Refrigeration capacity control

There are various options for capacity control of reciprocating refrigeration compressors ... 

Capacity Control The simplest way to regulate the capacity of most steam vacuum refrigeration systems is to furnish several primary boosters in parallel and operate only those required to handle the heat load. It is not uncommon to have as many as four main boosters on larger units for capacity variation. A simple automatic on-off type of control may be used for this purpose. By sensing the chilled-water temperature leaving the flash tank, a controller can turn steam on and off to each ejector as required. 

Figure 11-21D. Helical rotors refrigerant compressors. (1) Cutaway of a 100-ton intermediate compressor. The intermediate Helirotor compressor has only three moving parts the two rotor assemblies and the capacity controlling slide valve. The general purpose Helirotor compressor has only four moving parts two rotor assemblies, the variable unloader valve, and the step unloader valve. Unlike reciprocating compressors, the Trane Helirotor compressor has no pistons, connecting rods, suction and discharge valves, or mechanical oil pump.

Figure 12-2E. 250 psig working pressure cylinder used in refrigeration service. Auxiliary stuffing box for added sealing on shutdown. Manual fixed volume clearance pockets for capacity control. (Used by permission Bui. L-679-BIA, 1957. Dresser-Rand Company.)... 

Some small cooling circuits have reversing refrigerant flow (i.e. cooling/ heat pump) and may work at reduced gas flow for capacity control. Under such conditions it may not be possible to maintain the minimum velocity to carry oil back to the compressor, and it will settle in the circuit. Arrangements must be made to increase or reverse the gas flow periodically to move this oil. 

It may be noted that refrigeration system operating on high-pressure ammonia is to be operated and maintained very carefully to prevent any accident. Capacity control facility to run the compressor at lower load if required. 

Figure 1 shows the major elements of a multiplex refrigeration system. Multiple compressors operating at the same saturated suction temperature are mounted on a skid, or rack, and are piped with common suction and discharge refrigeration lines. Using multiple compressors in parallel provides a means of capacity control, since the compressors can be selected and cycled as needed to meet the refrigeration load. 

The operators will slow the compressor down on hot days. This will control the refrigerant accumulator pressure below the relief valve popping pressure. Slowing the compressor down also reduces refrigeration capacity. 

Dry bulb sensors may be bi-metal strips, thermistors or refrigerant-filled phials or bellows responding to pressure differences caused by temperature change. These, in turn, provide an electrical or mechanical signal. The mechanical items are used to alter the value of potentiometers or make-or-break contacts. The signals are transmitted to amplifiers, which respond to the degree of error. An important feature is the proportional band of temperature over which the controls call for up to full plant capacity. 

A refrigeration system will be designed to have a maximum duty to balance a calculated maximum load, and for much of its life may work at some lower load. Such variations require capacity reduction devices, originally by speed control (when steam driven) or in the form of bypass ports in the cylinder walls. 

The refrigerant charge of such a system is critical, since it must not exceed the working capacity of the evaporator. It is not possible to have a receiver in circuit and this control cannot feed more than one evaporator, since it cannot detect the needs of either. 

Solenoid valves are used in refrigeration and air-conditioning systems for refrigerant lines, oil pressure pipes (to control oil return and capacity reducers), and water and compressed air lines. Four-port changeover valves (Figure 9Ab) are used to reverse flow in defrosting and heat pump circuits. A de-energized expansion valve will act as a solenoid valve. 

To carry out measurements at a fixed temperature, the refrigerator temperature must be kept constant for a suitably long time. The problem of the temperature control depends not only on the refrigerator itself, but on the thermal characteristics of the experiment. Let us now consider an oversimplified case in which heat capacities are neglected the mixing chamber temperature of a dilute refrigerator (DR) is to be controlled by a resistive heater HR and a d.c. power supply. 

From Fig. 10.13, we see the latter condition is fulfilled in the first three cases, but not in the fourth case. The most stable situation is obtained with Rx. The choice R = RcosL is however usually adopted when the power supplied to the resistor must be measured. The control of temperature in the real (dynamic) case is much more complex. The problem is similar to that encountered in electronic or mechanical systems. The advantage in the cryogenic case is the absence of thermal inductors . Nevertheless, the heat capacities and heat resistances often show a steep dependence on temperature (i.e. 1 /T3 of Kapitza resistance) which makes the temperature control quite difficult. Moreover, some parameters vary from run to run for example, the cooling power of a dilution refrigerator depends on the residual pressure in the vacuum enclosure, on the quantity and ratio of 3He/4He mixture, etc. 

Vendor shall supply a completely piped and wired skid mounted packaged refrigeration system, which shall include but not be limited to the following all components, piping, valves, instruments, alarms, shutdowns, control, control panels, special tools and equipment necessary to operate the unit at its rated capac ity. ... 

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