Successful refrigeration systems depend on good piping design and knowing the required accessories
Refrigeration is the process of moving heat from one location to another by the use of refrigerant in a closed cycle. The refrigeration cycle consists of oil management; gas and liquid separation; subcooling, superheating, and piping of refrigerant liquid and gas; and two-phase flow. Applications include air conditioning, commercial refrigeration and industrial refrigeration.
There are many desired characteristics of a refrigeration system, which may include:
-Year-round operation, regardless of outdoor ambient conditions.
-Possible wide load variations during short periods without serious disruption of the required temperature levels.
-Frost control for continuous performance applications.
-Oil management for different refrigerants under varying loads and temperatures.
-System efficiency, maintainability and operating simplicity.
A successful refrigeration system depends on a good piping design and an understanding of the required accessories. Probably 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 the proper performance of a system is often overlooked. It's not uncommon for a technician to add gallons of oil, and it seemingly disappears without a trace. It is, of course, lying in the bottom of tubing in the system, usually in the evaporator or suction line. This oil must be removed when it returns to the compressor after operating conditions change. It's important to note that the same oil must be removed or the compressor will fail.
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 any fluid, whether it's a gas or liquid, and the inter-relationship of velocity, pressure, fiction, density, viscosity and the work required to cause the flow. The design of a refrigeration piping system is a continuous series of compromises. It's desirable to have maximum capacity at minimum cost and 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 lines, discharge lines and evaporator.
The design of a refrigerant piping system should:
-Ensure proper refrigerant feed to evaporators.
-Provide practical refrigerant line sizes without excessive pressure drop.
-Prevent excessive amounts of lubricating oil from being trapped in any part of the system.
-Protect the compressor at all times from loss of lubricating oil.
-Prevent liquid refrigerant or oil slugs from entering the compressor during operating and idle time.
-Maintain a clean and dry system.
Refrigerant Line Velocities
Elements that establish feasible design velocities in refrigerant lines include economics, pressure drop, noise and oil entrapment. The velocities of various lines are:
Suction line .......... 700 to 4,000 fpm
Discharge line ........ 500 to 3,500 fpm
Liquid line ........... 125 to 450 fpm
Higher gas velocities sometimes are found in relatively short suction lines on comfort air conditioning or other applications where the operating time is only 2,000 to 4,000 hours per year and where low initial cost of the system may be more significant than low operating cost. Industrial or commercial refrigeration applications, where equipment runs almost continuously, should be designed with low refrigerant velocities for efficient compressor performance and low equipment operating cost.
The liquid line from the condenser to receivers should be sized for 100 fpm or less to ensure positive gravity flow without incurring backup of liquid flow. Liquid lines from receivers to the 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.
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 usage. 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 don't exceed a pressure differential equivalent to a corresponding change in the saturation boiling temperature. The primary measure for determining pressure change 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 2° F or less for each segment of the discharge, suction and liquid lines.
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's almost impossible to oversize such a line, under sizing must be avoided. An undersized line can restrict the flow of refrigerant to the extent that some of it's held in the condenser. If some of the condenser surface is flooded, the capacity is reduced. This causes the head pressure to rise, decreasing 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:
-The distance between the condenser and receiver should be kept as short as possible.
-The condenser must be located above the receiver.
-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 positive means should be provided to isolate the receiver from the condenser during cold weather shutdown, such as a combination check and relief valve.
Receivers
Refrigerant receivers are vessels used to store excess refrigerant circulated throughout the system. Receivers perform the following functions and are designed to:
-Provide pump down 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 and drain the condenser of liquid to maintain an adequate effective condensing surface on systems 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 increases, the operating charge in the evaporator decreases and the receiver must store excess refrigerant.
-Hold the full charge of the idle circuit on the system with multicircuit evaporators that shut off the liquid supply to one or more circuits during reduced load, and pump out the idle circuit.
-The receiver must be kept close to the condenser.
-If there is any doubt about the line size, the larger of the two lines' sizes should be used.
-The minimum vertical dimension required to overcome friction always should be adhered to.
-A pressure relief device on top of each receiver and on the condenser.
-The surge receiver pressure-relief device is piped together with the condenser.
-Size to 80 to 125 percent of the refrigerant charge.
