What Do You Need to Know About the Compressor for the Cpr Valve
Caput Pressure Control, Solenoid Valves, and Hot Gas By-Laissez passer
In Figure 1, you see a system with period control pressure regulator controls. Non all of these devices are on every arrangement. (Dryers, sight glasses, etc., have been omitted.) A TXV is shown, as it is a flow command device, but has been discussed in Info-Tec 7.
There are only iii kinds of pressure level regulator catamenia controls:
• Inlet regulators (also chosen upstream regulators)
• Outlet regulators (also called downstream regulators)
• Differential regulators
Annotation the sensing line in relation to the direction of menstruation. An inlet regulator only "cares" nearly what is happening upstream. Information technology adjusts the flow according to inlet pressure. Information technology doesn't "care" what happens downstream. An outlet regulator adjusts the catamenia according to downstream pressure level. Information technology doesn't "care" what's happening upstream. A differential regulator senses both upstream and downstream pressure level and adjusts the flow to maintain a set divergence between inlet and outlet pressures.
HEAD PRESSURE CONTROL VALVES
Refer to effigy i. Particular 1 is a head pressure control valve or inundation valve. As was noted in Info-Tec 9, fan cycling can control head pressure merely has drawbacks. Systems that use dampers to control airflow through the condenser are very expensive and leak refrigerant. There are few condenser damper command systems in use. The flood valve system is the best head pressure control system for shine, accurate, and trouble-free low ambient control.
Head pressure command is needed on air-conditioning at 60°F ambience and on commercial refrigeration at fifty°F ambient and lower.
Control of head pressure is needed in guild to maintain:
1. Acceptable pressure drop over a TXV for the refrigerant effect
2. To prevent flash gas in the liquid line
3. To provide pressure level for hot gas by-pass, or hot gas defrost, if nowadays
Flooded condenser systems are able to maintain pressure level within 5 to ten psig. They tin can operate efficiently to very low ambients encountered in our cold winters. By flooding a condenser with refrigerant, we truly reduce the condenser'due south capacity.
Up to nigh 15 tons capacity, the HP Alco Headmaster tin can exist used.
See Figure 3. B is the connexion for the discharge line from the compressor. R is the connectedness to the receiver, and C is from the condenser.
The dome is charged and exerts pressure level on the top of the diaphragm. At high ambient temperatures, by-pass gas (B) pushes on the lesser of the diaphragm and counteracts the dome pressure level. The upward forcefulness of this gas seals the seat disc confronting the tiptop seat. Flow is then from the condenser (C) to the receiver (R). Every bit the ambient temperature drops, head force per unit area decreases. The by-pas gas can no longer overcome the dome pressure and the diaphragm moves downwards, moving the seat disc towards the bottom seat. This allows belch by-pass gas to be metered direct into the receiver, creating higher pressure at the condenser outlet. This pressure reduces the menstruum from the condenser. The stay time of the refrigerant in the condenser is increased, and liquid refrigerant begins to rising in the condenser, flooding it. This reduces the constructive condensing surface resulting in acceptable loftier side pressure level.
Annotation: Head pressure valves can simply be used on systems with TXV'south, and must have a receiver big enough to hold the arrangement's regular accuse of refrigerant and the boosted 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 alluvion the condenser in cold weather condition would back up in the condenser and cause besides high a head pressure.
The receiver has to exist big enough to store this actress refrigerant during warm weather condition.
To calculate the amount of extra refrigerant we demand to approximate the book 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 2 feet. Count the number of tubes in the condenser. In this instance it is seven. Count the number of return bends. In our example, this is 6. Determine the O.D. of the tube. Condensers are built with 3/eight, 1/two, or five/eight O.D. tubing. We'll utilize 3/eight O.D. for our example. Now, multiply the length of tube by the number of tubes: seven x 2 = xiv feet of 3/8 O.D. tube. Come across Figure 5.
Figure five shows the amount of specific refrigerant per tube size, per foot, to add for the everyman expected ambient temperature encountered. (In Wisconsin, that is commonly -20oF.) Let's say our instance condenser is in an R-22 system. From the chart, we see that one foot of iii/eight O.D. tube at -20°F volition concur .055 lbs. of R-22. Each return bend is the equivalent of .two of a foot of tube. We have 6 return bends, or 6 x .2 = 1.ii additional feet of tube. Seven, 2 human foot long tubes are equal to fourteen anxiety of tube, plus i.2 feet of return bends gives a grand total 15.ii feet of 3/8 O.D. tube. 15.2 x .055 = .836 lbs. of R-22 that must exist added to the regular accuse to flood this condenser. (This case would be a very small condenser.) If the arrangement accuse was 4 lbs., we add the .836 lbs. (round off to five lbs.), and select a receiver rated to hold 5 lbs. when it is 90% full of refrigerant. Never select a receiver that would exist more than 90% full.
