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Service Application Manual SAM Chapter 620-41

Section 5E

AUTOMATIC SYSTEM CAPACITY CONTROLS By: Richard Baseler

Emerson Electric Company, Alco Controls Division INTRODUCTION

The purpose of this article is to review the fundamentals of Automatic System Capacity control methods, based on new developments and experience in the refrigeration and air conditioning industry. The basic principles are not new and there is very little reference material available concerning the selection and application of external capacity control devices. Every effort, however, has been made to include all possible updated information as an assist to the Service Engineer in the solution of his ever-increasing problems of maintaining proper operation throughout a wide range of partial loading.

SYSTEM CAPACITY CONTROL OPTIONS

Continued operation at partial load conditions whether for cooling or dehumidifying can produce undesirable conditions such as low suction pressure, short cycling, evaporator frosting, compressor lubrication, etc. If these conditions can be anticipated in the initial design stage, the following counteracting methods might be considered:

1. Installation of multiple units.

2. Single units with multiple compressors.

3. Variable speed compressors, such as the gas engine drive unit.

4. Integral compressor cylinder unloading.

5. Artificial evaporator coil loading.

6. Compressor loading by means of external automatic control devices.

It is the last of these methods which requires proper attention to application details since it is most commonly used or recommended by equipment manufacturers for smaller units having hermetic or semi-hermetic compressors or as a supplement to integral cylinder unloading devices.

BASIC SYSTEM DESIGN

Hot gas bypass for capacity modulation has been used for years with many variations from simple manual valve operation or solenoid valve and pressure switch control (Figure 18F29) to very elaborate balance loader applications (Figure 18F34). Basically the system must provide a means of bypassing high pressure refrigerant to the system low pressure side so as to maintain operation at a given minimum suction pressure. Proper bypass control can be accomplished by a modulating type pressure regulator which opens on a decrease in valve outlet pressure.

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Emerson Electric Company, Alco Controls Division

Hot Gas Bypass With Solenoid and Hand Throttling Valve

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Typical Balance Loader Application

Therefore, when suction pressure begins to drop because of reduced load conditions, the modulating valve in bypass line will open, allowing high pressure refrigerant to flow to the system low side, thus increasing suction pressure and providing an artificial load for compressor. When suction pressure increases to bypass valve setting, the valve will throttle and finally close off.


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Emerson Electric Company, Alco Controls Division HOT GAS BYPASS TO COMPRESSOR SUCTION

Figure 18F30 shows perhaps the most common hot gas bypass system. In the system the bypass line is taken directly from compressor discharge line, through a bypass regulator, and into suction line at compressor.

Typical Schematic for Hot Gas Bypass to Suction Line

With this system, hot gas should enter the suction line so as to provide a good mixing of hot gas being bypassed and suction gas entering the compressor. Figure 18F30 shows the bypass line entering on a 45° angle, however, line size and individual system piping may suggest an equivalent method.

Prolonged bypass operation may necessitate use of a liquid injection expansion valve for the purpose of de-superheating or quenching, again depending on system design, compressor design, etc.

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Emerson Electric Company, Alco Controls Division HOT GAS BYPASS TO THE EVAPORATOR

Figure 18F31 shows an alternate method of hot gas bypass to the system low side, which is rapidly gaining acceptance. Here the bypass line from compressor discharge line, through the bypass regulator, is connected to a tee between expansion valve feeding the evaporator and refrigerant distributor.

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Figure 18F31 Typical Schematic for Hot Gas Bypass to Evaporator Inlet

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CAUTION:

The refrigerant distributor must be a venturi type design or if it is the pressure drop type distributor it must be equipped with side outlet connection, downstream of distributor orifice or nozzle.

On serpentine coils, plates or shell and tube chillers, this need not be considered since a refrigerant distribution device is not generally used.

With this system the hot gas is mixed in evaporator and, therefore, not only raises the low side pressure, but also provides a false load, defrosts the coil, increases refrigerant velocity within the evaporator, and generally eliminates the need for de-superheating with a liquid injection valve. The expansion valve feeding the evaporator has its remote bulb and equalizer connected to the suction line leaving the evaporator. Thus, when superheat increases, it will open and feed liquid refrigerant into the evaporator along with hot gas until such time as the load increases to permit bypass valve to close.

Although there are many advantages to this system, it is not generally used on a multiple coil system or where evaporator sections may be located a distance from the compressor. Separate regulators must be used for each evaporator when bypassing into more than one. Coil design must incorporate free draining circuiting to prevent the increase in velocity from forcing a large quantity of trapped liquid out of the low side which in some cases may have enough volume to flood compressor crankcase.

Hot gas bypass into evaporator is suggested when evaporator elevation is BELOW the compressor. This assures proper oil return at low loads.

