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Refrigeration Pressure Regulators-Flow controls Parts 1 and 2

Head Pressure Control, Solenoid Valves, and Hot Gas By-Pass



In Figure 1, you see a system with flow control pressure regulator controls.  Not all of these devices are on every system.  (Dryers, sight glasses, etc., have been omitted.)  A TXV is shown, as it is a flow control device, but has been discussed in Info-Tec 7.



There are only three kinds of pressure regulator flow controls:


•   Inlet regulators (also called upstream regulators)

•   Outlet regulators (also called downstream regulators)

•   Differential regulators




Note the sensing line in relation to the direction of flow.  An inlet regulator only “cares” about what is happening upstream.  It adjusts the flow according to inlet pressure.  It doesn’t “care” what happens downstream.  An outlet regulator adjusts the flow according to downstream pressure.  It doesn’t “care” what’s happening upstream.  A differential regulator senses both upstream and downstream pressure and adjusts the flow to maintain a set difference between inlet and outlet pressures.




Refer to figure 1.  Item 1 is a head pressure control valve or flood valve.  As was noted in Info-Tec 9, fan cycling can control head pressure but has drawbacks.  Systems that use dampers to control airflow through the condenser are very expensive and leak refrigerant.  There are few condenser damper control systems in use.  The flood valve system is the best head pressure control system for smooth, accurate, and trouble-free low ambient control.


Head pressure control is needed on air-conditioning at 60°F ambient and on commercial refrigeration at 50°F ambient and lower. 


Control of head pressure is needed in order to maintain:


1.   Adequate pressure drop over a TXV for the refrigerant effect

2.   To prevent flash gas in the liquid line

3.   To provide pressure for hot gas by-pass, or hot gas defrost, if present


Flooded condenser systems are able to maintain pressure within 5 to 10 psig.  They can operate efficiently to very low ambients encountered in our cold winters.  By flooding a condenser with refrigerant, we truly reduce the condenser’s capacity.


Up to about 15 tons capacity, the HP Alco Headmaster can be used.


See Figure 3.  B is the connection for the discharge line from the compressor.  R is the connection to the receiver, and C is from the condenser.




The dome is charged and exerts pressure on the top of the diaphragm.  At high ambient temperatures, by-pass gas (B) pushes on the bottom of the diaphragm and counteracts the dome pressure.  The upward force of this gas seals the seat disc against the top seat.  Flow is then from the condenser (C) to the receiver (R).  As the ambient temperature drops, head pressure decreases.  The by-pas gas can no longer overcome the dome pressure and the diaphragm moves down, moving the seat disc towards the bottom seat.  This allows discharge by-pass gas to be metered directly into the receiver, creating higher pressure at the condenser outlet.  This pressure reduces the flow from the condenser.  The stay time of the refrigerant in the condenser is increased, and liquid refrigerant begins to rise in the condenser, flooding it.  This reduces the effective condensing surface resulting in adequate high side pressure.


NOTE:   Head pressure valves can only be used on systems with TXV’s, and must have a receiver big enough to hold the system’s regular charge of refrigerant and the additional refrigerant needed to flood the condenser.  Care should be exercised to insure adequate receiver capacity.  If the receiver is too small, in warm weather, the extra refrigerant added to flood the condenser in cold weather would back up in the condenser and cause too high a head pressure. 


The receiver has to be big enough to store this extra refrigerant during warm weather.


To calculate the amount of extra refrigerant we need to estimate the volume of the condenser, see Figure 4.



To determine the length of the tube in the condenser, measure the tube length between the return bends.  In our example, this is two feet.  Count the number of tubes in the condenser.  In this case it is 7.  Count the number of return bends.  In our example, this is 6.  Determine the O.D. of the tube.  Condensers are built with 3/8, 1/2, or 5/8 O.D. tubing.  We’ll use 3/8 O.D. for our example.  Now, multiply the length of tube by the number of tubes:  7 x 2 = 14 feet of 3/8 O.D. tube.  See Figure 5.




Figure 5 shows the amount of specific refrigerant per tube size, per foot, to add for the lowest expected ambient temperature encountered.  (In Wisconsin, that is usually -20oF.)  Let’s say our example condenser is in an R-22 system.  From the chart, we see that one foot of 3/8 O.D. tube at -20°F will hold .055 lbs. of R-22.  Each return bend is the equivalent of .2 of a foot of tube.  We have 6 return bends, or 6 x .2 = 1.2 additional feet of tube.  Seven, 2 foot long tubes are equal to 14 feet of tube, plus 1.2 feet of return bends gives a grand total 15.2 feet of 3/8 O.D. tube.  15.2 x .055 = .836 lbs. of R-22 that must be added to the regular charge to flood this condenser.  (This example would be a very small condenser.)  If the system charge was 4 lbs., we add the .836 lbs. (round off to 5 lbs.), and select a receiver rated to hold 5 lbs. when it is 90% full of refrigerant.  Never select a receiver that would be more than 90% full. 


Tech Tip:       Select a size that will result in the receiver being between 75% and 90% full when storing the charge.  If the receiver will be in a warmer ambient than the condenser, a check valve should be installed in the drain line to the receiver to prevent reverse flow.


CAUTION:      Disconnect any fan cycling controls when using a flood valve.  If you don’t, the large sudden change in head pressure due to fan cycling will soon destroy the flood valve.  The system must also employ pump down (pump down will be discussed later in this Info-Tec.), when using a flood valve.


