REFRIGERATING CAPACITY

The measure of the job done by a refrigerating machine is the rate of heat transfer to its evaporator or cold part, from the bodies or fluids which are being refrigerated. This rate of heat transfer will be called the refrigerating capacity.

1 Ton of Refrigeration = 12,000 Btu/h = 200 Btu/min

In terms of other units:

1 Ton of Refrigeration = 3516.85 W = 3023.95 kcal/h

Plant capacities in practice range from microwatts in very low temperature refrigerators to several megawatts in large plants.

The "Ton" is often used in general terms to indicate the order of size of a plant, e.g. a 100 Ton plant, or "fractional tonnage" plants (small commercial ones).

It also happens that at the temperature of evaporation and condensation used in air-conditioning, 1 Ton of refrigeration requires a power of about 1 horse-power, which gives a useful way of visualizing the size of a plant.

Other names for what is called refrigerating capacity above are also to be met, and one of these will be used from time to time, namely the refrigerating load. This also means the rate of heat transfer to the evaporator or cold part of the plant, but it places the emphasis on what the plant is asked to do rather than on what it can do. It will be said, for example, that the load is increased or decreased, that the temperature of the things being cooled, or the rate of heat transfer from them is increased or decreased. The refrigerating plant reacts to match the load, by an increase or decrease of the refrigerating capacity. This use of the word "load" is the same as its use in connection with engines. Applying load to an engine means making a demand for more power by increasing the torque, i.e. by attempting to stop the shaft turning. The engine reacts by giving more power, providing it is within its capacity to do so. At a steady state, with no acceleration, the power equals the load of course. But the difference of emphasis is clear: load is the demand which is made of the engine, power is what the engine gives.

Another term in use is refrigeration duty. It has at least two meanings. One is the same as refrigerating capacity, meaning the rate of heat transfer at a particular instant to the evaporator. Another meaning is the specified refrigerating capacity, i.e. what is asked for by the buyer of the plant, usually at a specified set of conditions. The conditions may be internal ones, such as evaporating and condensing temperatures, temperature of liquid at the expansion valve, and temperature of the vapour leaving the evaporator and entering the compressor. On the other hand, the conditions may be external ones: temperature of cooling water, rate of flow, temperature of cooled fluid and rate of flow, for example.

Finally, the term refrigerating effect is also in use, meaning either refrigerating capacity or the heat transfer to the evaporator associated with the passage of a certain amount of refrigerant.

Lesson C.

Speech Practice

AT AN OFFICE. В ОФІСІ.

Exercises: