R e f r i g e r a t i o n Pi Pi p i n g
&
Charging
R e s i d e n t i a l A i r Co C o n d i t i o n i n g RR -2 2 R og e r D . Hol de r , C M , MSME
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Piping & Tubing Refrigeration Piping
Refrigeration is the process of moving heat from one location to another by the use of refrigerant in a closed cycle. The piping and tubing system must be designed, installed and maintained to provide proper flow of refrigerant in both liquid and gaseous states. A successful refrigeration system depends on a good piping design and an understanding of the required accessories. The first skill that any refrigeration apprentice mechanic learns is to make a soldered joint. Running pipe is so common a task that its critical importance in system performance is often overlooked.
Tubing Inspection & Leak Check The object of a good visual inspection of system tubing design is to note obvious oil traps. Also look for long vertical suction lengths without p traps and inadequate OD tubing size If the system is known to be leaking or if oil is present around mechanical fittings, solder joints, gaskets or seals, recover the refrigerant and repair the leaks. Pressurize the system with a residual amount of refrigerant and dry nitrogen using the recommended test pressure on data plate. Maximum test pressures should be approximately 150 psi for high-pressure AC/R systems. In chiller applications, controlled hot water or heater blankets will raise pressure adequately for a leak check, which should never exceed 10 psi for low pressure chillers. After a thorough inspection with a good leak detector, apply a deep vacuum to 500 microns. A good triple evacuation, with dry nitrogen and then deep vacuum is the preferred method.
Piping Basic Principles Refrigeration piping involves extremely complex relationships in the flow of refrigerant and oil. Fluid flow is the study of the flow of a gas or a liquid, and the inter-relationship of velocity, pressure, friction, density and the work required to cause the flow. The design of a refrigeration piping system is a continuous series of compromises. It is desirable to have 3
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maximum capacity at minimum cost, as well as proper oil return. Since oil must pass through the compressor cylinders to provide lubrication, a small amount of oil is always circulating with the refrigerant. Oil and refrigerant vapor, however, do not mix readily, and the oil can be properly circulated through the system only if the mass velocity of the refrigerant vapor is great enough to sweep the oil along. To ensure oil circulation adequate velocities of refrigerant must be maintained in the suction and discharge lines, and in the evaporator.
The design of refrigerant piping systems should: •
Ensure proper refrigerant feed to evaporators.
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Provide practical refrigerant line sizes without excessive pressure drop.
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Prevent excessive amounts of lubricating oil from being trapped in any part of the system.
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Protect the compressor at all times from loss of lubricating oil.
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Prevent liquid refrigerant or oil slugs from entering the compressor during operating and
idle time. •
Maintaining a clean and dry system.
Refrigerant Refrigerant Line Velocities Economics, pressure drop, noise, and oil entrapment require establishing feasible design velocities in refrigerant lines. •
Suction line 700 to 4000 fpm
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Discharge line 500 to 3500 fpm
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Condenser drain line 100 fpm or less
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Liquid line 125 to 450 fpm
Higher gas velocities are sometimes found in relatively short suction lines, on comfort air conditioning or other applications where the operating time is only 2000 to 4000 hours per year and where low initial cost of the system may be more significant than low operating cost. In the Industrial refrigeration applications where equipment runs continuously, should be designed with low refrigerant velocities for most efficient compressor performance and low equipment operating cost. Care must be taken that the velocities is not to low that oil is taped in the refrigeration lines.
Liquid line from condenser to receivers should be sized for 100 fpm or less to ensure positive 4
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gravity flow without incurring backup of liquid flow. Liquid lines from receivers to evaporator should be sized to maintain velocities below 300 fpm, thus minimizing or preventing liquid hammer when solenoids or other electrically operated valves are used.
Refrigerant Line Sizing In sizing refrigerant lines, cost considerations favor keeping line size as small as possible. However, suction and discharge line pressure drops cause loss of compressor capacity and increased power use. Excessive liquid line pressure drops can cause the liquid refrigerant to flash, resulting in faulty expansion valve operation. Refrigeration systems are designed so that friction pressure losses do not exceed a pressure differential equivalent to a corresponding change in the saturation boiling temperature. The primary measure for determining pressure drop is a change in saturation temperature. Pressure drop in refrigerant lines causes a reduction in system efficiency. Correct sizing must be based on minimizing cost and maximizing efficiency. Pressure drop calculations are determined as normal pressure loss associated with a change in saturation temperature of the refrigerant. Typically, the refrigeration system will be sized for pressure losses of 2ºF or less for each segment of the discharge, suction, and liquid lines. An HFC refrigerant liquid line is sized for pressure losses of 1ºF or less.
