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

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Pressure Regulators


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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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When the outdoor temperature is lower, the condenser acts like it is a lot larger

Temperature = 20F

Temperature = 90F


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•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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•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


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



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



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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•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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•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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•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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•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



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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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•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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•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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•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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•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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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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Find the maximum suction pressure

Step 3Select the capacity that is above the required capacityStep 4


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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

pressure regulators

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