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R E F R I G E R A T I O N A N D A I R C O N D I T I O N I N G KVR + NRD Presentation
2004.05.04., Jbm - Page 1
Pressure Regulators
R E F R I G E R A T I O N A N D A I R C O N D I T I O N I N G KVR + NRD Presentation
2004.05.04., Jbm - Page 2
Pressure Regulators Overview
• CONDENSER PRESSURE REGULATORS
• EVAPORATOR PRESSURE REGULATORS
• CRANKCASE PRESSURE REGULATORS
• HOTGAS BY-PASS REGULATORS– Direct feed of hot gas– Use of Liquid/Vapour mix
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Some Functions of Regulators
• Maintain required temperatures through control of pressure
• Maintain pressure to components that are within operating envelopes
• Ensure adequate pressure differential across metering devices in all ambients
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Condenser Pressure Regulators(CPR)
Application Information
The main purpose of a condenser pressure regulator is to maintain sufficient pressure at the TEV during low ambient (Outdoor) temperatures
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Location of CPR
Used in the liquid line before the receiver
Condenser
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Condenser Pressure Regulator
•Condensers are designed for a certain difference between the outside ambient temperature and the condensing temperature
•Because the condenser must be able to reject heat even at high loads, the condenser must be large enough to reject this heat even when it is hot outside i.e. 90F
90 degree Fahrenheit Air Inlet Temperature
Condensing Temperature = 110 Fahrenheit
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Condenser Pressure Regulator
When the outdoor temperature is lower, the condenser acts like it is a lot larger
Temperature = 20F
Temperature = 90F
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Condenser Pressure Regulator
•When the condenser effectively becomes larger, the condensing pressure can drop substantially ,causing the TEV to operate erratically
•The CPR acts by reducing the area available for heat rejection, effectively making the condenser smaller
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Condenser Pressure Regulator
•The CPR accomplishes this by backing up the liquid in the condenser, using up free volume
•The condenser then has a smaller area available to reject heat from the refrigerant
This the way the condenser behaves when liquid is backed up in it
Only a portion of it can reject heat
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Hot discharge gas inlet
Sub-cooled liquid outlet
Reduction in Latent Heat Capacity
• If the coil is full of liquid, then the majority of the coil is only capable of sensible heat removal
• This can reduce the capacity of the condenser by 300%
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Heat (Energy)Enthalpy
Tem
pera
ture
Enthalpy is the heat in BTUs per pound added to or removed from a substance, in this case water.
superheatingvapor
heatingwater (liquid)
212 °F
boiling water(liquid + vapor)
970 BTU/lb
180 BTU/lb
Heat Energy - Enthalpy
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CPR Functionality
• The CPR closes on a fall in inlet pressure and opens on a rise in inlet pressure
• The regulator is in turn set to operate over a given pressure range or Proportional band (P-band) with the opening pressure controlled by a chosen setting
Capacity in %
100
psi225 37530015075
25
50
75
P-bandFactory setting
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Capacity in %
100
psi
225 30015075
25
50
75
P-bandFactory setting
Relation of Capacity and P-band
Below are the values between the start of the valve opening and the valve being fully open
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Capacity in %
100
psi
225 30015075
25
50
75
P-bandFactory setting
Relation of Capacity and P-band
By subtracting the value of the opening pressure from the pressure @ fully open we arrive at the P-band
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Capacity in %
100
psi
225 30015075
25
50
75
P-bandFactory setting
Relation of Capacity and P-band
225 -150 = 75 psi
The P-band is 75psi
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Capacity in %
100
psi
225 30015075
25
50
75
OffsetFactory setting
Relation between Capacity and Offset
Offset is the permissible difference between the operating pressure and the minimum pressure allowed.
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Capacity in %
100
psi
225 30015075
25
50
75
OffsetFactory setting
Relation between Capacity and Offset
In this example the operating pressure is 175 psi and the minimum pressure desired is 155 psi
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Use of Pressure Differential Valve
•Under very low ambient conditions, even back filling the condenser with liquid may not be adequate to maintain sufficient condensing pressure
-10 F with ambient air movement
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Differential Pressure Valve(DPV)
The DPV uses a pressure differential to inject hot gas into the receiver to boost it’s pressure It opens when the pressure difference between the compressor discharge and the receiver is greater than the spring pressure holding the valve closed
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Differential Pressure Valve(DPV)
A pressure differential of 20 psi will begin to open the DPV. It will inject hot gas into the receiver to boost it’s pressure. The valve will be 100% open when the differential pressure is 43psi.
