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View Subscription Details: Letter ( 09/05/2006 ) - HTML Format

Temperature and level controls in industrial refrigeration

The majority of control loops in industrial refrigeration plants are two positions or on-off. In most storage spaces for frozen food, for example, when temperature in the space rises to the set point, a solenoid valve opens allowing liquid refrigerant into the coil. The additional refrigeration capacity thus provided will drop the temperature. But this method of temperature control sometimes creates instability of refrigeration plant and reduces its efficiency.

Refrigeration load of holding freezers are usually pretty steady. But the load on refrigeration plant is fluctuating very significantly. Very often we can see continuous loading and unloading of refrigeration compressors. Refrigeration plant is loosing efficiency when screw compressors are operating at capacity lower than 100 %. To improve efficiency and stability of refrigeration plant, we have to minimize the fluctuation of the load on refrigeration plant. How can we do that?

Example. Holding freezer has 4 evaporator coils. Assume that these coils are operating at optimum suction pressure (October, November 2005 newsletters). Winter refrigeration load is 40% of summer (design) refrigeration load.

During the summer, all 4 coils are running. Operating time of these coils are 90-100 %. We have a stable operation of refrigeration plant.

During the winter, operating time of these coils will decrease to 35-40 %. Refrigeration plant will have instable operation. Sometimes 4 coils are running, sometimes just 1 coil is running. To minimize this instability, total capacity of evaporator coils should be slightly higher than refrigeration load. In our case, we can turn off 2 coils, for example 2, 4, and run just coils 1, 3. Operating time of coils 1, 3 will be 70-80% and fluctuation of refrigeration load will be reduced. Every day we can switch the pair of operating coils.

Another source of refrigeration plant instability is a level control. To keep certain level in the vessel, we should supply liquid from high pressure vessel. Liquid refrigerant at high pressure passes though a solenoid and an expansion valve. After expansion valve we have mixture of liquid and flash-gas vapor. This flash-gas vapor can significantly increase the load on refrigeration compressors. Operating time of liquid supply solenoids is usually less than 50 %. When solenoid is open, significant amount of flash-gas vapor is going into the vessel and refrigeration load on the compressors is jumping up. Modulating level controls will help you to keep a precise level in the vessel and will prevent instability of the refrigeration plant.

 

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