SPECIAL APPLICATION WHERE EVAPORATOR PRESSURE REGULATOR IS USED

When evaporator pressure must be regulated by an EPR valve to prevent evaporator saturation pressure from going below a given setting, hot gas can still be passed into the evaporator, however, it is important to remember that the bypass regulator external equalizer line must be connected on compressor side of evaporator pressure regulator. See Figure 18F32.

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Typical Application Hot Gas Bypass to Evaporator Inlet when EPR Valve is Used

SPECIAL BYPASS APPLICATIONS

Some manufacturers of package refrigeration systems have designed and tested other configurations which suit their individual requirements. Since these are generally tailored to a specific type of equipment design, it is not recommended that they be used in general practice.

One such system is shown in Figure 18F35.

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Typical Schematic for Hot Gas Bypass to Evaporator Outlet

Here the hot gas bypass is piped into suction line just ahead of thermostatic expansion valve remote bulb. Superheat gain causes expansion valve to overfeed evaporator, thus eliminating the need for a liquid injection valve for de-superheating. Although there are certain applications where it can be used successfully, it is not generally recommended because of the possibility of residual liquid in the evaporator when bypass valve closes. This may result in a momentary flooding condition which sometimes cannot be tolerated.

Bypass to flooded evaporators and suction line accumulators also present special cases. Detailed information can be obtained from equipment manufacturer or bypass control valve manufacturer.


Section 5E


Emerson Electric Company, Alco Controls Division A typical schematic for two-stage systems is shown in Figure 18F33.

Typical Piping – Two Stage Application for Bypass Pressure Regulators

BALANCE LOADER APPLICATIONS

Figure 18F34 shows a typical balance loader where liquid refrigerant is bypassed from the liquid line through an automatic expansion valve into a heat exchanger in compressor discharge line. Liquid refrigerant is boiled off in the heat exchanger and superheated gas can then flow into suction line or suction line accumulator.

Although this system is usually more costly, it nevertheless provides an excellent means of capacity control.

GENERAL RECOMMENDATIONS FOR HOT GAS BYPASS APPLICATIONS

For all hot gas bypass applications, the problem of head pressure control must be considered. Where low condensing temperatures occur during low load periods when bypass capacity control is required, head pressure must be maintained well above required hot gas bypass valve suction pressure setting.

Piping of the hot gas into system low side should be done in such a manner as to provide a homogeneous mixing of the gases. Pipe sizes usually dictate the best method to use. A tee may be used, a 45° Y connection or an elbow turned upstream to the flow of suction gas. Some manufacturers are recommending baffles or mixing chambers.

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A tee in the bypass valve outlet is recommended so that a liquid injection valve might be installed if required. Bull head mixing of liquid quench can then be accomplished in the bypass line before it enters the low side. A typical example is shown in Figure 18F37.

Typical Piping Arrangement for Pilot Operated Bypass Pressure Regulator

Most major equipment manufacturers have definite and specific instructions concerning these applications. It is suggested that you consult the application manual provided by the compressor or unit manufacturer.

OPERATION OF BYPASS VALVES

Bypass pressure regulators can be grouped into the following categories.

1. Direct acting unbalanced port valves.

2. Direct acting balanced port valves.

3. Pilot operated valves.

Any of these three categories may be obtained with either an adjustable setting or a fixed nonadjustable setting.

For most field installations, it is recommended that a direct acting balanced port valve or pilot operated characterized port valve be used, and that they have an adjustable setting.

Following is a brief discussion of the operation of various types of valves.


Section 5E


Emerson Electric Company, Alco Controls Division DIRECT ACTING VALVES

Direct acting valves, see Figure 18F36, either with or without a balanced port, operate with a spring loading above a diaphragm which will directly move valve stem and seat open or closed depending on equalizer pressure below the diaphragm.

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Typical Balanced Port Bypass Pressure Regulator

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Emerson Electric Company, Alco Controls Division When equalizer (either internal or external) pressure falls below spring setting, the valve begins to move in an opening direction. As equalizer pressure rises, it overrides the spring setting above diaphragm and the valve closes. Direct acting valves are usually designed with a 6 degree "gradient.” (Gradient being defined as the change in saturation pressure required to move the valve from closed position to the opening needed to deliver rated flow.) This is, of course, an average figure and it will vary with spring setting, sizing, and actual operating conditions.

The balanced port design will provide a smooth flow modulation from the instant of opening to full open position. Valves without a balanced port may sometimes chatter or hunt slightly at the instant of opening or at periods where very low flow is required. Aside from equalizer pressure and spring setting which are the forces which must operate the bypass regulator, there is always a high pressure differential across valve port, which may tend to override valve control in an unbalanced valve. Balanced port valves have two internal ports which are unbalanced in opposite directions. The two equal area ports are so arranged as to have one port unbalanced in the closing direction, the other in the opening direction. When these two forces cancel each other, the valve can then control smoothly from the equalizer pressure and diaphragm spring loading without other influence from high side to low side pressure differentials.