To size an Alco Headmaster HP valve, you need to know what refrigerant the system uses and how many tons it is.




Figure 6 shows the nominal capacities of the Headmasters in tons.  The table is based on a liquid line temperature of 100°F and an evaporator at 40°F.  For conditions other than 100°F liquid temperature and 40°F evaporator, the nominal capacities shown in Figure 6 should be adjusted by the multiplier in Figure 7.




After adjusting the nominal rating from Figure 6 by the multiplier from Figure 7, select the HP valve from Figure 6.  Do not exceed the 5 lb. psig rating for any HP valve.


Example: Given: R-502 system, 3 1/2 tons, and -20°F evaporator.  From Figure 7, the multiplier of .845 is applied to the 3 1/2 ton rating.  3.5 x .845 = 2.958 or approximately 3 tons.  From Figure 6, we see that a HP5 at 4 lb. psig is 3 tons on R-502. 


NOTE:            Unless an actual liquid temperature is known, the 100°F figure is a good average liquid temperature to use.


After selecting the proper HP valve, Figure 8 completes the selection.  The “A” valve is for R-12 (95 lb. dome charge pressure), “B” for R-22 and R-502 (170 lb. dome charge pressure).




On systems where adjustable head pressure is desired or capacities of the HP are exceeded, an Inlet Pressure Regulator (IPR) and Outlet Pressure Regulator (OPR) can be used.  (The HP valve is a combination IPR, OPR, with a fixed setting).  See figure 9.




The IPR is set for the head pressure you want to maintain and the OPR is set approximately 20 lbs. higher than the IPR.  Flow tonnage capacities of IPR’s and OPR’s are shown in Figure 10.






See Figure 1.  Item 2 is a solenoid valve.  Solenoid valves are on/off flow controls.  Solenoid valves are sized only by flow tonnage, not by line size!  This is critical.  Oversized pilot operated valves may not open when energized and undersized valves will cause excessive pressure drops. To properly size a refrigeration solenoid valve, you need to know the refrigerant, the flow tonnage, liquid or gas service, normally open or normally closed, and electrical characteristics for the coil.  Charts are printed in the catalogs to show the liquid and gas capacities in tons.  If the valve line size does not match the desired line size, use bushings or couplings to adapt to the line size.


The most prevalent use of refrigeration solenoids is in “pump down” systems.  Pump down should be on every system that uses an expansion valve.  Pump down does one thing and one thing only, but it is important.  Pump down is used to prevent refrigerant migration to the low side of a system on the off cycle.  The solenoid valve is installed in the liquid line at the entrance to the TXV. A thermostat in the refrigerated space controls the solenoid.  When the thermostat opens, the solenoid is de-energized and closes, stopping refrigerant flow.  The compressor is controlled by a low-pressure control.  As the compressor pumps all the refrigerant into the high side of the system, the suction pressure drops to the low-pressure control setting and the low-pressure control opens and stops the compressor.  The entire refrigerant is now held between the discharge valves of the compressor and the liquid line solenoid valve.  As long as the valves on the compressor or the liquid line solenoid valve don’t leak, there will be no refrigerant migration to the oil in the crankcase of the compressor or to the suction piping of the system.  Excess refrigerant in the oil of a compressor reduces the oil’s lubricity and causes excess wear on start-up.  Liquid refrigerant in the suction piping can cause liquid floodback, even slugging on startup, severely damaging the compressor.  If there is a hot gas by-pass, a parallel-wired solenoid valve and liquid line solenoid must be installed in the hot gas line in order to accomplish pump down.


Even if the solenoid valve or compressor valves leak a little, the pump down system will be effective.  A small leak will cause the suction pressure to rise to the low-pressure control’s cut-in setting, starting the compressor and pumping down again.  (If this “short run” to pump down occurs frequently during the off cycle, the leak should be repaired.)


To start up a pump down system, the temperature control closes, energizing the solenoid valve.  The solenoid valve opens, and refrigerant flows to the low side raising suction pressure, closing the low-pressure control, and starting the compressor.  Solenoid valves and temperature controls can operate two or more evaporators to refrigerate separate spaces using one condensing unit.




See Figure 1.  Item 3 is the hot gas by-pass valve.  A hot-gas by-pass valve is an O.P.R., or downstream regulator.  It responds to outlet pressure.  It meters discharge gas into a system’s low side in a proportion that will balance capacity to load demand.  They modulate and can be the last step of unloading.


A portion of the refrigerant is by-passed around the condenser and TXV.  This reduces the amount of refrigerant available for refrigerant effect.  The heat content of the hot discharge gas adds load, further reducing the refrigerant effect of an evaporator.


Air-conditioning and commercial refrigeration systems are sized for maximum expected load, and then the design engineer adds safety factors to the calculated loads “to be sure” the system will perform as expected.  Studies have shown that most systems operate at full load only 10 to 20 percent of run time.  Systems that operate at or near minimum load run into many problems:


Coils frost, chillers freeze, compressors overheat, poor oil circulation is encountered, short cycling occurs, etc.  Many on/offs are hard on the equipment.


Hot gas by-pass can alleviate all of the above and result in less temperature variation, good control, less maintenance, and longer equipment life.  Hot gas by-pass artificially loads the compressor, but great cost savings still result.  Demand charges are less; lubrication problems that are 85% of compressor failures are eliminated.  Maintenance time and electrical problems are reduced.


All compressors are designed to operate continuously.  Process systems that employ a constant load and are seldom turned off last for years and years with no problems.


A properly designed and installed hot gas by-pass system will result in acceptable suction pressure throughout a range of loads, even to no load.  It will not cause excessive super heat, and will not allow liquid return via the suction line.


There are various methods of doing hot gas by-pass systems.  The most common is by-pass to the evaporator inlet.


This is the only approved method if the evaporator is below the compressor, due to problems encountered with good oil return.  In this system, the evaporator acts as an excellent mixing chamber.  The injection of the hot gas into the evaporator will cause the TXV to open to de-superheat the hot gas.  The by-pass gas will increase velocity in the evaporator, which facilitates oil return.  It is easy to add, and is usually low-cost compared to other methods of hot gas by-pass.  It does have limitations.  All hot gas lines should be insulated to prevent condensation of the hot gas before it reaches the evaporator.  Even insulated, these systems are limited to 35 to 40 feet of hot gas lines.  A venturi type distributor must be used or a special hot gas tee used with orificed distributors.  When used on a multi-evaporator system, care must be exercised to insure that the evaporator used for by-pass is large enough to handle the complete by-pass and unused TXV capacity, and be the last active evaporator on line.If the vertical riser of the hot gas line exceeds 4 to 5 feet, oil may be held up in the vertical riser, necessitating constructing an oil return line at the base of the hot gas riser.  (See Figure 11, oil return line.)  The oil return line should be made out of 5 feet of 1/8" O.D. copper tube.  This causes enough pressure drop so hot gas will not pass through it but oil will drain.  If this oil drain line has to span more than 5 feet to connect to the suction line, add 1/4 O.D. tube to make up the span.




Figure 12 is by-pass to suction line.  It has the advantage of short lines and everything located at the compressor.  If an orifice distributor cannot be replaced with a venturi type or special hot gas tee available for the orifice distributor, this system must be used.




Depending on the length of the hot gas line in a by-pass system to the entrance of an evaporator, this system may or may not be cheaper to install, due to the addition of an extra de-superheat TXV and liquid injection solenoid valve needed.  Special care has to be used with by-pass to suction systems.  Make sure the evaporator is free draining to the compressor.  Use of this system will tend to trap oil in the evaporator and suction line since we are not injecting the hot gas into the evaporator to keep up, or increase, velocity.  This is the reason this system must never be used when the evaporator is below the compressor.  Not having the evaporator to act as a mixing chamber and de-superheating device, we must add a liquid injection TXV to de-superheat the hot gas.  The liquid line to the de-superheat TXV will require another solenoid valve.  All the solenoid valves: liquid line pump down, hot gas, and liquid line de-superheat, are wired in parallel to open and close together.  To insure good mixing of the hot gas and suction gas, certain practices should be observed in teeing the hot gas line into the suction line.


Suction lines 7/8 O.D. or smaller can use a standard tee.   1-1/8 O.D. and up should be teed in at a 45°angle, the hot gas flow opposing the suction line flow.  See Figure 13.  Dimension “X” should always be at least 5 feet.  The longer X is, the better the mixing will be.  If unable to construct the 45° angle tee, a regular tee can be used to elbow the suction line and bring the hot gas line into the tee so the hot gas opposes the flow of the suction gas.




A suction line accumulator, as shown in Figure 14, makes an excellent mixing chamber.




On all hot gas by-pass systems, the sensing line of the hot gas valve must be downstream of the hot gas connection into the suction line and as close to the compressor suction as possible.  Always make the connections on top of the suction line tube to prevent oil blocking the tube.  For ease of servicing, installation, and to keep cost down, keep all the hot gas by-pass components as close to the compressor as possible.  The sensing lines, or pilot lines, are 1/4 O.D. tube up to 20 feet long.  If, for some reason, the hot gas by-pass valve was installed over 20 feet from the suction connections for the valve, use a 5/16 O.D. or 3/8 O.D. tube.


The liquid injector or de-superheat TXV is a special high superheat TXV.  They are usually internally equalized valves.  The sensing bulb of this TXV should be attached to the suction line downstream of the mixing tee or accumulator.  (See Figure 13 and 14.)  The liquid line to this TXV is a 3/8 O.D. tube, up to 50 tons, and 1/2 O.D. to 100-ton systems.


If an Evaporator Pressure Regulator (EPR) is used, make sure the sensing line of the by-pass valve is downstream of the EPR.  We want to sense suction pressure, not evaporator pressure.


There are two types of hot gas by-pass valves.  Direct actuated, such as Alco DGRE’s and CPHE’s.  The other hot gas by-pass valve type is pilot operated, such as FA8’s.  Direct actuated are low tonnage valves.  Pilot operated valves are large tonnage valves.  Both kinds have adjustable set points between 0 to 80 lbs. psig, and respond to outlet pressure only.


In order to select the proper components to add hot gas by-pass to an existing system, the following information must be gathered:










The question that poses the most problems is the “tons to be by-passed”.  This is seldom the system’s entire tonnage.  Just because the system is rated 50 tons does not mean the by-pass system has to be rated to handle all 50 tons.  After unloaders have been activated and TXV’s have throttled to their minimum, the balance of the load is what the by-pass system needs to handle.