Discharge lines should be designed to: •
Avoid trapping oil at part-load operation.
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Prevent condensed refrigerant and oil from draining back to the head of the compressor.
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Have carefully selected connections from a common line to multiple compressors.
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Avoid developing excessive noise or vibration from hot-gas pulsation, compressor
vibration, or both.
When sizing discharge lines, considerations similar to those applied to the suction line are observed. Pressure loss in discharge lines increases the required compressor power per unit of refrigeration and decreases the compressor capacity by increasing the compression ratio. While the discharge line pressure drop is not as critical as that of the suction line, the accepted maximum values are 4 psi for R-12 and 6 psi for R-22. The same minimum gas velocities of 500 feet per minute in horizontal runs and 1000 feet per minute in vertical runs with upward gas flow are observed. The maximum acceptable gas velocity, based on noise considerations, is 4000 feet per minute.
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Condenser Drain Line The line between a condenser (not providing liquid subcooling) and a liquid receiver, when such an arrangement is used, must be carefully sized. While it is almost impossible to oversize such a line, under sizing is to be avoided. An undersized line can restrict the flow of refrigerant to the extent that some of it is held in the condenser. If some of the condenser surface is flooded, the capacity is reduced. These causes the head pressure to rise and decrease the overall system capacity. At the same time, the power to drive the compressor rises.
There are a few points that the piping designer should keep in mind. •
Condenser drain line velocity should be 100 fpm or less.
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The distance from the condenser to receiver should be as short as possible.
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The condenser must be located above the receiver.
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If the system is equipped with an air-cooled condenser and a liquid receiver, it is good
practice to locate the receiver within the building. Some means should be provided to isolate the receiver from the condenser during cold weather shutdown, such as a combination check and relief valve.
Receiver Refrigerant receivers are vessels used to store excess refrigerant while still allowing circulation throughout the system. Receivers perform the following functions:
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Provide pumpdown storage capacity when another part of the system must be serviced or
the system must be shut down for an extended time. In some water-cooled condenser systems, the condenser also serves as a receiver if the total refrigerant charge does not exceed its storage capacity. •
Handle the excess refrigerant charge that occurs with air-cooled condensers using the
flooding type condensing pressure control. •
Accommodate a fluctuating charge in the low side.
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Drain the condenser of liquid.
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Maintain an adequate affective condensing surface on system where the operating charge in
the evaporator and or condenser varies for different loading conditions. When an evaporator is fed with a thermal expansion valve, hand expansion valve, or low-pressure float, the operating charge in the evaporator varies considerably depending on the loading. During low load, the evaporator requires a larger charge since the boiling is not as intense. When the load 6
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increases, the operating charge in the evaporator decreases, and the receiver must store excess refrigerant. •
Hold the full charge of the idle circuit on systems with multicircuit evaporators that shut off
the liquid supply to one or more circuits during reduced load.
Receiver Design Considerations •
Receiver should be close to the condenser.
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If there is any doubt about the line size, use the larger of the sizes.
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Always adhere to the minimum vertical dimension required to overcome friction.
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Install a pressure relief device on top of each receiver and on condenser.
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The surge receiver pressure relief device is piped together with condensers.
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Size the receiver to hold 40 to 125% of refrigerant charge depending on system load
variance.
When a through type receiver is used, the liquid must always flow from the condenser to the receiver. The piping must provide free flow of liquid from the condenser to the receiver by equalizing the pressure between the two. If a vent is not used, the piping between condenser and receiver is sized so that liquid flows in one direction and gas flows in the opposite direction. Sizing the condensate drain line for 100 fpm liquid velocity is usually adequate to attain this flow. Piping should slop at least 0.25 in/ft and eliminate any natural liquid traps. The condensate drain line should be sized so that the velocity does not exceed 100 fpm.
Please consult the manufacturer’s literature for making receiver capacity comparisons when changing refrigerants.
Liquid Lines Pressure drop should not be so large as to cause gas formation in the liquid line or insufficient liquid pressure at the liquid feed device. Systems are normally designed so that the pressure drop in the liquid line, due to friction, is not greater than that corresponding to about a 1º to 2ºF change in saturation temperature.
Pressure drop (in psig) for a change of 1ºF saturation at 100ºF condensing pressure: (R-508B is at 10ºF): 7
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Liquid subcooling is the only method of overcoming the liquid line pressure losses in order to guarantee liquid at the expansion device in the evaporator. If the subcooling is insufficient, flashing will occur within the liquid line and degrade the efficiency of the system. Friction pressure drops in the liquid line are caused by accessories such as solenoid valves, filter driers, and hand valves, as well as by the piping and fittings between the receiver outlet and the refrigerant feed device at the evaporator. Liquid line risers are also a source of pressure loss. The loss due to a riser is approximately 0.556 psi per foot of liquid lift. The total loss is the sum of all friction losses plus the pressure loss from liquid risers. Refrigeration systems that have no liquid risers and have the evaporator below the condenser and/or receiver benefit from a gain in pressure due to liquid weight. They can thus tolerate larger friction losses without flashing. When flashing occurs, the overall efficiency is reduced and the system may malfunction. The only way to reduce the effect of pressure loses and friction is by subcooling the refrigerant.
Suction Lines
Suction lines are more critical than liquid and discharge line from a design and construction standpoint. They should be designed to: •
Provide a minimum pressure drop at full load.
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Return oil from the evaporator to the compressor under minimum load conditions.
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Prevent oil from draining from an active evaporator into an idle one.
A pressure drop in the suction line reduces a system’s capacity by forcing the compressor to operate at a lower suction pressure, in order to maintain a desired evaporating temperature in the coil. As the suction pressure is decreased, each pound of refrigerant returning to the compressor occupies a greater volume, and the weight of the refrigerant pumped by the compressor decreases. For example, a typical low 8
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temperature R-502 compressor at -40ºF evaporating temperature will lose almost 6% of its rated capacity for each 1 psi suction line pressure drop. Normally accepted design practice is to use a suction line pressure drop equivalent to a 2ºF change in saturation temperature.
Of equal importance in sizing the suction line is the necessity of maintaining adequate velocities to properly return oil to the compressor. Studies have shown that oil is most viscous in a system after the suction vapor has warmed up a few degrees from the evaporating temperature, so that the oil is no longer saturated with the refrigerant. This condition occurs in the suction line after the refrigerant vapor has left the 9
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evaporator. Movement of the oil through suction lines is dependent on both the mass and velocity of the suction vapor. As the mass or density decreases, higher velocities are required to force the oil along.
Nominal minimum velocities of 700 fpm in horizontal suction lines and 1500 fpm in vertical suction lines have been recommended and used successfully for many years as suction line sizing design standards. Use of the one nominal velocity provided a simple and convenient means of checking velocities. However, tests have shown that in vertical risers the oil tends to crawl up the inner surface of the tubing, and the larger the tubing, the greater velocity required in the center of the tubing to maintain tube surface velocities that will carry the oil. The exact velocity required in the vertical line is dependent on both the evaporating temperature and the line size, and under varying conditions, the specific velocity required might be either greater or less than 1500 fpm. An HFC refrigerant, however, is designed for 1500 fpm or greater.
Always pitch vapor lines in the direction of flow, 1/2 inch per ten-foot of suction line. “P” traps for uphill oil return should be used after the first 6- foot and every 12-foot thereafter. It is good practice to use an inverted trap just before entering the compressor.
Double Risers On systems equipped with capacity control compressors, or where tandem or multiple compressors are used with one or more compressor cycled off for capacity control, a single suction line riser may result in either unacceptably high or low gas velocities. A line properly sized for light load conditions may have too high a pressure drop at maximum load, and if the line is sized based on full load condition, then velocities may not be adequate conditions to move oil through the tubing at light load. On air conditioning applications where somewhat higher pressure drops at maximum load conditions can be tolerated without any major penalty in overall system performance, it is usually preferable to accept the additional pressure drop imposed by a single vertical riser. However, on medium or low temperature applications where pressure 10
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drop is more critical and where separate risers from individual evaporators are not possible, a double riser may be necessary to avoid an excessive loss of capacity.
A typical double riser has a small and large riser. The two should be sized so that the total cross-sectional area is equivalent to the cross-section area of a single riser that would have both satisfactory gas velocity and acceptable pressure drop at maximum load conditions. The larger is trapped and the smaller line must be sized to provide adequate velocities and acceptable pressure drop when the entire minimum load is carried in the smaller riser.
Another method of suction oil return is the use of a double riser, as shown in Figure 1. Oil return is accomplished with this method at minimum loads. In addition, excessive pressure drop at full load is avoided. The small riser “A” is sized to return oil under minimum capacity conditions. Riser “B” which, may be larger, is sized so pressure drop through both risers during full load conditions is adequate. Traps with minimum oil holding capacity are recommended.