Some direct acting valves are non-adjustable, that is, they have a fixed inert gas charge pressure above the diaphragm in place of a spring. Valves of this design may be ordered with a specified fixed setting. These are more frequently used by package equipment manufacturers who are in a position to tailor a fixed setting to their particular requirements. Although a wide ambient change may affect the fixed charge setting, it is usually negligible. Temperature compensated models are available to satisfy this special requirement.

Adjustable direct acting valves may be obtained with 0 to 80 psig adjustable range. Turning adjusting screw to the right (clockwise) will increase spring tension and RAISE valve setting. Turning adjusting screw to the left (counterclockwise) will decrease spring tension and LOWER valve setting. Valve manufacturers supply instructions for obtaining settings below 0 psig for vacuum range applications.

Some valve manufacturers of holdback or crankcase pressure regulating valves have tested and modified their design to make them suitable for hot gas bypass applications. They are similar in design to direct acting valves described above and usually differ only in the use of a bellows in place of a diaphragm.

PILOT OPERATED BYPASS REGULATORS

Larger size regulators are generally of the pilot operated design. See Figure 18F37. They use a small direct acting regulator to operate a larger valve, generally called the main regulator. The pilot valve must have an external equalizer which is connected to the system low side at the point where pressure control is required. It operates exactly as described under the paragraph covering direct acting regulators. When equalizer pressure drops below diaphragm spring setting, the pilot valve port opens, admitting high pressure gas into the cylinder above the main regulator piston.

The piston is driven downward thus opening main regulator port. When main regulator port opens, high side gas is bypassed to the system low side. As low side pressure increases, the pilot valve closes. The main regulator piston has a small bleed hole which permits the pressure above piston to bleed to system low side thus permitting main regulator spring to close the valve.

Pilot operated bypass valve main regulators have a long stroke stem with a restrictor plug characterized with either a parabolic or vee port restrictor plug design. This prevents the valve from operating close to the seat where pressure differential unbalance may occur, and thus eliminating the need for a balanced port design. The characterized port will provide smooth bypass flow modulation. Pilot operated valves usually have the extra features of a manual opening stem for testing or emergency operation, flanged

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Emerson Electric Company, Alco Controls Division connections, synthetic tight seating seats, and replaceable parts. Application of pilot operated valves does not differ from that of the direct acting valves.

VALVE SELECTION

Manufacturers of Hot Gas Bypass regulators provide catalogs with simple selection procedures. The following steps outline the necessary information required to use published catalog information.

1. Determine system refrigerant.

2. Determine bypass capacity in tons. Compressor capacity minus minimum system load equals tons to be bypassed.

3. Determine minimum evaporator temperature permissible. This determines suction pressure at which bypass valve is to begin to open.

4. Select the bypass line size to establish valve connection size and style.

5. Determine if regulator is to be adjustable or have a fixed set point. If a fixed set point is to be used, it should be as found in step 3.

NOTE:

If the adjustable model is required, normal adjustment range is 0 to 80 psig.

6. Determine required voltage for bypass line solenoid. If a pilot operated type with integral solenoid is used, this will have to be specified also.

Now the catalog tables can be used to select the proper valve type number. Instructions for selection and ordering as outlined in the manufacturer's catalog should be followed. Generally selections are shown for actual valve capacity based on a 6 degree gradient. This means that the valve will deliver its rated flow capacity at a suction pressure equivalent to six degrees lower than selected set point at which the valve begins to open.

If a compressor has cylinder unloading devices and you require a bypass valve to balance compressor capacity below the last step of cylinder unloading, you should select a valve having the required capacity to balance the difference between compressor capacity at the last stage of cylinder unloading and lowest permissible load.

The application may dictate continuous compressor operation in which case required valve capacity will equal compressor capacity or the remaining compressor capacity other than the last stage of cylinder unloading.

EXAMPLE OF VALVE SELECTION

1. A refrigeration system has a compressor capacity of 20 tons R-22 and must operate with a load as low as 15 tons R-22. The evaporator temperature must not drop below 30°F. For an evaporator temperature of 30°F, select a valve having five tons R-22 capacity (20-15 = 5 tons bypass).

2. A compressor with 30 tons R-12 capacity has integral cylinder unloading to 15 tons or 50% capacity reduction. There are conditions where this system must operate with a load as low as

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five tons with a 50°F evaporator. Select a valve having a capacity of 10 tons R-12 (15-5 = 10 tons bypass) at 40°F evaporator.

SELECTION OF LIQUID INJECTION VALVE

If the application requires hot gas bypass into the suction line you will also have to select a liquid injection valve to de-superheat (quench) hot gas being bypassed to protect compressor from overheating. Most bypass valve manufacturers provide a simple selection table that permits choosing a valve having an adjustable superheat range to provide for 45°F, 65°F, or 85°F suction gas temperature at compressor. Normally the expansion valve power assembly charge is tailored to provide control at 65°F suction gas temperature where most compressor capacity ratings are established but special models are available to provide control within any required suction gas superheat.

APPLICATION OF LIQUID INJECTION VALVE

Figure 18F37 shows an effective method to achieve good mixing of bypass gas, liquid injection and suction gas. There are a variety of methods that may be suited for a particular application or line size situation. Some equipment manufacturers may recommend mixing chambers, spray nozzles or baffles to provide proper homogeneous mixing of the three refrigerant conditions.

A solenoid valve in the liquid line to liquid injection valve is recommended to prevent the flow of liquid into suction line during periods when the hot gas bypass valve is not operating, such as high load or pull-down periods.

HOT GAS BYPASS LINE SOLENOID

Just as a solenoid is recommended for the liquid injection valve, it is also important to install a solenoid valve in hot gas bypass line ahead of bypass regulator. This will permit automatic pump down and will permit unwanted high side to low side leakage. It also prevents bypass during unwanted periods. The solenoid may be wired into system control circuit, parallel with liquid line solenoid, wired into the cylinder unloader circuit, wired to a pressure switch, or outdoor thermostat, etc. If the bypass valve is piped to suction line, the bypass line solenoid must be wired in series with a discharge line thermostat set to terminate bypass if discharge temperature exceeds 300°F. Consult compressor manufacturer for specific recommendations.

The selection of the bypass line solenoid may be based on required tons at a 10 to 15 psi Δpd drop in pressure, as this additional pressure drop will not seriously affect bypass regulator capacity.

Considerable cost savings can be achieved when using a pilot operated valve as a small solenoid is used in the pilot circuit, integral with pilot operated valve.

PNEUMATIC CONTROL

Most adjustable direct acting or pilot operated hot gas bypass regulators are available with an external air connection so that they might be adapted to a standard pneumatic control circuit. See Figure 18F39.

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Pneumatic Compensation for Hot Gas Bypass Regulator

One (1) psi increase in air pressure will increase the bypass valve setting one (1) psi. A decrease of one (1) psi air pressure will decrease the setting one (1) psi. Standard pneumatic control systems will normally provide a 2 to 15 psig or 2 to 20 psig control range. If bypass valve spring is set for the lowest required setting, the pneumatic controller or thermostat can then raise the setting by 13 or 18 psi depending on available control range. Thus, the very sensitive pneumatic thermostat may sense a load change (bulb in entering air or water line) and in anticipation of a drop in suction pressure, apply air pressure to the bypass valve to raise the setting so it will start to open prematurely. This provides excellent control and can reduce valve gradient to any amount desired (Figure 18F39).

Pneumatic relays may be used to provide an even greater range of pressure control. Multiplier relays can provide control for high pressure air to boost the bypass regulator well above its adjustable range for special applications.

BALANCING SYSTEM LOAD BY PROPER BYPASS VALVE ADJUSTMENT

The following steps for valve adjustment may be used as a recommended method to properly balance system load.

1. When bypass regulator is properly installed, start the system and operate at normal design load.

2. Adjust regulator to its minimum setting.

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3. Begin reducing the load, by closing dampers, covering evaporator, reducing air flow, shutting off fans, or in the case of a water chiller, throttle or bypass water.

4. As suction pressure goes down, raise the bypass valve adjustment until desired constant suction pressure is reached. This may be a setting high enough to prevent short cycling or prevent frosting or freezing of the evaporator.

5. Raise evaporator load and observe pressure at which bypass valve closes to be sure that bypass does not occur during normal operation.

6. Reduce load again to verify suction pressure control.

7. Permit system to operate at low load condition long enough to establish suction superheat rise at compressor.

NOTE:

Use a thermocouple or thermistor type temperature measuring instrument to establish suction superheat again.

8. If a liquid injection valve is used, adjust superheat to required amount for optimum system performance.

SUMMARY

It is significant that most major equipment manufacturers are now offering hot gas bypass controls as a standard or alternate method of effectively providing compressor capacity control. A once specialized application is rapidly becoming standard operating procedure and although it cannot compete in operating economy with many methods of capacity reduction, it does present a simple and effective means to provide a method of coping with more exact design requirements without resorting to more expensive or specially designed equipment. Whether elimination of short cycling or steady suction pressure control will prove to increase compressor life remains a question, but a smooth running compressor and a balanced system load is sure to create customer satisfaction.

Copyright © 1972, 2009, By Refrigeration Service Engineers Society.

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