Applying hot gas by-pass to a reciprocating compressor water chiller differs from a system using a finned airflow evaporator only in the control system and setup of the by-pass valve.  The only by-pass method that can be used is by-pass to the evaporator, not by-pass to suction line.  A hot gas valve modulates over an 8 lb. range.  On a chiller, these 8 lbs. after the last stage of unloading can’t be tolerated, because the tubes of the chiller barrel might freeze.  The hot gas valve is adjusted to already pass the necessary hot gas when the compressor is unloaded to its minimum capacity.  The valve is set to begin opening above the activation of the last stage of unloading.  When the last stage of unloading is activated, the hot gas valve is already open enough to pass a sufficient volume of gas to prevent freezing the chiller tubes.  A chilled water temperature controller initially energizes the hot gas solenoid valve.  When the controller opens the solenoid, the hot gas imposes a load on the chiller, resulting in increased water temperature.  Sensing this, the controller will cycle the hot gas valve in an effort to stabilize the water temperature.  The compressor will continue to operate until some other control stops it.  This arrangement prevents the chiller tubes from freezing at minimum capacity load conditions.


The size of the hot gas line can be the same size as a properly selected hot gas by-pass valve if the equivalent line length is less than 20 feet.  Equivalent line length takes into consideration the pressure drop imposed by fittings and valves expressed as feet of a specific pipe size.  A hot gas line should be sized for a pressure drop of approximately 10 psig.  Figure 16 is a chart showing the equivalent length in feet of various components used in copper tube lines.  For instance, a 7/8 O.D. long radius elbow’s pressure drop is equal to 5.3 feet of a 7/8 O.D. tube.


There is a bit of an art to line sizing.  That is, we will select a line size we think will work and then do calculations to see if it is going to work.  Let’s say we had to size a by-pass for 20 tons capacity, R-22 refrigerant, and an air-conditioner.  The line will have three long radius ells in it and 20 feet of tubing.


We think the line may be 7/8 O.D. or 1-1/8 O.D.  We’ll try 7/8 O.D. first.  From Figure 16, three 7/8 O.D. long radius ells are equal to 15.9 feet, or rounded off, 36 feet.



NOTE:  Figures in bold face type are maximum recommended tonnages at pressure drops calculated to minimize suction line temperature penalty.  Shaded areas are for general information only.



Tonnage Capacities of Discharge Lines Delivering Hot Refrigerant-22* 


*Based on 40 degrees F suction and 105 degrees F condensing.  For other conditions apply correction factor


 from Table 6-25 to design tons before entering this table.


Figure 17 shows the capacity of lines for R-22 discharge gas at various pressure drops, at 40 °F suction and 105°F condensing temperatures, standard for air conditioning.  Note that there is no 10 lb. pressure drop line, the pressure drop we are using as our design drop.  The tonnage capacity for a 10 lb. drop is about 1.8 times the capacity for 3 lbs. for the same equivalent length.  (Square root of 10/3.)  Therefore, we can use the 3 lb. drop multiplied by 1.8 to get a capacity at 10 lbs.  There is no need to try and interpolate line length.  Simply use the next longer length of your calculated length. 


In our example we’ll use the 40 ft. line.  Given: 3 lb., 40 ft., 7/8 O.D. we find 7.1 tons.  7.1 x 1.8 = 12.8 tons.  Not big enough.  Trying 1-1/8 O.D.:  5.7 + 20 = 25.7 30 ft. equivalent length column, 3 lb. drop shows 16.8 tons.  16.8 x 1.8 = 30.2 tons.  Plenty of capacity.  The line should be 1-1/8 O.D.


If our evaporator temperature or condensing temperature are not 40°F and 105°F, a correction factor should be applied to the tonnage ratings.  (See Figure 18.) Apply the correction factor to the tonnage found in Figure 17, before multiplying by 1.8.



Selecting the proper hot gas valve, solenoid valve, and liquid injection TXV (if by-pass to suction), has been made quite simple by Alco.  Alco’s catalog 24-D, dated September 1988, “Pressure Regulators”, has charts in it, beginning on page 30.  Make selections from these charts.


The direct acting DGRE valve can be used up to 3 tons by-passed for R-12, 10 tons for R-22 and R-502.  Because of the DGRE’s low cost, this should be your selection, where applicable.  Larger tonnage capacities will require the FA8.  CPHE valves are take-a-part versions of the DGRE.  They are more expensive than the DGRE, a lot more in the larger sizes.


Once you know the tons to be by-passed, the refrigerant, the evaporator temperature, and the type of by-pass system, you need only to go to the charts.  In our example, R-22, 20 tons, 40 °F evaporator, by-pass to evaporator we find a FA8-13H is required.  No solenoid valve is shown since the FA8 has a solenoid built into it.  To complete the FA8, we need a coil and a set of flanges.  In our example, RX174 1-1/8 O.D. flanges.  If this had been by-pass to suction, the liquid injector TXV would have been shown along with an LCL4A (to complete the number of the LCL4A, see P. 16 and P. 17 of the catalog).  We also need a liquid line solenoid valve, for by-pass to suction, in the liquid injector TXV feed line.  On P17 are shown the solenoid valves to use.  We see the 200 RB4 is used with the LCL4A on R-22.  If dealing with a system where the DGRE applies, note that the charts show the correct hot gas solenoid to use.  DGREs do not have a solenoid built in.


Once the hot-gas system is installed, it must be set to operate.  The hot gas valves do not come set at a specific pressure; they must be adjusted for what is required.  They are adjustable from 0 to 80 lbs.


