REFRIGERANT CHARGE

  Refrigerant R-12 is used in most transport system at the present time, but R-502 is well suited for low temperature applications, and its use is increasing. Since R-502 creates a greater power requirement for a given compressor displacement than R-12, the motor-compressor must be properly selected for the refrigerant to be used. Different expansion valve are required for each refrigerant, so the refrigerant are not interchangeable in a given system and should never maximum working pressures than those used with R-12, so normally it is not feasible to attempt to convert an existing R-12 unit for the use of R-502.

 The refrigerant charge should be held to the minimum required for satisfactory operation. An abnormally high refrigerant charge will create potential problems of liquid refrigerant migration, oil slugging, and loss of compressor lubrication due to bearing washout or excessive refrigerant foaming in the crankcase.

  Systems should be charged with the minimum amount of refrigerant necessary to insure a liquid seal ahead of the expansion valve at normal operating temperatures. For an accurate indication of refrigerant charge, a sight glass is recommended at the expansion valve inlet, and a combination sight glass and moisture indicator is essential for easy field maintenance checking. It should be born in mind that bubbles in the refrigerant sight glass can be caused by pressure drop or restrictions in the liquid line, as well as inadequate nominal working charge data should be used only as a general guide, since each installation will vary in its charge requirements.

ELECTRICAL PRECAUTIONS

  Electrical failures are a common field maintenance problem due to the wet environment, shock and vibration, and the possibility of improper power from an engine generator set.

  For the safety of operating and maintenance personnel, the electrical system should be grounded to the frame, and the frame in turn grounded by means of a chain or metal link to the ground if a generator  set in is mounted on the vehicle. All components should be grounded from one to the other, such as the generator set to condensing section to evaporator section. Cables to remote sources of power should carry an extra wire for grounding purposes at the supply plug.

  At the time of manufacture, each system should be given a high potential test to insure against electrical flaws in the wiring. All relays and terminals should be protected against the weather, and all wiring should be covered with protective loom to guard against abrasion. All switches should be of the sealed type, recommended by the manufacturer for use in wet environments. Plus type line connectors should be of the waterproof type. Electrical cables connecting split units should have a watertight cable cover, or should be run in conduit. All wiring should be fastened securely to prevent chafing, and should be clearly identified by wire marking and/or following the color code specified by the National Electrical Code.

  Adequately sized extension cords, plugs, and receptacles must be used to avoid excessive voltage drop. Voltage at the compressor terminals must be within 10% of the nameplate rating, even under starting conditions. Many single phase starting problems on small delivery trucks can be traced to the fact that power is supplied to the compressor from household type wiring circuits through long extension cords, neither of which are sized properly for the electrical load. Single phase and type motors which are used for belt driver a compressor during over-the-road operation must be equipped with a relay to break the capacitor circuit, rather than a centrifugal switch. The variable speed operation experienced during truck operation may cause a centrifugal switch to fail because of excessive wear at low operating speeds. All start capacitors must be equipped with bleed resistors to permit the capacitor charge to bleed off rapidly. preventing arcing and overheating of the relay contacts.

  When units are operated from several power sources be sure all plugs and receptacles are wired in the same sequence, so that the compressor rotation will not be reversed.

COMPRESSOR SPEED

  Open type compressor operating from a truck engine by means of a power take-off or by a belt driver are subject to extreme speed ranges. A typical truck engine may idle at 500 RPM to 700 RPM, run at 1,800 RPM at 30 MPH, and run at 3,600 RPM over the highway at high speed. Whatever the power take-off or belt ratio, this means the compressor must operate through a speed ratio range of 6 to 1 or greater unless it is disconnected from the power source by some means.

  The compressor speed must be kept within safe limit to avoid loss of lubrication and physical damage. Operation within the physical limitations of the compressor may be possible for example from 400 RPM to 2,400 RPM. It may be possible to use a cut-out switch ta disconnect the compressor from the power source at a given speed. The compressor manufacturer should be contacted for minimum and maximum speeds of specific compressors.

  If the compressor is of  the accessible-hermetic type, there is no problem concerning speed so long as the electrical source is operating at the voltage and frequency for which the motor was designed. If the speed of the generator is varied in order to obtain variable speed operation, the voltage and frequency on the normal alternating current generator will vary proportionally. Since the compressor speed and motor load will vary directly with the frequency, it is often possible to operate over a wide speed range with satisfactory results.