Tech Tip: Select a size that volition event in the receiver existence between 75% and 90% full when storing the accuse. If the receiver will be in a warmer ambient than the condenser, a bank check valve should be installed in the drain line to the receiver to prevent opposite flow.
Caution: Disconnect any fan cycling controls when using a inundation valve. If yous don't, the large sudden modify in head pressure level due to fan cycling will soon destroy the flood valve. The system must too utilize pump down (pump down will be discussed afterwards in this Info-Tec.), when using a overflowing valve.
To size an Alco Headmaster HP valve, y'all need to know what refrigerant the organization 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 Effigy half dozen should be adjusted by the multiplier in Figure 7.
Later adjusting the nominal rating from Figure 6 past the multiplier from Figure 7, select the HP valve from Figure vi. Do non exceed the v lb. psig rating for any HP valve.
Example: Given: R-502 organization, three 1/ii tons, and -twenty°F evaporator. From Figure seven, the multiplier of .845 is applied to the 3 1/2 ton rating. 3.5 ten .845 = 2.958 or approximately 3 tons. From Figure six, nosotros run into 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 viii 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 level).
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 stock-still setting). See figure nine.
The IPR is set up for the head pressure yous want to maintain and the OPR is set approximately twenty lbs. higher than the IPR. Menstruum tonnage capacities of IPR's and OPR's are shown in Figure 10.
SOLENOID VALVES
Meet Effigy one. Particular ii is a solenoid valve. Solenoid valves are on/off menses controls. Solenoid valves are sized simply by menstruum tonnage, non past 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 period 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, employ bushings or couplings to suit to the line size.
The nearly prevalent apply of refrigeration solenoids is in "pump down" systems. Pump down should be on every system that uses an expansion valve. Pump down does one matter and i thing only, merely information technology is important. Pump down is used to preclude refrigerant migration to the low side of a arrangement on the off wheel. The solenoid valve is installed in the liquid line at the archway 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 period. The compressor is controlled past a depression-pressure level control. As the compressor pumps all the refrigerant into the loftier side of the system, the suction force per unit area drops to the low-pressure command setting and the depression-pressure command opens and stops the compressor. The entire refrigerant is at present held between the discharge valves of the compressor and the liquid line solenoid valve. Equally long every bit the valves on the compressor or the liquid line solenoid valve don't leak, at that place will be no refrigerant migration to the oil in the crankcase of the compressor or to the suction piping of the organisation. Excess refrigerant in the oil of a compressor reduces the oil'due south lubricity and causes excess vesture on commencement-up. Liquid refrigerant in the suction pipe can crusade liquid floodback, fifty-fifty slugging on startup, severely damaging the compressor. If there is a hot gas by-laissez passer, 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 volition be effective. A small leak volition cause the suction pressure to rise to the low-pressure level control'southward cut-in setting, starting the compressor and pumping downwards again. (If this "short run" to pump down occurs frequently during the off cycle, the leak should be repaired.)
To first up a pump downward organisation, the temperature control closes, energizing the solenoid valve. The solenoid valve opens, and refrigerant flows to the low side raising suction force per unit area, closing the low-force per unit area control, and starting the compressor. Solenoid valves and temperature controls tin can operate 2 or more evaporators to refrigerate separate spaces using one condensing unit of measurement.
HOT GAS BY-Pass VALVES
See Effigy 1. Detail 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 arrangement'southward low side in a proportion that will balance chapters to load demand. They attune and can be the terminal pace of unloading.
A portion of the refrigerant is past-passed around the condenser and TXV. This reduces the amount of refrigerant bachelor for refrigerant consequence. The rut content of the hot belch gas adds load, further reducing the refrigerant effect of an evaporator.
Air-conditioning and commercial refrigeration systems are sized for maximum expected load, then the design engineer adds condom factors to the calculated loads "to be certain" the system will perform as expected. Studies accept shown that most systems operate at full load only 10 to 20 percent of run fourth dimension. Systems that operate at or almost minimum load run into many problems:
Coils frost, chillers freeze, compressors overheat, poor oil circulation is encountered, brusque cycling occurs, etc. Many on/offs are hard on the equipment.
Hot gas past-pass tin can alleviate all of the above and result in less temperature variation, good control, less maintenance, and longer equipment life. Hot gas past-pass artificially loads the compressor, but dandy cost savings still result. Demand charges are less; lubrication problems that are 85% of compressor failures are eliminated. Maintenance time and electrical issues are reduced.
All compressors are designed to operate continuously. Procedure systems that use a constant load and are seldom turned off terminal for years and years with no problems.
A properly designed and installed hot gas by-pass arrangement will result in acceptable suction pressure throughout a range of loads, fifty-fifty to no load. It volition non cause excessive super oestrus, and will not permit liquid return via the suction line.
There are diverse methods of doing hot gas by-laissez passer systems. The virtually mutual is by-pass to the evaporator inlet.
This is the merely approved method if the evaporator is below the compressor, due to problems encountered with adept oil return. In this arrangement, the evaporator acts as an splendid 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 render. It is like shooting fish in a barrel to add, and is usually low-cost compared to other methods of hot gas by-pass. It does accept limitations. All hot gas lines should be insulated to foreclose condensation of the hot gas earlier it reaches the evaporator. Even insulated, these systems are limited to 35 to twoscore 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 organization, care must be exercised to insure that the evaporator used for by-pass is large enough to handle the consummate by-pass and unused TXV chapters, and be the last active evaporator on line.If the vertical riser of the hot gas line exceeds 4 to 5 anxiety, oil may exist held up in the vertical riser, necessitating constructing an oil return line at the base of the hot gas riser. (Run across Effigy 11, oil render line.) The oil return line should be made out of v feet of 1/8" O.D. copper tube. This causes enough pressure drop so hot gas will not pass through it but oil will bleed. If this oil drain line has to span more than 5 anxiety to connect to the suction line, add 1/iv O.D. tube to make upwards the span.
Figure 12 is by-laissez passer to suction line. Information technology has the advantage of short lines and everything located at the compressor. If an orifice distributor cannot be replaced with a venturi blazon 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 organization to the archway 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-laissez passer to suction systems. Make sure the evaporator is free draining to the compressor. Use of this organization 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 beneath 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 exist observed in teeing the hot gas line into the suction line.
Suction lines 7/8 O.D. or smaller can use a standard tee. one-1/8 O.D. and up should be teed in at a 45°angle, the hot gas flow opposing the suction line period. See Figure 13. Dimension "X" should e'er be at to the lowest degree v anxiety. The longer X is, the amend 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 splendid 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 every bit close to the compressor suction as possible. Always make the connections on summit of the suction line tube to prevent oil blocking the tube. For ease of servicing, installation, and to keep cost down, go on all the hot gas by-pass components as close to the compressor as possible. The sensing lines, or pilot lines, are 1/four O.D. tube up to 20 feet long. If, for some reason, the hot gas by-pass valve was installed over 20 anxiety 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 loftier superheat TXV. They are usually internally equalized valves. The sensing bulb of this TXV should be fastened to the suction line downstream of the mixing tee or accumulator. (See Figure 13 and xiv.) The liquid line to this TXV is a 3/8 O.D. tube, upward to 50 tons, and 1/2 O.D. to 100-ton systems.
If an Evaporator Pressure level 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 level, not evaporator pressure.
There are two types of hot gas by-laissez passer valves. Direct actuated, such as Alco DGRE'south and CPHE's. The other hot gas past-pass valve type is pilot operated, such as FA8'southward. Direct actuated are low tonnage valves. Pilot operated valves are large tonnage valves. Both kinds have adaptable fix 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 data must exist gathered:
• REFRIGERANT?
• EVAPORATOR TEMPERATURE?
• HOT GAS TO EVAPORATOR INLET OR SUCTION LINE?
• PUMP DOWN System?
• VOLTAGE OF CONTROL Circuit?
• TONS TO BE By-PASSED?
The question that poses the most problems is the "tons to be past-passed". This is seldom the arrangement'due south entire tonnage. Just because the arrangement is rated 50 tons does not mean the past-pass system has to be rated to handle all 50 tons. After unloaders have been activated and TXV's accept throttled to their minimum, the balance of the load is what the by-laissez passer organisation needs to handle.
Applying hot gas by-pass to a reciprocating compressor h2o chiller differs from a organisation using a finned airflow evaporator but in the control system and setup of the by-pass valve. The only by-pass method that can exist used is by-pass to the evaporator, not by-pass to suction line. A hot gas valve modulates over an viii lb. range. On a chiller, these eight lbs. later the last stage of unloading can't be tolerated, because the tubes of the chiller barrel might freeze. The hot gas valve is adapted 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 final stage of unloading. When the last stage of unloading is activated, the hot gas valve is already open enough to laissez passer a sufficient book of gas to prevent freezing the chiller tubes. A chilled h2o 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 bicycle the hot gas valve in an effort to stabilize the h2o temperature. The compressor volition continue to operate until another control stops it. This organisation prevents the chiller tubes from freezing at minimum capacity load conditions.
The size of the hot gas line can be the same size equally a properly selected hot gas by-laissez passer valve if the equivalent line length is less than 20 feet. Equivalent line length takes into consideration the force per unit area drop imposed by fittings and valves expressed as anxiety 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 seven/viii O.D. tube.
There is a bit of an art to line sizing. That is, we will select a line size we think volition piece of work and then do calculations to see if information technology is going to work. Let's say we had to size a by-laissez passer for twenty 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 exist 7/viii O.D. or 1-1/8 O.D. Nosotros'll endeavor 7/8 O.D. first. From Effigy 16, three 7/viii O.D. long radius ells are equal to 15.nine anxiety, or rounded off, 36 anxiety.
NOTE: Figures in assuming face type are maximum recommended tonnages at pressure drops calculated to minimize suction line temperature penalty. Shaded areas are for general data just.
Tonnage Capacities of Discharge Lines Delivering Hot Refrigerant-22*
*Based on xl degrees F suction and 105 degrees F condensing. For other conditions apply correction factor
from Tabular array 6-25 to design tons earlier inbound this table.
Figure 17 shows the capacity of lines for R-22 discharge gas at diverse pressure drops, at twoscore °F suction and 105°F condensing temperatures, standard for air-conditioning. Note that there is no 10 lb. pressure drop line, the pressure level drib we are using equally our pattern drop. The tonnage capacity for a 10 lb. drop is virtually 1.8 times the capacity for 3 lbs. for the aforementioned equivalent length. (Square root of 10/3.) Therefore, we tin can use the 3 lb. driblet multiplied by 1.8 to get a chapters at 10 lbs. There is no demand to try and interpolate line length. Simply use the next longer length of your calculated length.
In our instance nosotros'll use the 40 ft. line. Given: 3 lb., 40 ft., vii/viii O.D. nosotros find 7.ane tons. 7.1 x 1.eight = 12.8 tons. Not big enough. Trying ane-1/viii O.D.: v.7 + 20 = 25.7 30 ft. equivalent length column, iii lb. driblet shows xvi.eight tons. 16.viii ten 1.8 = thirty.2 tons. Enough 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 gene should be applied to the tonnage ratings. (Meet Figure 18.) Use the correction factor to the tonnage found in Figure 17, earlier 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 folio 30. Make selections from these charts.
The direct acting DGRE valve can exist used up to three tons by-passed for R-12, 10 tons for R-22 and R-502. Considering of the DGRE'southward depression cost, this should be your option, where applicable. Larger tonnage capacities will crave the FA8. CPHE valves are take-a-part versions of the DGRE. They are more than expensive than the DGRE, a lot more in the larger sizes.
In one case you lot know the tons to be past-passed, the refrigerant, the evaporator temperature, and the type of by-pass system, you demand only to get to the charts. In our instance, R-22, 20 tons, 40 °F evaporator, by-pass to evaporator we notice 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 gear up of flanges. In our example, RX174 1-ane/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, encounter P. 16 and P. 17 of the catalog). Nosotros too demand a liquid line solenoid valve, for past-pass to suction, in the liquid injector TXV feed line. On P17 are shown the solenoid valves to use. We encounter the 200 RB4 is used with the LCL4A on R-22. If dealing with a system where the DGRE applies, note that the charts testify the correct hot gas solenoid to employ. DGREs do not have a solenoid congenital in.
Once the hot-gas organisation is installed, it must be set to operate. The hot gas valves do not come up set at a specific pressure; they must be adjusted for what is required. They are adjustable from 0 to 80 lbs.
To gear up a organization, kickoff remove the seal cap over the aligning screw. Dorsum the screw all the way out, counter-clockwise. Block airflow over the evaporator anyway yous can. Shut off the fan, block off the evaporator with cardboard, or whatever. We need to simulate a no load or depression load condition. When the suction force per unit area begins to become about 5 lbs. below the desired set point, begin to turn the adjustment spiral clockwise until the valve begins to open. Y'all tin can hear the hot gas going through the valve, or feel the hot gas line at the outlet of the valve. Turn the aligning screw slowly until the suction pressure level is increased to the desired set point. Each consummate plow of the aligning screw is approximately 4 lbs. Allow enough fourth dimension between each adjustment for the organization to stabilize. Past-laissez passer valves attune from closed to wide open over an eight lb. range. They can exist very precisely set. If the compressor is equipped with unloading, fix the by-laissez passer valve to begin to open 2 to three lbs. beneath the last stage of unloading. In order to be able to obtain the proper setting, the loftier side pressure level should be maintained at a minimum pressure corresponding to 85 to 86°F condensing temperature. This is well-nigh 90 lbs. for R-12, 150 lbs. for R-22, and 170 lbs. for R-502.
If the by-laissez passer to suction line organisation is used, the liquid injection TXV must be adjusted. (No instructions come with this valve.) Again, decrease the organisation'south load to brand the hot gas past-laissez passer go fully open. Adjust the superheat of the injection TXV then 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 lot may need to "tweak" the superheat setting. Turning the superheat adjustment on the injector valve clockwise increases superheat, counter-clockwise decreases superheat. Recycle the organization once or twice to make sure of the settings. In brusk, we are trying to maintain twenty°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 i, number four. It is an inlet or upstream force per unit area regulator. The EPR is designed to sense inlet pressure level and arrange the menses to maintain a fix force per unit area. They are sometimes chosen back pressure regulators. They are bachelor 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 mutual suction system. The EPR's will prevent the temperature from dropping in evaporators that are warmer than others in the aforementioned system, while the condensing unit(southward) continues to run to satisfy the coldest evaporators. They tin can prevent the freezing of a chiller by keeping the saturation force per unit area above the freezing temperature of the h2o.
EPR'southward should exist installed every bit close to the evaporator outlet as possible, although they can be located some altitude downstream from the evaporator. If a airplane pilot is used, connect a pilot line from the evaporator to the EPR.
Alco manufactures a large selection of EPR's. Alco's itemize 24-D "Pressure Regulators", dated 9/88, describes all of the EPR's and the options available.
The commencement and most important part of selecting an EPR is determining the port size needed which determines the EPR'due south capacity. Like refrigeration solenoid valves, EPR's should not be selected past line size.
Five basic arrangement weather condition must be adamant before a selection tin can exist made. They are:
ane. Refrigerant
2. Evaporator design capacity in tons.
3. Evaporator blueprint temperature (or pressure)
four. Minimum evaporator temperature (or force per unit area)
5. Options
6. Available pressure drop across the EPR at design capacity
Conditions1 through 5 are like shooting fish in a barrel to determine. Number 6 causes the well-nigh problems. Depending on the specific system involved, the pressure drop across an EPR can vary from 2 to xx lbs. at blueprint load. When the load is to a higher place design load (such equally at start-up or afterward a after a defrost period) the EPR has to open wide enough to handle the load and so command at design load or light load once more. EPR's are selected using design capacity, non the startup or whatever other abnormal load.
Tech Tip: When applying an EPR to a single evaporator organisation, select an EPR for a two lb. pressure drop. The
2 lb. pressure level drop is desirable and should result in good control.
An example: An R-502 organization with one evaporator designed for 12,000 BTU at 0°F. We want to prevent the evaporator from going beneath 0°F. No suction cutoff is desired. Cost is a factor. 12,000 BTU is 1 ton of refrigeration; 0°F is 31.2 psig on R-502.
Using the Alco catalog 24-D, nosotros can select an EPR from the 2 lb. drop column considering we are dealing with only one evaporator. Selecting just on the basis of capacity, we'd find an EPRB-12 (folio 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 toll, merely even without considering those additions, we'd notice the IPR-ten-0 to 50 lb. direct acting EPR to be the lowest cost. Direct acting EPR's will ever be the lowest cost EPR. Our selection would be the IPR-10, 0 to fifty lb. range.
When no other options are needed, such as suction stop solenoid valve, or pilot arrangement, e'er consider the IPR valve first. IPR's are available in 3 pressure adjustment ranges: 0 to 50 psig, 30 to 100 psig, and 65 to 225 psig. Line sizes from 5/8 O.D. to one-3/viii O.D. are also available. The IPR has a pressure level tap on the inlet (evaporator) side for connection to a approximate for convenient fix point adjusting.
Multiple evaporators in a arrangement present a slightly unlike trouble when computing pressure drib. With evaporators operating at different temperatures, i or more than EPR's may be required to maintain pressure college than a common suction line. (EPR's can never maintain pressures lower than the suction line.)
In an R-502 system (figure two), 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 driblet for evaporator A is then 72.6 - 58.7 or thirteen.9 psig. For evaporator B, information technology is 68.2 - 58.seven or 9.5 psig.
Evaporator A's pattern chapters is i.8 tons, and B's is ane ton. Evaporator A's selection is an EPR that volition be rated i.8 tons on R-502 at 35°F DP 13.9 psig. If no other options were specified, the IPR-6 adjustment range xxx to 100 psig is the option. 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, once again, the selection. The Alco charts evidence the maximum tonnage capacity the EPR can pass. Ever brand your selection with a capacity rating the same or more than required (never less), merely exercise not deliberately oversize. In our case for evaporator A, the IPR-ten would accept about 5 to vi 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 lawmaking 12, a i/2" orifice, would exist the correct selection for evaporator A, and the port lawmaking eleven, three/eight" orifice, for evaporator B.
All the "other" evaporators mentioned in our example would announced not to need EPR'southward since the pattern suction pressure is their blueprint pressure. Even so, 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 contumely sweat continued pilot-operated EPR used mainly on refrigeration "racks". Piloting with discharge gas assists the valve'southward functioning and consequently, it has very depression pressure level drop. The EPR is a larger capacity cast fe, flange connected, EPR bachelor with various pilots. It is bachelor in vacuum range as low as 25" Hg.
Calculation an "S" to the EPRB and EPRV model number adds a built-in suction cease solenoid valve to the devices.
The FA1 is an EPR that is a dual pressure level regulator. By energizing a built-in solenoid valve, the FA1 will command evaporator pressure level at a lower setting than the prepare 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 airplane pilot acts every bit a low limit. The solenoid valve is optional.
The EPR (Five) can exist piloted with a pneumatic control indicate. This airplane pilot, the EAC, is bachelor in two ranges, 2 to 110 psig, or 25" Hg vacuum to 95 psig.
Alco's 722 EPR functions the same every bit an EPRV, except it must be used with an external pilot of some kind. What pilot is used determines how the 722 will office.
With the broad variety of pilots and options bachelor from Alco, an EPR can be configured for about any application. No EPR should e'er be used with a design DP over 20 psig. It is all right to parallel EPR's if needed.
Crankcase Pressure level Regulators
The Crankcase Pressure Regulator (CPR) is every bit shown in figure i, number five. These are outlet pressure regulators, often called "holdback" valves. They are downstream regulators. Their sole function is to limit crankcase pressure to preclude over-loading the compressor. They are sensitive only to their outlet pressure and the regulator will shut in the upshot of an outlet pressure increase. As long every bit the valve outlet pressure is greater than the valve's pressure setting, the valve volition stay closed. As the compressor reduces the outlet pressure, the valve will open up and permit refrigerant vapor into the compressor. As outlet pressure level drops, the valve will open up to its rated position.
A crankcase pressure-regulating valve should be applied to any organization on which the compressor could be overloaded by high suction force per unit area. It should exist installed every bit close to the compressor'due south suction port as possible, downstream of any other controls. An accumulator may exist downstream of a crankcase force per unit area regulator.
Note: To select a crankcase pressure level regulator, we need to know the system's refrigerant, capacity in tons, design suction pressure, maximum allowable suction pressure level, 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 commanded force per unit area drop. Maximum pressure level drop for medium or high temperature systems is two psig, for depression temperature systems ane/2 to 1 psig. Nigh chapters tables will only show pressure drops of one/2, 1, and 2 psig. Meet Alco's catalog 24-D on folio 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 force per unit area. This is the valve setting.
Case: 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. -xx°F, R-502, is 15.5 psig saturation pressure. The AH2511 is rated to +ten°F or 41.i psig. Our option is based on a valve for R-502, with a setting of twoscore psig and a suction pressure of 15.5 psig. This is low temperature, and so a 1/ii or 1 psig DP should exist used. Run across Effigy three beneath, from Alco's itemize 24-D.
Since we want to proceed the DP every bit low as possible to foreclose loss of capacity, we'll try the i/ii psig drop. Enter the chart at the valve point setting of 40 psig. Our blueprint suction pressure of 15.5 psig is about half way betwixt ten and 20 psig. When blueprint gauge pressures fall betwixt pressures on the chart, utilize the lower pressure; in this instance 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'south. If we use the 1 psig DP column, an OPR-half-dozen is rated .ix-ton capacity and could exist our selection. The OPR-6 volition impose almost a 2°F temperature punishment, the ORP10 about 1°F. (Systems are seldom designed so close to bodily load they can't afford an extra degree or two of temperature penalization.) If a suction solenoid stop had been required, or a pilot operated crankcase regulator, other than the depression cost straight interim 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.five psig (-20°F), DP ii psig, the port is 3/iv" or lawmaking thirteen. FA5's are very expensive compared to OPR'south. They are usually used only when capacity exceeds the OPR ratings. Crankcase force per unit area regulators tin be parallel when more than capacity is needed than the largest regulator can supply. Like EPR'southward 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 adapted after installation so they operate together, equal amounts passing through the valves.
If a crankcase force per unit area regulator is used in a system with hot gas by-pass, the setting of the crankcase regulator must be higher than the past-laissez passer valve setting, as the crankcase force per unit area regulator will be constantly throttling the menstruation to endeavour and protect the compressor.
Accumulators
While accumulators are not exactly pressure regulators, they tin can be described as a flow regulator. They are protective devices that meter refrigerant and oil back to the compressor to preclude liquid return directly to the compressor. Like pump downward, they should be on every system, merely seldom are, except on low temperature systems. Selection of an accumulator is based on force per unit area drop, oil return, and the total corporeality of charge to exist held.
Tecumseh has simplified accumulator choice to just determining the arrangement charge. They have taken pressure drop and oil return into consideration in the blueprint of their accumulators. They accept patented their design. To use 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 non be applied to compressors with a suction size less than 1/2" O.D. If the organization's charge is unknown, a rule of thumb to determine the accuse can be used to judge the corporeality of refrigerant in a system, but it is a final resort. It is: Air conditioning and other loftier temperature systems: 3 lbs. per horsepower. Medium temperature: 5 lbs. per horsepower. Depression temperature: 7 lbs. per horsepower. (Note: horsepower, not tons). One time the system's charge has been determined, an accumulator large enough to concord 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 employ, as long as the compressor's suction port was iii/iv" O.D. or larger.
On systems besides large for Tecumseh accumulators, it may not be cost effective or practical to try and apply an accumulator large plenty to agree the entire charge. In fact, information technology may be difficult to find a manufacturer that makes i. Big accumulators can be sized to hold a minimum of fifty% of a system's charge. Manufacturers' charts for accumulators volition normally show maximum and minimum tonnage capacities at various evaporator temperatures based on a pressure drib equivalent to 1/ii°F. When selecting an accumulator other than a Tecumseh, read the manufacturer'southward chart carefully when making your selection to ensure you are staying within the parameters set downward 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. I type of system where all these devices will be installed is in a supermarket rack servicing freezers, coolers, cases, etc. At that place they are necessary to brand the system function and to protect the expensive compressors existence used.
You may be asked how long purge timing should be for the plug-in purge carte du jour. Purges are supposed to be long plenty to make four air changes earlier lighting the burner. If you lot knew the CFM rating of the blower, and the book of air in the boiler and chimney, yous could calculate the purge timing needed to accuse the air 4 times. Since most of the R4795'southward volition exist replacements, use the aforementioned timing every bit the existing card.
Depression voltage controllers cannot be used with R4795'south. The T & T terminals on an RA890 are now 6 & 7, where an airflow switch is connected.
Purge timing does non offset 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 and then we accept safe first check. If a flame simulating condition is present during purge, the flame relay gyre, 2K, will energize preventing ignition, simply 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 status, or real flame, goes out, the start-up will go along. If a purge card fails or is non installed correctly, the burner motor, on a call for estrus, volition run but pre-purge cannot exist 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 once more, the burner motor volition run only no ignition can accept place.