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Suction Line Double Risers Sizing Single Riser Size Double Riser Sizes 2/3, 1/3 1 1/8 = .83 7/8 and 3/4 = .83 1 3/8 = 1.26 1 1/8 and 7/8 = 1.31 1 5/8 = 1.78 1 3/8 and 7/8 = 1.74 2 1/8 = 3.10 1 5/8 and 1 3/8 = 3.04 2 5/8 = 4.77 2 1/8 and 1 5/8 = 4.88 3 1/8 = 6.81 2 5/8 and 1 5/8 = 6.55
Defrost Gas Supply Lines Sizing refrigeration lines to supply defrost gas to one or more evaporator is an estimate at best. The parameters associated with sizing the defrost gas lines are related to allowable pressure drop and refrigerant flow rate during defrost. Design professionals typically use approximately two times the evaporator load for effective refrigerant flow rate to determine line-sizing requirements. The pressure drop is not as critical during the defrost cycle, and many engineers have used velocity as criterion for determining line size. The effective condensing temperature and average temperature of the gas must be determined. The velocity determined at saturated conditions will give a conservative line size. It is recommended that initial sizing be based on twice the evaporator flow rate and that velocities from 1000 to 2000 fpm be used for determining the defrost gas supply line size.
Refrigerant Line Capacity Tables & Equivalent Lengths Of Valves & Fittings Refrigerant line capacity tables are based on unit pressure drop per 100 ft length of straight pipe or per combination of straight pipe, fitting, and valves with friction drop equivalent to a 100 ft length of straight pipe. Generally, pressure drop through valves and fittings is determined by establishing the equivalent straight length of pipe of the same size with the same friction drop. Alternately, one rule of thumb is to add 50% to the calculated pipe length to account for pressure drops from fittings and valves.
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Expansion & Contraction Temperature change will expand and contract all refrigeration piping material. Techniques must allow for expansion and contraction changes to prevent stresses that may buckle, bend or rupture the refrigerant piping. he two common methods of taking care of expansion and contraction in copper piping are the “expansion loops” or “pipe offsets”. During the installation of the line, care must be taken that the line maintains a perfect alignment.
On average, copper’s coefficient of expansion is 0.0000104 inch per inch per degree Fahrenheit. Thus, expansion of copper is 1.25 inch per 100 feet per 100ºF change. For example, a copper compressor discharge line of 75 feet long at 225ºF could have a temperature change of 150ºF in a 70ºF room. Therefore, 1.25 X 1.55 (temperature change per 100ºF) X .75 (length per 100 feet) will equal 1.453 inches of expansion. The 75 foot long line would now be approximately 75 feet, 1-1/2 inches long.
Location & Arrangement of Piping Refrigerant lines should be as short and direct as possible to minimize tubing and refrigerant requirements and pressure drops. Plan piping for a minimum number of joints, using as few elbows and other fitting as possible, but provide sufficient flexibility to absorb compressor vibration and stresses due to thermal expansion and contraction.
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Arrange refrigerant piping so that normal inspection and servicing of the compressor and other equipment is not hindered. Do not obstruct the view of the oil level sight glass, or run piping so that it interferes with the removal of compressor cylinder head, end bells, access plates, or any internal parts. Suction line piping to the compressor should be arranged so that it will not interfere with removal of the compressor for servicing.
You must provide adequate clearance for insulation installation between the piping, wall, and hangers. Use sleeves that are sized to permit installation of both pipe and insulation through floor, walls, or ceilings. Set the sleeves prior to pouring of concrete or erection of brickwork. Piping must not interfere with passages or obstruct headroom, windows, or doors. Refer to ASHRAE Standard 15, Safety Code for Mechanical Refrigeration, and other governing local codes for restrictions that may apply.
Protection Against Damage To Piping Protection against damage is necessary, particularly for small lines, which have a false appearance of strength. Where traffic is heavy, provide protection against impact from pedestrian and motorized traffic.
Piping Insulation All piping joints and fittings should be thoroughly leak tested before insulation is sealed. Suction lines should be insulated to prevent sweating and heat gain. Insulation covering lines on which moisture can condense or lines subjected to outside conditions must be vapor sealed to prevent any moisture travel through the insulation, or condensation in the insulation. Although the liquid line ordinarily does not require insulation, the suction and liquid lines can be insulated together. The liquid line should be insulated to minimize heat gain if it passes through an area of higher temperature. Hot gas discharge lines usually are not insulated, however, they should be insulated if the heat dissipated is objectionable or if necessary to prevent injury from high temperature surfaces.
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Vibration & Noise in Piping Two undesirable effects of vibration of refrigerant piping are: 1) physical damage to the piping, which may result in the breaking of brazed joints and consequent loss of charge; and 2) transmission of noise through the piping itself and through building construction with which the piping may come into direct physical contact. Both can be eliminated or minimized by proper piping design and installation.
Using Manufacturer’s Pressure Drop & Velocity Charts
Always size for pressure drop first, then velocity. On the top right of the pressure drop chart in Figure 8-3, you will find tons of refrigeration or cooling capacity calibrated in Btu per hour up to 1 ton, and in tons of cooling from 1 ton to 100 tons. You start the sizing procedure by drawing a straight line from your system’s designated capacity through the diagonal lines on the right side of the chart.
The diagonal lines represent: •
The “evaporator temperature”, used to size the suction line.
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The “discharge line” temperature, used to size the hot gas discharge.
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The “liquid lines” diagonal line, used to size the liquid line.
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The diagonal lines on the left of the chart represent the actual tubing sizes that will be derived by your calculation. Starting at the bottom line, representing 3/8" OD Type L copper tubing and increasing in size up through 6-1/8. Draw a horizontal line from each intersection of tonnage and each of the three diagonal lines, horizontally across through the tubing sizes.
On the bottom left of the chart you will find the “pressure drop graph.” The three horizontal lines represent condenser coil temperature applications. The curved diagonal lines are pressure drop in psi per 100 ft. When you have determined necessary condenser coil temperature follow the horizontal line to the required pressure drop. Draw a line straight up until it intersects with the horizontal line used to determine tubing size. If it falls in between two sizes then your size is the tubing size up and to the left. For example, let’s size for a 6-ton R-134a medium temperature walkin refrigerator using the chart in your book:
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First, find 6 tons on the top right of your chart and draw a vertical line straight down
through all of the diagonal lines.
Medium temperature requires a 20°F evaporator so draw a line from the diagonal line designated 20°F horizontally all the way across the chart.
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Do the same from the diagonal line labeled “Discharge Line.”
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And again from the diagonal line labeled “Liquid Line.”
The suction line pressure drop maximum for medium temperature is 1-1/2 psi so the suction line size will be 1-5/8". We require a 3psi pressure drop for our liquid line so the line size is 5/8". The hot gas discharge line is a bit more forgiving, 10 psi or less, so 1-1/8" will work adequately. The velocity chart is very similar to the pressure drop chart and is used in the same way. The idea is to confirm the sizes you found on the pressure chart by cross checking with the applicable velocities on the velocity chart. If you plot the same temperature variables on the velocity chart, you’ll find the sizes chosen will fall between minimum and maximum velocities recommended for each refrigeration line. There are exceptions and sometimes economical compromises on many close-coupled and field fabricated systems. It is these exceptions that may need careful consideration before retrofit.
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Charging Residential Air Conditioning R-22 Different types of metering devices have different ways of charging. A Thermostatic Expansion Valve (TXV) is charged to the subcooling of the liquid line leaving the condenser. A fixed orifice is charged to the superheat of the suction line leaving the evaporator. To under stand why this is, it requires an understanding of the physical properties of the refrigeration cycle. The four main components of the refrigeration cycle include: •
Compressor
•
Condenser
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Metering Devices
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Evaporator
These four components are divided into sections and explained in depth as follows.
Compressor The Compressor compresses a low-pressure superheated gas into a high-pressure superheated gas.
If the suction gas is not superheated, the compressor can be
damaged. The compressor pulls the refrigerant out of the evaporator and pushes it though a condenser. The act of compression is performed by any one of the following six types of compressors: a reciprocating piston, rotary, scroll, screw, centrifugal, and sonic compressors. Of the six, the reciprocating and scroll compressors are the two most frequently found in a residential air conditioning system.
The mass flow rate produced by a compressor is equal to the mass of the suction gas pulled in by the compressor. The compressor’s out put is equally only to its intake because the mass flow must be equal. The process of compression, through mass flow, raises the temperature and pressure of the refrigerant.
The result of the
temperature increase is superheat. Pressure and temperature of the refrigerant must be higher than the condensing temperature. 33
The refrigerant temperature must be
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higher so heat can flow into the condensing medium.
This process explains the
necessary relationship between the increased pressure and the rise in temperature. If the pressure and temperature is not increased through compression, there is no heat transferred from the refrigerant to the condensing medium. The compressor has a maximum inlet temperature of about 70
degrees and outlet of about 225
degrees.
Inlet refrigerant gas cools the compressor motor.
Desuperheating (heat leaving the refrigerant gas) of refrigerant begins as it is discharged. From a compressor and pushed into a condenser.
Condenser The condenser removes heat and changes a high-pressure vapor into a high-pressure liquid.
As the superheated (high-pressure) gas is pushed into the condenser, it is
desuperheated, that is the temperature is reduced to saturated pressure-temperature.
The refrigerant does not start to change state until the temperature reaches saturated pressure-temperature.
The only variable that can change the temperature is a
pressure change. (See table 1) At saturation pressure-temperature point, the change of state becomes latent heat (invisible or hidden heat). Latent heat is a lack of rise or fall of temperature during a change of state (saturation). When the temperature does not rise or fall it is at saturation and the change of state process begins. Refrigerant continues to change state at one pressure-temperature. The only variable that can change a temperature is a pressure change.
If a temperature change occurs a
pressure change occurs. If a pressure change occurs a temperature change occurs. At the change of state the refrigerant liquid and vapor are at the same temperature. This is defined as equilibrium contact. The temperatures of the liquid and vapors will stay the same until the temperature of the refrigerant starts to drop. Temperature of the refrigerant start to drop once 98% to 99% of the refrigerant becomes a liquid. This is called subcooling.
Subcooling is a temperature below saturated pressure-
temperature. (See table 1) Subcooling is a measurement of how much liquid is in the condenser. In air conditioning, it is important to measure subcooling because the longer the liquid stays in the condenser, the greater the sensible (visible) heat loss. 34
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Low subcooling means that a condenser is empty.
High subcooling means that a
condenser is full. Over filling a system increases, pressure due to the liquid filling of a condenser that shows up as high subcooling.
To move the refrigerant from
condenser to the liquid line, it must be pushed down the liquid line to a metering device.
If a pressure drop occurs in the liquid line and the refrigerant has no
subcooling, the refrigerant will start to re-vaporize (change state from a liquid to a vapor) before reaching the metering devise.
Refrigerant –22 Liquid line Saturated temperate - Temperature = subcooling 200 psig = 101 degrees -
96 degrees
= 5 degrees
210 psig = 105 degrees -
90 degrees
= 15 degrees
240 psig = 114 degrees -
98 degrees
= 16 degrees
(Table 1)
Metering Devices A metering device is a pressure drop point, which has two jobs: 1. Holds refrigerant back in a condense; and 2. Feeds refrigerant into the evaporator.
When high-pressure liquid enters a metering device, pressure starts to drop, as the temperature remains the same until it reaches saturation pressure-temperature.
At
this time both the pressure and temperature continues to drop to evaporator pressuretemperature. (See table 2) Low-pressure liquid that is leaving the metering device is boiling at saturated pressure-temperature. The process of a refrigerant changing its state (from a liquid to a vapor) in the metering device is called flash gas. Flash gas is what cools the refrigerant liquid in the metering device. A system with no subcooling 35
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has more gas that is flashed and less capacity.
Evaporators The refrigerant enters the evaporator as a boiling low-pressure liquid at saturated pressure-temperature. It continues to boil at one temperature as long as the pressure remains the same. If there is not a pressure change in the evaporator, there will not be a temperature change in the refrigerant changing state.
At saturation, refrigerant
absorbs latent heat, which is a change of state heat. The refrigerant changes state at one temperature (for any one pressure) from the beginning of the evaporator until the entire liquid refrigerant has become a vapor. The only variable that can change a temperature is a pressure change. If a temperature change occurs a pressure change occurs.
In latent heat, the liquid and vapor are at the same temperature due to
equilibrium contact.
When heat is added to the gas, past saturation pressure-
temperature, it is called superheat. (See Table 2) Superheat is an indication of how full the evaporator is of liquid refrigerant. High superheat means the evaporator is empty. Low superheat means the evaporator is full. refrigerant can still be boiling with 2
There have been reports that liquid
degrees of superheat. Superheat should never
be observed below 4 degrees or a compressor failure may occur. The superheat gas is pulled into the compressor were it starts the cycle again.
Refrigerant –22 suction line Saturated temperate - Temperature = superheat 58 psig = 32 degrees -
44 degrees
= 12 degrees
64 psig = 37 degrees -
47 degrees
= 10 degrees
70 psig = 41 degrees -
50 degrees
=
(Table 2)
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9 degrees
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Charging Methods Before charging of a residential air conditioning system, two temperatures must be recorded: 1. Condensing air inlet dry bulb temperature. 2. Evaporator air inlet wet bulb temperature. Wet bulb temperature is a measurement of the heat contained within air. Air may have many different wet bulb temperatures for one dry bulb temperature, depending on relative humidity of the air.
Different types of metering devices have different ways of charging.
Thermostatic Expansion Valve R-22 A/C with a Thermostatic Expansion Valve (TXV) is charged to the subcooling of the liquid line leaving the condenser because the superheat is fixed. The superheat is fixed at 8 to 12 degrees in most residential air conditioning systems. Subcooling is the amount of liquid held back in the condenser. This allows the liquid to give up more heat, below saturated pressure- temperature. For every one degree of subcooling at the same condensing pressure, capacity will increase .5 percent.
Increasing
subcooling with an increase of discharge pressure and compression ratio, decrease capacity. Add 5 degrees of subcooling for every 30 feet of liquid line lift.
To measure subcooling: 1. Obtain refrigerant saturation pressure-temperature. Take a pressure reading of the liquid line leaving the condenser. Refrigerant saturation temperature is the pressuretemperature when the refrigerant is turning from a high-pressure vapor into a highpressure liquid (giving up heat). At saturation pressure-temperature, both liquid and vapor are at the same temperature. 2. Convert pressure to temperature with a pressure temperature chart. 3. Take a temperature reading at the leaving liquid line of the condenser. 4. Compare both, the saturated temperature and leaving liquid line temperature. Subtracting one from the other, the difference is the amount the refrigerant has cooled past saturated temperature. This is subcooling. (See table 1)
This four-step procedure is known as subcooling. Manufacturers should be able to 37
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identify the amounts of subcooling they have designed into a system. A low charge will give a low subcooling.
An overcharge will give a high subcooling along with a
high compression ratio. Do not worry about a few bubbles in the sight glass. Sight glasses will not always be clear with a full charge. The zeotropes refrigerant group is known for their fractionation. It is possible to never have a clear sight glass. To determine what the subcooling should be in a system (see table 3).
Subcooling for A/C with TXV R-22 Evaporator Inlet Air Temperature Fahrenheit Wet Bulb Outside Air Temperature DB 75 80 85 90 95 100 105 110 115
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65
67
69
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73
25 24 23 22 21 20 19 17
24 23 22 21 20 19 18 16
23 22 21 20 19 18 17 15
22 21 20 19 18 17 16 13
21 20 19 18 17 15 14 12
20 19 18 16 15 13 12 10
19 18 17 15 13 12 10 8
18 17 16 14 12 10 8 6
17 15 14 12 10 8 6 4
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10
8
6
4
2
(Table 3) + --2 degrees
Fixed Orifice R-22 A/C with a fixed orifice is charged to the superheat of the suction line leaving the evaporator. Superheat is the gas temperature above the saturated temperature.
Superheat can be split into two types of heat: 1. Superheat of the evaporators; and 2. Total superheat entering the compressor.
The evaporators superheat must be figured at the evaporator outlet not at the compressor inlet. Total superheat is figured at the compressor inlet.
To measure evaporator superheat: 1. Take a pressure reading of the suction line-leaving evaporator to get refrigerant saturation pressure-temperature. Refrigerant saturation temperature is the pressure38
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temperature when the refrigerant is turning from a low-pressure liquid to a lowpressure vapor (absorbing heat). At saturation pressure-temperature, both liquid and vapor are at the same temperature. 2. Convert pressure to temperature with a pressure temperature chart. If reading is obtained at the compressor, not at the evaporator leaving line, you may have to add a few pounds of pressure due to pressure drop in the suction line. 3. Take a temperature reading at the leaving suction line of the evaporator. 4. Compare both, the saturated temperature and the leaving suction line temperature. Subtracting one from the other, the difference is the amount the refrigerant gas has heated past saturated temperature. This is superheat. (See table 2)
This four-step procedure is known as superheat. Manufacturers should be able to identify the amounts of superheat they have designed into a system. A low charge will give a high superheat. An overcharge will give a low superheat along with a higher compression ratio. To determine what superheat in a system should be, see (table 4).
Superheat for A/C with fixed Orifice R-22 Evaporator Inlet Air Temperature Fahrenheit Wet Bulb 54 Outside Air Temperature DB 60 13 65 11 70 8 75 5 80 85 90 95 100 105 110 115
56
58
60
62
64
66
68
70
72
74
17 13 11 7 4
18 15 12 10 6 4
20 17 14 12 8 6 4
24 18 16 14 12 8 6 4
26 22 18 16 14 12 9 7 5 4
28 25 22 18 16 14 12 11 8 6 5
30 28 25 23 18 17 15 13 11 8 7 5
33 30 28 26 23 20 18 16 14 12 11 8
36 33 30 28 27 25 22 20 18 15 14 13
39 36 33 30 28 27 25 23 20 19 18 16
(Table 4) +-- 2 degrees
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Total Superheat Method Some residential air-conditioning system with fixed orifice may be charged by the total superheat method. Various equipment manufacturers furnish charts with their units that explain the proper procedures to the installing or servicing technician.
This method, similar to evaporator superheat method, is effective only when the indoor conditions are within 2F
of desired indoor comfort conditions and the suction
pressure and temperature is stabilized.
To measure total superheat: 1. Read and record the outdoor ambient air-dry bulb temperature entering the condenser.
2. Read and record suction line pressure and temperature at the suction service valve or service port at compressor.
3. From Table 5, the reading at the intersection of vapor pressure and outdoor ambient temperature should coincide with the actual vapor line temperature.
4. If the vapor line temperature is not the same, adjust the refrigerant charge. Adding R-22 will raise suction pressure and lower suction line temperature. Removing R-22 will lower suction pressure and raise suction line temperature.
Caution: If adding R-22 increases both suction pressure and temperature, the unit is overcharged.
This method is very useful when performing preventive maintenance or corrective 40
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service on residential air conditioning.
Remember to always refer to the
manufacturer’s recommendations whenever possible.
Vapor pressure at service valve 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 Outdoor T
Vapor line T at compressor F
100+ 100 95 90 85 80 75 70 65
43 45 46 47 49 50 51 53 54 55 44 45 47 48 49 51 52 53 55 56 57 45 47 48 50 51 52 54 55 56 58 59 60 49 51 52 54 55 56 58 59 60 62 52 53 55 56 58 59 61 62 63 53 55 56 58 59 61 62 63 65 55 56 58 59 61 62 64 65 66 55 57 58 60 61 63 64 66 67 57 58 60 61 63 64 66 67 69
Refrigeration The use of sight glass for charging is common in refrigeration. It is better to charge a system first by measuring the operating condition (discharge and suction pressures, suction line temperature, compressor amps, super heat, subcooling and coils temperature deferential) before using the liquid line sight glass. If the sight glass is close to the exit of the condenser or if there is very little subcooling at the sight glass, bubbles may be present even when the system is properly charged. If a system is charged to full sight glass, overcharging may be the result, decreasing efficiency.
Note: Follow the manufacturer recommendation for superheat and subcooling. 41
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A Thermostatic Expansion Valve (TXV) is designed to maintain a constant superheat. Over charging a TXV will rise subcooling, increases system pressures, and decreases system efficiency.
Under charging a TXV will decrease subcooling, increases
superheat, decrease system capacity, and lower refrigerant velocity leaving oil in the evaporator. An Automatic Expansion Valve (AXV) is a constant evaporator pressure valve and not normally used in A/C. A fixed orifices is the simplest metering devise made and the most critical to charge.
Over charging fixed orifices will lower
superheat, increases pressures, decrease efficiency, and flood the compressor with liquid refrigerant. Under charge, the fixed orifices will raise superheat, lower pressure, lower capacity, and lower refrigerant velocity leaving oil in the evaporator. Always refer to the manufacturer recommendations on charging fixed orifices.
The process of charging to superheat and subcooling improves an air conditioning system’s efficiency, capacity and lessens equipment failures.
Always let system
stabilize (10 to 20 minutes) after adjusting the charge, this takes time but improves efficiency and capacity.
Remember when changing refrigerants all superheat and subcooling adjustments have to be checked and recorded. Baselining.
The procedure of recording adjustments is called
This procedure not only saves time, money, and aggravation but it is a
sign of a professional.
Roger D. Holder, CM, BSME, Is the owner of R D Holder Eng in Bakersfield CA, 661665-8893,
He also is a Refrigeration and Air Conditioning specialist at National
Technical Transfer Inc., P. O. Box 4558 Englewood, CO 80155 (800) 922-2820
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