When a through-type receiver is used, the liquid must always flow from the condenser to the receiver. The receiver and its associated piping provide free flow of liquid from the condenser to the receiver by equalizing the pressure between the two so that the receiver cannot build up a higher pressure than the condenser. If a vent isn't used, the piping between the condenser and receiver is sized so that liquid flows in one direction and gas flows in the opposite direction. Sizing the condensate line for 100-fpm liquid velocity is usually adequate to attain this flow. Piping should slope at least 0.25 inches per feet and eliminate any natural liquid traps. The condensate line should be sized so that the velocity does not exceed 150 fpm.
Liquid Lines
Pressure drop should not be so large as to cause gas formation in the liquid line, insufficient liquid pressure at the liquid feed device, or both. 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 1° to 2° F change in saturation temperature, as shown in Table 1.
Liquid subcooling is the only method of overcoming the liquid line pressure loss 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 actual piping and fittings between the receiver outlet and the refrigerant feed device at the evaporator.
Liquid line risers are a source of pressure loss and add to the loss of the liquid line. The loss due to a riser is approximately 0.5-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/receiver benefit from a gain in pressure due to liquid weight and can tolerate larger friction losses without flashing.
Regardless of the routing of the liquid line when flashing takes place, the overall efficiency is reduced and the system may malfunction. The only way to reduce the effect of pressure loss and friction is by subcooling the refrigerant.
Suction Lines
Suction lines are more critical than liquid and discharge lines from a design and construction standpoint.
A refrigerant line should be sized to:
-Provide a minimum pressure drop at full load.
-Return oil from the evaporator to the compressor under minimum load conditions.
-Prevent oil from draining from an active evaporator into an idle one.
A pressure drop in the suction line reduces a system's capacity because it forces the compressor to operate at a lower suction pressure 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-temperature R-502 compressor at -40° F evaporating temperature will lose almost 6 percent of its rated capacity for each 1-psi suction line pressure drop, as shown in Table 2. A normally accepted design practice is to use a suction line pressure drop equivalent to a 2° F change in saturation temperature.
Of equal importance when sizing a suction line is the necessity of maintaining adequate velocities so oil is properly returned to the compressor. Studies show 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 evaporator.
Oil movement through the 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 1,500 fpm in vertical suction lines have been recommended and successfully used 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 vertical lines is dependent on both the evaporating temperature and the line size. Under varying conditions, the specific velocity required might be greater or less than 1,500 fpm.
Double Risers
On systems equipped with capacity control compressors, or where tandem or multiple compressors are used with one or more compressors 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 on the basis of full load conditions, then velocities may not be adequate at light load conditions to move oil through the tubing.
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's usually preferable to accept the additional pressure drop imposed by a single vertical riser. But on medium- or low-temperature applications where pressure drop is more critical and where separate risers from individual evaporators are not desirable or possible, a double riser may be necessary to avoid an excessive loss of capacity.
A typical double riser has a smaller and larger riser. The two lines should be sized so that the total cross-sectional area is equivalent to the cross-sectional area of a single riser that would have both a satisfactory gas velocity and acceptable pressure drop at maximum load conditions. The larger riser 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.
Discharge Lines
A hot gas line should be designed to:
1. Avoid trapping oil at part-load operation.?
2. Prevent condensed refrigerant and oil from draining back to the head of the compressor.
3. Have carefully selected connections from a common line to multiple compressors.
4. Avoid developing excessive noise or vibration from hot-gas pulsations, compressor vibration or both.
When sizing discharge lines - lines that conduct refrigerant vapor from the compressor to the condenser - considerations similar to those applied to the suction line are observed. Pressure loss in hot gas 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 fpm in horizontal runs and 1,000 fpm in vertical runs with upward gas flow are observed. The maximum acceptable gas velocity, based on noise considerations, is 4,000 fpm.
Defrost Gas Supply Lines
Sizing refrigeration lines to supply defrost gas to one or more evaporators has not been an exact science. The parameters associated with sizing the defrost gas lines are related to allowable pressure drop and refrigerant flow rate during defrost. Engineers have used approximately two times the evaporator load for effective refrigerant flow rate to determine line sizing requirements. The pressure drop isn't as critical during the defrost cycle, and many engineers have used velocity as the 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's recommended that initial sizing be based on twice the evaporator flow rate and that velocities from 1,000 to 2,000 fpm be used for determining the defrost gas supply line size.
