BASIC KNOWLEDGE OF A REFRIGERATION PLANT

PROCESS PLANT
Process plants are usually industrial sites were raw material or natural products are used by a transformation processes to make them into useful and desirable goods. The end product can in these industrial processes go through a number of phase changes or travel through sets of conveyors to become what is desired it should be. In most process plants there is usually a lot of consumption of energy in any form or the other. Process plants are ever growing in terms of finding innovative ways to improve their end product.  Today a lot of these industries use PLC’s to do or perform most of the work which was previously done by relays and timers.

REFRIGERATION PLANT                                                     
Refrigeration works by changing the absorbed heat is state of the refrigerant. The majority of heat is removed from the temperature controlled space as the refrigerant absorbs heat when it changes state from a liquid to a gas in the evaporator. The majority of the absorbed heat is removed from the refrigeration system in the condenser as the refrigerant changes removed from the refrigeration system in the condenser as the refrigerant changes


Figure 1: Refrigeration Plant Cycle
OPERATING PRINCIPLE
The refrigeration plant installed services four areas:
  • Main freezer operating at -25°C.
  • Chiller rooms operation at <4°C.
  • Blast freezers at -30 to 35°C.
  • Receiving (Chiller° rooms) area operating at 5 and 10°C. 

The system is serviced by two Grasso single stage screw compressors each as follows. The screw compressors have a common open type flash gas economiser fitted with a suction line single back pressure regulator to control the economiser saturated temperature at -10°C. Each economiser compressor connection is fitted with an isolation valve strainer and check valve to stop reverse gas and oil flow. Each compressor is fitted with ammonia thermosyphon oil cooler.

Operation

Ammonia gas is drawn from the -33°C suction accumulator as well as via the economiser port on each compressor receiving ammonia gas from the common economiser which operates at a saturation pressure of -10°C degrees Celsius.

As the compressor unloads from 100% capacity the economiser port on each machine will revert to the compressor suction of -33°C. To prevent the economiser vessel pressure falling the common back pressure regulator is set to control this at all times at a saturation pressure/temperature of -10°C. This higher pressure ensures that the liquid in the economiser will flow to the -33°C accumulator.

The compressor discharges the gas at high pressure into the two evaporative condensers where it is cooled at constant temperature and phase changed into ammonia.

This liquid ammonia falls by gravity into a small vertical pot which then distributes this liquid preferentially to the consumers in the order listed below.
  • Feed 1 goes to the oil coolers on each compressor
  • Feed 2 goes to the receiving bay coolers.
  • Feed 3 goes to the economiser vessel via a Witt high pressure float valve.

From the economiser vessel the liquid is expanded through the second Witt high pressure float valve and into the -33°C accumulator.
Receiving Area


At high pressure liquid from the distribution pot is expanded via an orifice plate into each cooler where the resultant liquid is boiled off at -10°C and the gas returned to the economiser. These coolers are fitted with electric heated drip trays and hot gas defrost system described below.

Freezer Pods

Liquid ammonia at -33°C is pumped by the Hermetic ammonia pumps to each of the coolers mounted in the cooler pods. There are three cooler pods installed to service the freezer store. The flow of liquid is controlled by two liquid line orifices feeding each of the three coolers. One ammonia pump is a standby unit and normally it does not operate.

Oil Coolers

The liquid from the distribution pot (at condensing pressure) falls by gravity into each of the shell and tube oil coolers; this ammonia is on the tube side of the cooler. Gas and liquid return back to the gas pot mounted above the distribution vessel. This circulation is caused by thermosyphen effect. The gas is then re-condensed in the condenser and the liquid used to cool the other consumer in the system

When operating normally the ammonia in and out pipes are at the same temperature being around 32°C.

HP Float Valves

These valves will always try to empty the vessel in the inlet side on the valve, as the float chamber fills and float lifts the valve opens to expand the liquid and resultant flash gas into the next lower pressure vessel in the system. The liquid is monitored by two of these HP valves, one feeding the economiser and the other the -33°C

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