pressure drop
/ Potential for maldistribution
Wet operation!
FGB mass flow equals 9 % of the
total mass flow!
FLASH GAS HX
/ Gas cooler outlet to FG HX
� Higher temperature difference for IHX
� Lower gas cooler outlet temperature
� Lower vapour fraction inside medium pressure receiver
� No liquid sub-cooling
� Surface has to be selected for part load conditions with lower
temperature differences
� Potential for increased RGT for MT compressor stage during full
load conditions with high heat sink temperatures
OPERATING TEMPERATURES IN A BOOSTER
SYSTEM
/ High MT load and low LT load
� Low RGT for MT compressors
� Influence of FGB on RGT
/ Low MT and high LT load
� High RGT for MT compressors
Worst Case Scenarios on Load Conditions: MT / LT
140 °C
130 °C
40 °C
30 °C
140 °C
� CO2 introduced other system designs
� CO2 systems: Complexity is significant
� CO2 systems: Good SEER in cold and moderate climates
� Premise for a successful CO2 installation:
� Correctly balanced capacity regulation
� Observation of operating temperatures
� Good oil management
� Basic issues e.g. safety, cleanliness, dryness are more demanding
CONCLUSIONS