  However, it should be born in mind that increasing the frequency and voltage of the generator above the level for which the compressor motor was designed will increase the load on the compressor, may overload the motor, and can result in bearing or other compressor damage. Operation at speeds too low to provide adequate compressor lubrication must also be avoided, although normally lubrication can be maintained on Copelametic compressors down to 600 RPM and possibly lower speeds.

  Each new application involving operation of the compressor at a voltage and frequency differing from its nameplate rating should be submitted to the Copeland Application Engineering Department for approval.

  One other problem that may arise with operation from a variable speed generator is the operation of electrical contactors, relays, etc. on voltages below or above their nameplate rating. Field tests have shown that the winding design and physical construction of electrical components can cause wide variation in voltage tolerance. The drop-out voltage of various types of commercially available 220 volt contactors may vary from 145 volts to 180 volts depending on construction. If it is planned to operate at variable voltage and frequencies, the electrical  components which are to be used should be extensively tested at the electrical extremes in cooperation with the manufacture to insure proper operation

TWO STAGE COMPRESSION AND COMPRESSOR EFFICIENCY

  In order to increase operating efficiency at low evaporating temperature, the compression can be done in two steps or stages. for two stage operation, the total compression ratio is the product of the compression ratio of each stage. In other words, for a total compression ratio of 16 to 1, the compression ratio of each stage might be 4 to 1; or compression ratios of 4 to 1 and 5 to 1 in separate stage will result in total compression ratio of 20 to 1.

  Two stage compression may be accomplished with the use of two compressor with the discharge of one pumping into the suction of the second, but because of the difficulty of maintaining proper oil levels in the two crankcases, it is more satisfactory to use one compressor with multiple cylinder. On Copeland two stage compressors, the ratio of low stage ti high stage displacement is 2 to 1. The greater volume of the low stage cylinder is necessary because of the difference in specific volume of the refrigerant vapor at low and interstage pressure. While the compression ratios of the two stage are seldom exactly equal, they will be approximately the same. A typical 6 cylinder two the compressor with its external manifold and desuperheating expansion valve is shown in figure, and a typical 3 cylinder two stage compressor with external manifold is shown

  Shows a comparison of five different volumetric efficiency curves. The threes straight lines are typical single stage curves one for an air conditioning compressor, one for a typical multi-purpose compressor, and one for a low temperature compressor. There are some variations in compressor design involved, but the primary difference in characteristics is due to clearance volume.

  The two vertical curved lines represent the comparative efficiency of a two stage compressor. Actually each separate stage would have a straight line characteristic similar to the single stage curves, but to enable comparison with single stage compressor, the overall volumetric efficiency has been computed on the basis of the total displacement of the compressor, not just the low stage displacement.

SINGLE STAGE LOW TEMPERATURE SYSTEMS

  Low temperature single stage system become increasingly critical from a design and application standpoint as the desired evaporating temperature is decreased. The combination of high compression ratios, low operating temperatures, and rarified  return gas can cause lubrication and overheating problems, and make the compressor more vulnerable to damage from moisture and contaminants in the system.

  The compressor selection, suction temperature, and application must be such that the temperature of the discharge line measured within 1¨ to 6¨of the discharge line service valve does not exceed 230° F. for Refrigerants 12,22 and 502. Under these conditions, the estimated average temperature at the discharge port (measured at the valve retainer on the valve plate) will be approximately 310° F. for R-12 and R-502, and 320° F. for R-22.

  The compressor displacement, pressure limiting devices, and quantity of cooling air or water must be selected to prevent the motor temperature from exceeding the limits stated below:

 A. 210° F. when protected by inherent protectors affected by line current and motor temperature.

 B. 190° F. when protected by motor starters.

  The temperature of the motor should be determined by the resistance method and should be determined when the compressor is tested in the highest ambient in which it is expected to operate, at 90 per cent of rated voltage, with 90° F. return suction gas temperature of 170° F. to 190° F. are highly recommended.

  In order to prevent the discharge and motor temperatures from exceeding recommended limits, it is very desirable, and in some instances absolutely necessary, to insulated the suction lines and return the suction gas to the compressor at a lower than normal temperature. This is particularly important with suction cooled compressor when R-22 is used. (Approximately 30° F. superheat suggested.)

  Suction cooled compressors required auxiliary cooling by means of an air below 0° F. evaporator temperature.

  Either the evaporator must be properly designed, or a pressure limiting device such as a pressure limiting expansion valve or crankcase pressure regulating valve must be provided to prevent motor overloading during pulldown period, or after defrost.

  Copeland now recommends R-502 for all single stage low temperature applications where evaporating temperature of -20° F. and below may be encountered. Now that R-502 is readily available, R-22 should not be used in single stage low temperature compressor, 5 H.P. and larger. The lower discharge temperature of R-502 have resulted in much more trouble-free operation.