To set up a system, first remove the seal cap over the adjustment screw.  Back the screw all the way out, counter-clockwise.  Block airflow over the evaporator anyway you can.  Shut off the fan, block off the evaporator with cardboard, or whatever.  We need to simulate a no load or low load condition.  When the suction pressure begins to go about 5 lbs. below the desired set point, begin to turn the adjustment screw clockwise until the valve begins to open.  You can hear the hot gas going through the valve, or feel the hot gas line at the outlet of the valve.  Turn the adjustment screw slowly until the suction pressure is increased to the desired set point.  Each complete turn of the adjustment screw is approximately 4 lbs.  Allow enough time between each adjustment for the system to stabilize.  By-pass valves modulate from closed to wide open over an 8 lb. range.  They can be very precisely set.  If the compressor is equipped with unloading, set the by-pass valve to begin to open 2 to 3 lbs. below the last stage of unloading.  In order to be able to obtain the proper setting, the high side pressure should be maintained at a minimum pressure corresponding to 85 to 86°F condensing temperature.  This is about 90 lbs. for R-12, 150 lbs. for R-22, and 170 lbs. for R-502.


If the by-pass to suction line system is used, the liquid injection TXV must be adjusted.  (No instructions come with this valve.)  Again, decrease the system’s load to make the hot gas by-pass go fully open.  Adjust the superheat of the injection TXV so as to maintain 20°F superheat at the suction port of the compressor.  Load the system so the hot gas valve shuts off.  Make sure the injector valve does not feed now.  You may need to “tweak” the superheat setting.  Turning the superheat adjustment on the injector valve clockwise increases superheat, counter-clockwise decreases superheat.  Recycle the system once or twice to make sure of the settings.  In short, we are trying to maintain 20°F superheat at the compressor’s suction port.


In part 2, we will take up Evaporator Pressure Regulators (EPR) and Crankcase Pressure Regulators (OPR).




Evaporator Pressure Regulators

The Evaporator Pressure Regulator (EPR) is as shown in figure one, number four.  It is an inlet or upstream pressure regulator.  The EPR is designed to sense inlet pressure and adjust the flow to maintain a set pressure.  They are sometimes called back pressure regulators.  They are available with many options and in many configurations, but the sole function of any EPR is to maintain evaporator pressure at a predetermined setting.

Many are used on multiple evaporator systems where different evaporator temperatures are needed, such as in a supermarket with a “rack” or common suction system.  The EPR’s will prevent the temperature from dropping in evaporators that are warmer than others in the same system, while the condensing unit(s) continues to run to satisfy the coldest evaporators.  They can prevent the freezing of a chiller by keeping the saturation pressure above the freezing temperature of the water.

EPR’s should be installed as close to the evaporator outlet as possible, although they can be located some distance downstream from the evaporator.  If a pilot is used, connect a pilot line from the evaporator to the EPR.

Alco manufactures a large selection of EPR’s.  Alco’s catalog 24-D “Pressure Regulators”, dated 9/88, describes all of the EPR’s and the options available.

The first and most important part of selecting an EPR is determining the port size needed which determines the EPR’s capacity.  Like refrigeration solenoid valves, EPR’s should not be selected by line size.

Five basic system conditions must be determined before a selection can be made.  They are:

                1.  Refrigerant

                2.  Evaporator design capacity in tons.

                3.  Evaporator design temperature (or pressure)

                4.  Minimum evaporator temperature (or pressure)

                5.  Options

                6.  Available pressure drop across the EPR at design capacity

Conditions1 through 5 are easy to determine.  Number 6 causes the most problems.  Depending on the specific system involved, the pressure drop across an EPR can vary from 2 to 20 lbs. at design load.  When the load is above design load (such as at start-up or after a after a defrost period) the EPR has to open wide enough to handle the load and then control at design load or light load again.  EPR’s are selected using design capacity, not the startup or any other abnormal load.

Tech Tip:  When applying an EPR to a single evaporator system, select an EPR for a 2 lb. pressure drop.  The

                   2 lb. pressure drop is desirable and should result in good control.

An example:  An R-502 system with one evaporator designed for 12,000 BTU at 0°F.  We want to prevent the evaporator from going below 0°F.  No suction cutoff is desired.  Cost is a factor.  12,000 BTU is one ton of refrigeration; 0°F is 31.2 psig on R-502.

Using the Alco catalog 24-D, we can select an EPR from the 2 lb. drop column because we are dealing with only one evaporator.  Selecting only on the basis of capacity, we’d find an EPRB-12 (page 28) that would work.  EPRB’s require a pilot line that will add to installed cost.  The EPRV-13 requires that connection flanges be selected that add to the cost, but even without considering those additions, we’d find the IPR-10-0 to 50 lb. direct acting EPR to be the lowest cost.  Direct acting EPR’s will always be the lowest cost EPR.  Our selection would be the IPR-10, 0 to 50 lb. range.

When no other options are needed, such as suction stop solenoid valve, or pilot arrangement, always consider the IPR valve first.  IPR’s are available in three pressure adjustment ranges: 0 to 50 psig, 30 to 100 psig, and 65 to 225 psig.  Line sizes from 5/8 O.D. to 1-3/8 O.D. are also available.  The IPR has a pressure tap on the inlet (evaporator) side for connection to a gauge for convenient set point adjusting.

Multiple evaporators in a system present a slightly different problem when calculating pressure drop.  With evaporators operating at different temperatures, one or more EPR’s may be required to maintain pressure higher than a common suction line.  (EPR’s can never maintain pressures lower than the suction line.) 


In an R-502 system (figure 2), evaporator A is designed for 35°F (72.6 psig), evaporator B for 32°F (68.2 psig), and all other evaporators for 25°F (58.7 psig).  The design suction pressure is 58.7 psig.  The pressure drop for evaporator A is then 72.6 - 58.7 or 13.9 psig.  For evaporator B, it is 68.2 - 58.7 or 9.5 psig.

Evaporator A’s design capacity is 1.8 tons, and B’s is 1 ton.  Evaporator A’s selection is an EPR that will be rated 1.8 tons on R-502 at 35°F DP 13.9 psig.  If no other options were specified, the IPR-6 adjustment range 30 to 100 psig is the selection.  Evaporator B’s selection is an EPR rated 1 ton on R-502 at 32°F, DP 9.5 psig.  IPR-6 adjustment range 30 to 100 psig is, again, the selection.  The Alco charts show the maximum tonnage capacity the EPR can pass.  Always make your selection with a capacity rating the same or more than required (never less), but do not deliberately oversize.  In our example for evaporator A, the IPR-10 would have about 5 to 6 tons capacity and would operate erratically, probably on/off instead of modulating smoothly.  If the EPR’s had needed solenoids or some piloting arrangement, the port code 12, a 1/2" orifice, would be the correct selection for evaporator A, and the port code 11, 3/8" orifice, for evaporator B.

All the “other” evaporators mentioned in our example would appear not to need EPR’s since the design suction pressure is their design pressure.  However, on multi-temperature systems it is a good idea to put EPR’s on all the evaporators to insure proper refrigeration effect.

Alco has many configurations of EPR’s.  The BEPR, now relabeled the EPRB, is an all brass sweat connected pilot-operated EPR used mainly on refrigeration “racks”.  Piloting with discharge gas assists the valve’s operation and consequently, it has very low pressure drop.  The EPR is a larger capacity cast iron, flange connected, EPR available with various pilots.  It is available in vacuum range as low as 25" Hg.

Adding an “S” to the EPRB and EPRV model number adds a built-in suction stop solenoid valve to the devices.

The FA1 is an EPR that is a dual pressure regulator.  By energizing a built-in solenoid valve, the FA1 will control evaporator pressure at a lower setting than the set pressure when the solenoid is de-energized.  The FA6 is a remote bulb temperature piloted EPR with an optional solenoid suction stop.  The FA7 is an FA6 with a pressure pilot override.  This pilot acts as a low limit.  The solenoid valve is optional.

The EPR (V) can be piloted with a pneumatic control signal.  This pilot, the EAC, is available in two ranges, 2 to 110 psig, or 25" Hg vacuum to 95 psig.

Alco’s 722 EPR functions the same as an EPRV, except it must be used with an external pilot of some kind.  What pilot is used determines how the 722 will function.

With the wide variety of pilots and options available from Alco, an EPR can be configured for almost any application.  No EPR should ever be used with a design DP over 20 psig.  It is all right to parallel EPR’s if needed.

Crankcase Pressure Regulators

The Crankcase Pressure Regulator (CPR) is as shown in figure one, number five.  These are outlet pressure regulators, often called “holdback” valves.  They are downstream regulators.  Their sole function is to limit crankcase pressure to prevent over-loading the compressor.  They are sensitive only to their outlet pressure and the regulator will close in the event of an outlet pressure increase.  As long as the valve outlet pressure is greater than the valve’s pressure setting, the valve will stay closed.  As the compressor reduces the outlet pressure, the valve will open and let refrigerant vapor into the compressor.  As outlet pressure drops, the valve will open to its rated position.

A crankcase pressure-regulating valve should be applied to any system on which the compressor could be overloaded by high suction pressure.  It should be installed as close to the compressor’s suction port as possible, downstream of any other controls.  An accumulator may be downstream of a crankcase pressure regulator.

Note:   To select a crankcase pressure regulator, we need to know the system’s refrigerant, capacity in tons, design   suction pressure, maximum allowable suction pressure, and pressure drop across the valve.

Pressure drop across the valve should be kept to a minimum because suction pressure losses penalize system capacity.  The lower the evaporator temperature, the lower the allowable pressure drop.  Maximum pressure drop for medium or high temperature systems is 2 psig, for low temperature systems 1/2 to 1 psig.  Most capacity tables will only show pressure drops of 1/2, 1, and 2 psig.  See Alco’s catalog 24-D on page 29 (Figure 3).  Note the OPR tables.  For the FA5 the 2 psig columns should be used.


The compressor manufacturer determines the maximum allowable suction pressure.  This is the valve setting.

Example:  We want to select a crankcase pressure regulator for a Tecumseh AH2511KC unit with a -20oF evaporator.

A Tecumseh AH2511KC is an R-502 unit rated at 8500 BTU at -20°F suction, 90°F ambient.  8500 BTU is approximately 3/4 ton.  -20°F, R-502, is 15.5 psig saturation pressure.  The AH2511 is rated to +10°F or 41.1 psig.  Our selection is based on a valve for R-502, with a setting of 40 psig and a suction pressure of 15.5 psig.  This is low temperature, so a 1/2 or 1 psig DP should be used.  See Figure 3 below, from Alco’s catalog 24-D.