Refrigerant line capacity tables are based on unit pressure drop per 100-foot length of straight pipe or per combination of straight pipe, fitting and valves with friction drop equivalent to a 100-foot 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. Line sizing tables can then be used directly.
Piping Location and Arrangement
Refrigerant lines should be as short and direct as possible to minimize tubing and refrigerant requirements and pressure drops. When installing, it's best to plan for a minimum number of joints and use as few elbows and other fittings as possible. However, provide sufficient flexibility to absorb compressor vibration and stresses due to thermal expansion and contraction.
Arrange refrigerant piping so that normal inspection and servicing of the compressor and other equipment are not hindered. Do not obstruct the view of the oil level sightglass or run piping so that it interferes with the removal of compressor cylinder heads, 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.
Provide adequate clearance between piping and adjacent walls and hangers or between pipes for insulation installation. Use sleeves that are sized to permit installation of both pipe and insulation through the floor, walls or ceilings. Set these sleeves prior to the pouring of concrete or construction of brickwork. Run piping so that it does not interfere with passages or obstruct any headroom, windows and doors. Refer to ASHRAE Standard 15, Safety Code for Mechanical Refrigeration, and other governing local codes for restrictions that may apply.
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 carelessly handled hand trucks, overhanging loads, ladders and fork lifts.
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 doesn't require insulation, the suction and liquid lines can be insulated where the two lines are clamped together. When it passes through an area of higher temperature, the liquid line should be insulated to minimize heat gain. Hot gas discharge lines usually are not insulated; however, they should be if the heat dissipated is objectionable or to prevent injury from high-temperature surfaces.
Vibration transmitted through or generated in refrigerant piping can be eliminated or minimized by proper piping design and support. Two undesirable effects of vibration of refrigerant piping are:
-Physical damage to the piping, which may result in the breaking of brazed joints and, consequently, loss of charge.
-Transmission of noise through the piping itself and through building construction with which the piping may come into direct physical contact.
Finally, always follow the manufacturer's recommendations.
Refrigerant Receivers
Types:
• full size receivers - holds entire system charge (has queen valve and king valve)
• Oversized Receiver – holds excess system charge (condenser flooding)
• Partial Receiver – holds a portion of the system charge in the receiver and the rest in condenser
Components
• Inlet - Queen Valve – Full size
• Outlet – king valve – Quill - All receivers
• Liquid level indicators on large receivers (bulls eye)
• Liquid level gauge glass ( ball checks for protection if glass breaks)
Receiver Capacity As Required by B-52
• (4.6.2) most hold the entire refrigerant charge and only occupy 90% of its volume at 90 F
• if refrigerant side head pressure control is used receiver must be oversized by 50 %
Formula
- sizing of receivers Cr = Res + 20% / 0.9
cr = capacity of receiver
res = ref. charge in system (lb)
20% = extra ref. charge
• = 90% fill limit of a receiver
example Question: find the size of a receiver required for a system that has a requirement for 83 lbs of refrigerant
Cr = (83 + 20% or 83 lbs + 1.2) / 0.9 = 110.67 lbs ref.
High Pressure Safety
• purpose: a pressure responsive mechanism designed to stop the operation of the compression at predetermined safe pressure
• settings: water cooled 20% above condensing pressure
• settings: air 20% below high side design pressure
Relief Safety Devises
• purpose: a safety mechanism designed to open before dangerous pressure is reached
• usually mounted any place in the system where liquid refrigerant may be isolated by valves
Types
• fusible plug – temperature operated (non resetting)
• rupture disk – pressure operated (non resetting)
• pressure relief – valve pressure operated (resets when pressure drops to safe limit approximate. 10 to 20 % below setting
Approximate. Discharge Settings for PRV 's
• R 22 approximate 300 PSIG
• R 12 approximate 175 PSIG
Capacity of Relief devises
• the minimum required discharge capacity is found using the following formula
C = fDL
C = minimum discharge capacity lb/air/min
D = outside diam. Of vessel (ft.)
L = length of vessel
f = factor dependant on refrigerant (table 6)
Example question:
Select a PRV for the following Receiver
Receiver 36 “ diam.
Length 5 ft.
Ref. R 12
C = 1.6 x 3 ft. x 5 ft. = 24 lb/air/min