If the airflow switch opens during the run period, terminals 3, four, and 5 volition be de-energized, dropping out the main valve, pilot valve, and ignition. Final 8 will remain energized so the burner motor will continue to run. If the airflow switch closes, the purge timing will beginning and the start-up sequence will begin again. Note that no lockouts have occurred which take to be manually reset. Lockout requiring manual reset happens when no flame is detected later purge. Flame relay, 2K, will not energize and the safety switch will heat and lockout the control in about 15 seconds. If in that location is a flame failure during run, terminals 3, iv, and 5 are de-energized; pilot, ignition, and chief valve. If airflow is still proven, an R4795A will begin purge timing and attempt to re-light. It volition make only one try. An R4795D will non recycle. An R4795D will lockout on flame failure during run.
An R4795D differs from the A serial in safe start check. If a flame is detected during pre-purge (2K relay energizes), the purge will stop and rubber lock-out will occur in well-nigh 15 seconds—the fourth dimension it takes the safe switch to estrus upwardly. These 2 things are the only differences between R4795A and D.
The side by side upgrade of the R4795's was the R7795 series. The R7795 series used more solid-state engineering. The R7795's still used plug-in purge timers, ST795A'due south, merely the amplifier is not plug-in or interchangeable. Therefore, an R7795 has to be selected with the right amplifier to match the scanner. R7795A'due south are used with UV detectors and B's are flame rectification. A's and B's are intermittent pilot models. R7795C'southward and D's are interrupted pilot models, the C's for UV detectors, the D's with flame rectification detectors. R7795'due south require a Q795 sub-base of operations. Their operation is the same as the R4795's.
In lite of the RM7800 series, do not upgrade a client from an R4795 to an R7795. Always upgrade to the 7800 series. Honeywell is only keeping the R7795 available due to O.E.Grand. demand. To an O.E.M., the R7795 is less expensive than the 7800 series and OEMs are very, very toll conscious. With the demise of the R4795 serial, the 7800 series will be the service manufacture'south control of selection.
To select an RM7895 system to replace an R4795 organization, some decisions have to be fabricated. All R4795's were intermittent pilot. We tin now choose intermittent pilot, the RM7895A or B, or interrupted airplane pilot, the RM7895C or D. Intermittent pilot ways the pilot is on during the run flow. Interrupted pilot means the pilot is shut off during the run menstruation. All RM7895'southward 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 whatsoever of these tolerances are not met, the 10 2d initiate sequence volition go into a concord condition until the tolerances are met, and if non met the RM7895 volition lock-out in 4 minutes. If, at whatsoever fourth dimension 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 up-past. Stand-by can be whatever length of time. Stand-by simply ways the command is waiting for a call for heat. On a telephone 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 betwixt terminals vi and 7, must shut within the timing of the brusk timing purge cards, ii, 7, or 10 seconds, or within 10 seconds for longer timing purge cards. The purge timing does non commencement to count until the AFS closes. Should the AFS not close within the specified fourth dimension or 10 seconds, whichever is shorter, the command will recycle or lock-out, depending on jumper 3 being intact; recycle or cut; lock-out.
All RM7895'southward have three configuration jumpers. Jumper number 3 is the jumper that governs what happens if there is AFS failure. If the AFS opens at whatever time after information technology 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 command volition lock-out.
All RM7895'southward take iii jumpers that can be cut or left lonely. 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 ane 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 cutting, terminal 10 is powered for only four seconds. Jumper number 2 configures the command to exist 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 exercise non; 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 betwixt terminals 6 and 7. If this circuit is airtight, the RM7895B or D will lockout in 2 minutes. Call up this: In the "quondam 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 trouble. Patently, you tin't do this when dealing with the RM7895B or D.
The block diagram, Fig. six on page 11 of Honeywell's form 65-0086 on the RM7895 has an fault. The "Airflow Interlock" is shown as a closed excursion. It should exist shown equally an open excursion. Another fault is on the top of page iv. For the RM7895B under "Flame Establishing Menstruation" "primary" it says "yep". This should be "no". Under "AFSC", information technology says "no". This should exist "yes".
Fastened is the Gordon Piatt diagram that shows the results of converting from the T3 or T4 timer system to the R4795
Source: https://www.industrialcontrolsonline.com/training/online/refrigeration-pressure-regulators-flow-controls-parts-1-and-2
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