Since we want to keep the DP as low as possible to prevent loss of capacity, we’ll try the 1/2 psig drop.  Enter the chart at the valve point setting of 40 psig.  Our design suction pressure of 15.5 psig is about half way between 10 and 20 psig.  When design gauge pressures fall between pressures on the chart, use the lower pressure; in this case 10 psig.

An OPR6 is rated at .6 tons, a little under capacity at a 1/2 psig DP.  An OPR 10 is rated 1.4 ton.  It would be our selection, unless price is a big consideration.  OPR10’s cost over $20 more than OPR6’s.  If we use the 1 psig DP column, an OPR-6 is rated .9-ton capacity and could be our selection.  The OPR-6 will impose about a 2°F temperature penalty, the ORP10 about 1°F.  (Systems are seldom designed so close to actual load they can’t afford an extra degree or two of temperature penalty.)  If a suction solenoid stop had been required, or a pilot operated crankcase regulator, other than the low cost direct acting OPR, an FA5 would be considered.  Selection of the FA5 port code is made from the catalog page 38.  Using the R-502 table, 15.5 psig (-20°F), DP 2 psig, the port is 3/4" or code 13.  FA5’s are very expensive compared to OPR’s.  They are usually used only when capacity exceeds the OPR ratings.  Crankcase pressure regulators can be parallel when more capacity is needed than the largest regulator can supply.  Like EPR’s in parallel, careful piping is needed so the DP across both valves is the same.

In paralleling EPR’s or crankcase regulators, they should be adjusted after installation so they operate together, equal amounts passing through the valves.

If a crankcase pressure regulator is used in a system with hot gas by-pass, the setting of the crankcase regulator must be higher than the by-pass valve setting, as the crankcase pressure regulator will be constantly throttling the flow to try and protect the compressor.


While accumulators are not exactly pressure regulators, they can be described as a flow regulator.  They are protective devices that meter refrigerant and oil back to the compressor to prevent liquid return directly to the compressor.  Like pump down, they should be on every system, but seldom are, except on low temperature systems.  Selection of an accumulator is based on pressure drop, oil return, and the total amount of charge to be held.

Tecumseh has simplified accumulator selection to only determining the system charge.  They have taken pressure drop and oil return into consideration in the design of their accumulators.  They have patented their design.  To apply a Tecumseh accumulator, one needs to know the refrigerant type, amount of system charge, and the size of the suction port on the compressor.  Tecumseh accumulators should not be applied to compressors with a suction size less than 1/2" O.D.  If the system’s charge is unknown, a rule of thumb to determine the charge can be used to approximate the amount of refrigerant in a system, but it is a last resort.  It is:  Air conditioning and other high temperature systems:  3 lbs. per horsepower.  Medium temperature:  5 lbs. per horsepower.  Low temperature:  7 lbs. per horsepower.  (Note:  horsepower, not tons).  Once the system’s charge has been determined, an accumulator large enough to hold that amount should be selected.  See Figure 4.


As an example, if an accumulator is needed to hold 130 ounces of R-22, the model TK15007 would be the accumulator to use, as long as the compressor’s suction port was 3/4" O.D. or larger.

On systems too large for Tecumseh accumulators, it may not be cost effective or practical to try and apply an accumulator large enough to hold the entire charge.  In fact, it may be difficult to find a manufacturer that makes one.  Large accumulators can be sized to hold a minimum of 50% of a system’s charge.  Manufacturers’ charts for accumulators will usually show maximum and minimum tonnage capacities at various evaporator temperatures based on a pressure drop equivalent to 1/2°F.  When selecting an accumulator other than a Tecumseh, read the manufacturer’s chart carefully when making your selection to ensure you are staying within the parameters set down by the manufacturer.  Never parallel or series pipe accumulators!

While every refrigeration system would benefit from having all the devices shown in Figure 1, seldom will a system be seen with all of them, due to cost.  One type of system where all these devices will be installed is in a supermarket rack servicing freezers, coolers, cases, etc.  There they are necessary to make the system function and to protect the expensive compressors being used.













You may be asked how long purge timing should be for the plug-in purge card.  Purges are supposed to be long enough to make four air changes before lighting the burner.  If you knew the CFM rating of the blower, and the volume of air in the boiler and chimney, you could calculate the purge timing needed to charge the air four times.  Since most of the R4795’s will be replacements, use the same timing as the existing card. 