  An adequate supply of oil must be maintained in the crankcase at all times to insure continuous lubrication. If the refrigerant velocity in the system is so low that rapid return of the oil is not assured, on adequate oil separator must be used. The normal oil level should be maintained at or slightly above the center of the sight glass. An excessive amount of refrigerant or oil must not be allowed in the system as it may result in excessive liquid slugging and damage to the compressor valve, pistons, or cylinders.

VIBRATION AND NOISE

  No matter how well the compressor is isolated, some noise and vibration will be transmitted through the piping, but both can be minimized by proper design and support of the piping.

  On small units a coil of tubing at the compressor may provide adequate protection against vibration. On larger units, flexible metallic hose is supported by vibration absorbing mounts allowing compressor movement, refrigerant lines should not be anchored solidly at the unit, but at a point beyond the vibration absorber, so the vibration can be isolated and not transmitted into the piping system.

  The noise characteristics of a large refrigeration or air conditioning system, particularly when installed with long refrigerant lines and remote condensers, are not predictable. Variation in piping configuration, the pattern of gas flow, line sizes, operating, all can affect the noise generated by the system. Occasionnlly a particular combination of gas flow and piping will result in a resonant frequency to an undesirable level. Gas pulsation from the compressor may also be amplified in a similar manner.

  If gas pulsation or resonant frequencies are encountered on a particular application, a discharge line muffler may be helpful in correcting the problem. The purpose of a muffler is to damper the pulses of gas in the discharge line and to change the frequency to a level which is not objectionable. A muffler normally depends on multiple internal baffles and/or pressure drop to obtain an even flow of gas. In general, the application range of a muffler depends on the volume and density of the refrigerant gas discharged from the compressor are both factor in muffler performance.

  A given muffler may work satisfactorily on a fairly wide range of compressor sizes, but it is also quite possible that a given system may require a muffler with a particular pressure drop to effectively dampen pulsations. On problem applications, trial and error may be the only final guide. While large muffler are often more efficient in reducing the overall level of compressor discharge noise, in order to satisfactorily dampen pulsations, smaller muffles with  a greater pressure drop are usually more effective. Adjustable mufflers are often helpful since they allow tuning of the muffler pressure characteristics to the exact system requirement.

  Occasionally, a combination of operating conditions, mounting and piping arrangement may result in a resonant condition, which tends to magnify compressor pulsation and cause a sharp vibration, although noise may not be a problem. For large Copelametic compressors, discharge muffler plates haven been developed for use when necessary to dampen excessive pulsation. The muffler plate fits between the discharge valve and the compressor body and has a number of muffling holes break up the pattern of gas flow  and create sufficient restriction to reduce the gas pulsation to a minimum.

  When piping passes through walls or floors, precaution should be taken to see that the piping does not touch any structural members and is properly supported by hangers in order o prevent the transmission of vibration into the building. Failure to do so may result in the building structure becoming a sounding board.

PIPING DESIGN FOR HORIZONTAL AND VERTICAL LINES

  Horizontal suction and discharge lines should be pitched downward in the direction of flow to aid in oil drainage, with a downward pith of at least  ​1⁄2 inch in 10 feet. Refrigerant lines should always be as short and should run as directly as possible.

  Piping should be located so that access to system components is not hindered, and so that any components, which could possibly  require future maintenance are easily accessible. If piping must be run through boiler rooms or other areas where they will be exposed to abnormally high temperatures, it may be necessary to insulate both the suction and liquid line to prevent excessive  heat transfer into the lines.

  Every vertical suction riser greater than 3 to 4 feet in height should have a ¨P¨ trap at the base to facilitate oil return up the riser as shown in figure. To avoid the accumulation of large quantities of oil, the trap should be of minimum depth and the horizontal section should be as short as possible. Prefabricated wrought copper traps are available, or a trap can be made by using two street ells and one regular ell. Traps at the foot of hot gas riser are normally not required because of the easier movement of oil at higher temperatures. However it is recommend that discharge line from the compressor be looped to the floor prior to being run vertically upward to prevent the drainage of oil back to the compressor head during shut down periods.

  For long vertical risers in both suction and discharge lines, additional traps are recommended for each full length of pipe (approximately 20 feet) to insure proper oil movement.

  In general, trapped section of the suction line should be avoided except where necessary for oil return. Oil or liquid refrigerant accumulating in the suction line during the off cycle can return to the compressor at high velocity as liquid slugs on start up, and can break compressor valves or cause other damage.