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Location of CPR
Hot gas is injected directly into the receiver
Condenser
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SIZING THE KVR + NRD
Conditions
Refrigerant R-22Capacity 72 000 BTU/hrApplication Liquid line Desired Pressure drop 3 psiEvaporating temp. 20 FCondensing temp. +110FLiquid temp. +100FConnection KVR ½” flareNRD ½” solder
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Using the Application Guide
•The capacity chart is based on rated capacities which assume certain conditions
•In reality, the conditions are likely to be different and these differences need to be taken into consideration
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Using the Application Guide
•The actual required capacity must be determined by using a correction factor for actual evaporating temp
= .97 X 82 000 = 79 540 BTU/hr = ~6.6 tons
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Using the Application Guide
•The selection is made by locating the correct condensing temperature and the desired pressure drop across the valve and then picking the corrected capacity from the table
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Using the Application Guide
•The final code number selection is determined by the family model that fits the connection requirements
•In this example it is the 034L0091 with 1/2” flare connetions
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KVR & NRD Summary
• The KVR & NRD maintain sufficient condensing pressure during low loads and low ambient conditions
• They help to maintain a sufficient pressure differential across the TXV so it reliably controls according to it’s specifications
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Crankcase Pressure Regulators
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Crankcase Pressure Regulators
•The KVL limits the amount of refrigerant returning back to the compressor
•It is primarily used with low temperature compressors as they are designed for low density suction gas
Evaporator
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•During high loads, the returning gas can be at a high pressure and density and can damage the compressor motor
•The KVL will close on a rise in inlet pressure, limiting the amount of refrigerant that returns to the compressor
Crankcase Pressure Regulators
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Crankcase Pressure Regulator Selection
•Below are conditions from a system that we can use to make an example selection
•System capacity is 8000 btu/hr or .65 tons
•Refrigerant R404a
•Suction temperature is -10F
•Liquid temperature is 110F
•Desired pressure drop across valve is 3 psi
•Maximum suction pressure is 40 psi
•Connection size is 7/8 sweat
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Crankcase Pressure Regulator Selection
•The capacities given in this code number table is based on certain assumptions about the conditions of the system (rated conditions)
•Actual system conditions may be very different so the table capacities must not be used to make a selection
Tons
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Crankcase Pressure Regulator Selection
•Below are the steps that we need to take to make a selection
•Does this capacity exceed the required capacity?
•If yes, then choose the code number for the valve from the code number selection table based on the size and type of connections required
•If not, then go down to the next size of valves and repeat the process
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Crankcase Pressure Regulator Selection
•Here we see the correction table that corrects for the temperature of the liquid ahead of the TXV
•If the liquid temperature is different than 100F, then we need to use the appropriate conversion factor to correct the system capacity
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Crankcase Pressure Regulator Selection
•To correct the system capacity, we multiply this capacity by the correction factor
•Corrected capacity is .65 tons x 1.10 = .72 tons
•.72 tons is the capacity we use to make a selection from the capacity table
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Crankcase Pressure Regulator Selection
Using the system condition provided and the corrected capacity, make the selection from the capacity table
Find the evaporator temperature
Find the desired pressure drop
Step 1
Step 2
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Crankcase Pressure Regulator Selection
Find the maximum suction pressure
Step 3Select the capacity that is above the required capacityStep 4
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Crankcase Pressure Regulator Selection
Choose the code number for the valve from the code number selection table based on the size and type of connections required and the valve type
•The correct code number is 034L0045
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Evaporator Pressure Regulators
•An EPR is designed to maintain a desired pressure (temperature) in an evaporator
•It can be used with a single evaporator or in multiple evaporator arrangements
•An EPR will open when the inlet pressure rises to the desired set-point and it will close if it falls below the set point.
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EPR with Single Evaporator
Single Evaporator Application
Compressor
EPR
The KVP is located between the evaporator and the compressor and is designed keep the evaporator at a desired temperature
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Multi Evaporator Application
Evap #1 Evap #2
10F at 12psi
35F at 30psi
The EPR will keep evaporator Evap # 2 at it’s desired set-point regardless of the temperature of Evap #1KVP
EPR with Multiple Evaporators
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KVP Pipe Diagram
Multi Evaporator Application
Evap #1 Evap #2
10F at 12psi
35F at 30psi The compressor suction
pressure will be equal to the non-regulated evaporator (Evap #1)
KVP
12psi
12psi
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Hot Gas Regulator
•The KVC is designed to maintain a minimum pressure at the compressor suction inlet
•This is important when the compressor capacity is greater than minimum load requirements
•It opens when the outlet pressure drops below the desired set-point and it will close when the pressure rises above the set point.
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Simple Hot Gas System
Compressor
This image shows the hot gas regulator controlling hot gas from the compressor discharge to the inlet of the evaporatorCheck valve to
protect TXV
This adds a false load onto the compressor
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The suction pressure will follow the load on the evaporator, dropping on low load until the low pressure cut-out shuts off the compressor
Compressor on. Compressor off.
Suction pressure varies
The compressor cycles frequently on low pressure cut-out.
Suction Pressure
Time
Example ofCapacity Control - Air Drying
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By utilizing hot gas bypass, the addition of the discharge gas raises the suction pressure to a level where the low pressure control will not trip and cycle the compressor.
Suction pressure is relatively steady.
The compressor does not cycle.
Suction Pressure
Time
Suction pressure without hot gas bypass fluctuates with evaporator load
Example ofCapacity Control - Air Drying
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Capacity Control in Air Drying Applications
•In reality, there is a limit to the amount of additional load that can be added to the suction side of the system
•As the suction gas cools the compressor, an important limitation is the maximum allowable suction temperature of the compressor
30 F
180 F discharge
80 F
280 F discharge
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IMPORTANT!
Artificial load is often used in air driers without liquid injection, but remember :
If the artificial load exceeds 50 % of the total system load, it will normally be necessary to cool the suction gases.
This is done by injecting refrigerant into the suction line via a TEV, in series with a EVR solenoid valve mounted as close as possible to the TEV.
The TEV valve must be set to max. superheat and with sensor on suction line.
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Liquid & Hot Gas Mixing
Compressor
Liquid Gas Mixer
TEV sensing bulb controls superheat to compressor
Liquid from the TEV maintains the suction gas temperature.
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P-band and Offset