Low voltage controllers cannot be used with R4795’s.  The T & T terminals on an RA890 are now 6 & 7, where an airflow switch is connected.
Purge timing does not start counting until the airflow switch closes these contacts.  Once the purge has timed out, the lighting sequence is the same as the RA890’s.  The amplifier circuit is energized during purge so we have safe start check.  If a flame simulating condition is present during purge, the flame relay coil, 2K, will energize preventing ignition, but the burner motor will continue to run.  This will give continuous purge, a “safe” failure condition.  The relay will not “lock-out”.  If the flame simulating condition, or real flame, goes out, the start-up will proceed.  If a purge card fails or is not installed correctly, the burner motor, on a call for heat, will run but pre-purge cannot be completed so ignition cannot occur resulting in a continuous purge.
If the airflow switch doesn’t close, or opens during pre-purge, the purge cannot be completed, and once again, the burner motor will run but no ignition can take place.
If the airflow switch opens during the run period, terminals 3, 4, and 5 will be de-energized, dropping out the main valve, pilot valve, and ignition.  Terminal 8 will remain energized so the burner motor will continue to run.  If the airflow switch closes, the purge timing will start and the start-up sequence will begin again.  Note that no lockouts have occurred which have to be manually reset.  Lockout requiring manual reset happens when no flame is detected after purge.  Flame relay, 2K, will not energize and the safety switch will heat and lockout the control in about 15 seconds. If there is a flame failure during run, terminals 3, 4, and 5 are de-energized; pilot, ignition, and main valve.  If airflow is still proven, an R4795A will begin purge timing and attempt to re-light.  It will make only one try.  An R4795D will not recycle.  An R4795D will lockout on flame failure during run.
An R4795D differs from the A series in safe start check.  If a flame is detected during pre-purge (2K relay energizes), the purge will stop and safety lock-out will occur in about 15 seconds—the time it takes the safety switch to heat up.  These two things are the only differences between R4795A and D.
The next upgrade of the R4795’s was the R7795 series.  The R7795 series used more solid-state technology.  The R7795’s still used plug-in purge timers, ST795A’s, but the amplifier is not plug-in or interchangeable.  Therefore, an R7795 has to be selected with the correct amplifier to match the scanner.  R7795A’s are used with UV detectors and B’s are flame rectification.  A’s and B’s are intermittent pilot models.  R7795C’s and D’s are interrupted pilot models, the C’s for UV detectors, the D’s with flame rectification detectors.  R7795’s require a Q795 sub-base.  Their operation is the same as the R4795’s.
In light of the RM7800 series, do not upgrade a customer from an R4795 to an R7795.  Always upgrade to the 7800 series.  Honeywell is only keeping the R7795 available due to O.E.M. demand.  To an O.E.M., the R7795 is less expensive than the 7800 series and OEMs are very, very price conscious.  With the demise of the R4795 series, the 7800 series will be the service industry’s control of choice.
To select an RM7895 system to replace an R4795 system, some decisions have to be made.  All R4795’s were intermittent pilot.  We can now choose intermittent pilot, the RM7895A or B, or interrupted pilot, the RM7895C or D.  Intermittent pilot means the pilot is on during the run period.  Interrupted pilot means the pilot is shut off during the run period.  All RM7895’s have an initiate sequence that lasts at least 10 seconds on initial powering of the relay.  During this ten seconds, the relay is checking that the line voltage is within 132 VAC and 102 VAC and line frequency is within plus or minus 10%, or 66 HZ and 54 HZ.  If any of these tolerances are not met, the 10 second initiate sequence will go into a hold condition until the tolerances are met, and if not met the RM7895 will lock-out in four minutes.  If, at any time during this hold period the tolerances are met, the 10-second initiate sequence will restart checking voltage and frequency again.
After passing the initiate sequence, the relay goes into stand-by.  Stand-by can be any length of time.  Stand-by simply means the control is waiting for a call for heat.  On a call for heat, terminal 4 is powered; the blower motor and pre-purge begins.  Pre-purge timing is whatever ST7800A plug-in card was selected, from 2 seconds to 30 minutes.  The airflow switch (AFS), installed between terminals 6 and 7, must close within the timing of the short timing purge cards, 2, 7, or 10 seconds, or within 10 seconds for longer timing purge cards.  The purge timing does not start to count until the AFS closes.  Should the AFS not close within the specified time or 10 seconds, whichever is shorter, the control will recycle or lock-out, depending on jumper 3 being intact; recycle or cut; lock-out.
All RM7895’s have three configuration jumpers.  Jumper number 3 is the jumper that governs what happens if there is AFS failure.  If the AFS opens at any time after it has been made, that is in pre-purge, ignition trials, or during run, the RM7895 will recycle if jumper number 3 is left intact or if the jumper is cut the control will lock-out.
All RM7895’s have three jumpers that can be cut or left alone.  They are labeled JR1, JR2, and JR3.  Cutting a jumper enhances the level of safety.  Cutting a jumper never makes the control inoperative!  Jumper number 1 configures the PFEP (Pilot Flame Establishing Period).  Left intact, terminal10 will be powered for 10 seconds, the terminal that the ignition transformer is connected to.  If this jumper is cut, terminal 10 is powered for only 4 seconds.  Jumper number 2 configures the control to be a recycle or lockout control.  If left intact, the control will recycle on flame failure.  If cut, the control will lockout on flame failure.  Just like the RM7890, this jumper must be cut if an amplifier with 3 second flame response timing is used.  Jumper number 3 has been discussed.
The RM7895B and D have a feature the A and C series do not; an air flow switch check.  What this means is that on a call for heat or in stand-by, the control checks for a closed circuit between terminals 6 and 7.  If this circuit is closed, the RM7895B or D will lockout in 2 minutes.  Remember this: In the “old days”, to check R4795 nuisance shutdowns, we often jumped out the AFS for a while to see if bouncing contacts in the AFS were causing the problem.  Obviously, you can’t do this when dealing with the RM7895B or D.
The block diagram, Fig. 6 on page 11 of Honeywell’s form 65-0086 on the RM7895 has an error.  The “Airflow Interlock” is shown as a closed circuit.  It should be shown as an open circuit.  Another error is on the top of page 4.  For the RM7895B under “Flame Establishing Period” “main” it says “yes”.  This should be “no”.  Under “AFSC”, it says “no”.  This should be “yes”.
Attached is the Gordon Piatt diagram that shows the results of converting from the T3 or T4 timer system to the R4795
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