THE CHANGING MACHINE ROOM: MULTIPLEX …

1 In an effort to maintain a minimum common suction pressure, the rack s energy management system (EMS) will cycle compressors on/off as required. Cycling may involve a compressor cycling on/off, or a compressor of one size cycling off while a compressor of another size cycles on. The connected circuit that operates at the lowest tempera-ture determines the rack s common suction circuit temperature is typically controlled with an evaporator pressure regulator (EPR). By main-taining a constant pressure (refrigerant saturation tem-perature) in the evaporator, a fairly constant evaporator discharge air temperature will be maintained. Recently there has been a move from EPR valves to electroni-cally controlled step-motor suction line regulators. Un-like EPR valves, they respond to evaporator discharge air REFRIGERATION - PART IIBY DAVE DEMMAS ervice technicians who currently work on supermarket equip-ment are more likely to see MULTIPLEX racks than any other style.

2 This is CHANGING due to concerns over installation costs and THE CHANGING MACHINE ROOM: MULTIPLEX rack SYSTEMS refrigerant management, however it will be quite some time before they become extinct from the service techni-cian s world. The MULTIPLEX rack system employs several compressors of varying capacity, connected to common suction and discharge headers (see Figure 1). The refrigeration load consists of anywhere from five evaporator systems, to per-haps 20 on a large rack , all connected to the common suction the other options that may be present on the MULTIPLEX rack , the following are more common:(1) Split Suction Header. This allows the low tempera-ture and medium temperature rack to share the same refrigerant charge. The low temperature and medium compressors are isolated from each other by a suction header that is physically split, allowing for two distinct operating suction pressures.

3 They share a common dis-charge line, liquid line, and oil management system.(2) Liquid Subcooler. Subcooling the liquid refrigerant results in a denser liquid, and can allow for smaller liq-uid line pipe sizes. In addition, in systems where the me-dium temperature rack supplies the refrigeration to sub-cool the liquid refrigerant on the low temperature rack , an increase in efficiency can be realized. This comes from providing part of the Btu requirement to the low temperature rack (whose compressors operate between four and five horsepower per ton) from the medium tem-perature rack (whose compressors operate between two and three horsepower per ton).(3) Heat Reclaim. Diverting the rack s entire refrigerant discharge vapour mass flow through either a hot water reclaim tank, or reclaim coil in the HVAC ducting, will transfer some of the normal waste heat from the com-pression process to a useful process.

4 (4) Mechanical Head Pressure Control Valves. Compres-34 HPAC | MARCH 2007sors run more efficiently at lower compression ratios. This can be achieved by lower head pressure in the cool-er months of the year. However, if the head pressure is allowed to operate at too low a setting, it can negatively affect the system operation. These valves are designed to maintain a minimum head pressure for low ambient operation.(5) Split Condensers. Head pressure control valves main-tain minimum head pressure by flooding a portion of the condenser with liquid refrigerant. Splitting the condens-er in half during low ambient conditions will minimize the amount of extra refrigerant required to accomplish the minimum head pressure.(6) Energy Management System (EMS). A central proces-sor that monitors and maintains: fixture temperatures; minimum suction pressure by compressor cycling; de-frost schedules; condenser fan cycling; receiver refriger-ant level; store HVAC; display case anti-sweat heaters; and alarm conditions.

5 If set up correctly, this can alert monitoring personnel of problems before they become catastrophic, and can also result in substantial energy savings.(7) Satellite Compressors. These are used in applica-tions where there is a substantial difference between the temperature of the lowest operating circuit and the re-maining circuits. For example: ice cream display cases MULTIPLEX rack 3402/23/2007 10:39:36 PMtemperature and are controlled by the EMS (see sidebar for description). As the discharge temperature raises/low-ers below the set-point, the EMS will cause the step motor to open/close the valve. This will vary the satura-tion pressure of the refrigerant in the evaporator, which will in turn vary the discharge air MANAGEMENTIt is a known fact that reciprocating compressors pump some amount of oil.

6 There is no way to ensure that as this oil travels through the system piping, it will re-enter the suction header and be redistributed in proportion to the compressors from which it came. There-fore, MULTIPLEX racks utilize an oil management system, consisting of an oil separator, oil reservoir, oil filtration device and oil level controls for each oil management system separates a high percentage of oil from the discharge vapour before it enters the condenser. It is stored in the reservoir until required by an individual compressor. The function of the oil level control is to main-tain the desired level of oil in each compressor s crankcase. Like conventional/ MULTIPLEX systems, off-cycle and elec-tric defrost are possible choices for providing defrost cy-cles.

7 Gas defrost is another option. In the more common reverse flow method, the defrost gas (typically discharge gas, but it may also be saturated vapour from the top of the receiver) flows in a direction reverse that of the normal refrigerant flow during the refrigeration CONTROLThe defrost control sequence is as follows: When the time FIGURE 1 MULTIPLEX rack SYSTEMG raphics Parker Hannifin CorporationMARCH 2007 | HPAC 35operating at -35F saturated suction temperature, and the remaining frozen food display cases operating at -25F saturated suction temperature. The compressor s effi-ciency (capacity) at a saturated suction temperature of -35F is substantially less than at -25F. It would not make good sense to operate all of the com-pressors at -35F to satisfy the design requirement of the ice cream display cases, particularly since they will make up only a small percentage of the rack s total Btu satellite compressor would be selected to match the Btu requirement of the ice cream display cases, with its suction line directly piped to that load.

8 It shares the com-mon discharge header with the other compressors on the rack . Liquid refrigerant will be supplied to the ice cream display cases from the common liquid satellite compressor also shares the rack s oil man-agement system. Typically there is a check valve between the satellite compressor s suction line and the main suction header. This allows for the -25F compressors to pro-vide some measure of refrigeration during times when the satellite has cycled off on temperature. More importantly, if the satellite com-pressor suffers a failure, the -25F compressors can be temporarily set to operate at a lower suction pres-sure (saturated suction temperature) and prevent the loss of ice cream. The satellite compressor is in the same vein as a split suction header, although it typically involves one single compressor.

9 FIGURE 2 SATELLITE COMPRESSOR CONTINUED ON PAGE 3502/23/2007 10:39:57 PM36 HPAC | MARCH 2007 REFRIGERATION - PART IIc o n t i n u e d f r o m P A Ge 3 5clock initiates a defrost cycle for a particular circuit, it will simultaneously de-energize (close) the suction line regula-tor and liquid line solenoid valve, and energize the gas defrost solenoid valve and the differential pressure regula-tor. The differential pressure regulator must be installed somewhere between the gas supply for defrost and the liq-uid header. Set the regulator to maintain an approximate 20 psi pressure differential between the defrost gas pres-sure and the liquid header pressure. Defrost gas flow will immediately commence in a reverse direction through the from the defrost gas is transferred to the evapora-tor surface, melting the accumulated frost.

10 In the process, most of the refrigerant vapour condenses into a liquid. Check valves piped in parallel with the TEV and liquid solenoid valve facilitate reverse flow past these same com-ponents. This liquid/vapour mixture will enter the liquid header at a pressure slightly higher than the liquid refrig-erant pressure, and assist in supplying the required refrig-erant mass flow to the remaining circuits in the refrigera-tion :The MULTIPLEX rack has an inherent potential to be much more efficient than the conventional or multi-plex systems. This is due to the utilization of several compressors of varying capacity along with the energy management system that determines when they op-erate. Once the store is properly commissioned, in-cluding setting all fixture TEVs, the suction pressure can be set to operate at the highest possible level for maintaining design fixture temperatures.