THE HONG KONG AND FAR EAST BUILDER
REVERSE CYCLE REFRIGERATION-THE HEAT PUMP
By J. Tausz, B.Sc. (Brno)
of
Gilman and Co. Ltd. Hong Kong.
Much has been said and written in recent years about reverse cycle refrigeration. The term "Heat Pump" is probably more often used in connection with this process. The first term applies more to the apparent reversal of a refrigeration process, though this is not entirely true. The second name more clearly applies to the idea of obtaining heat, when heat is required. It may be required or used for any purpose. but we will concern ourselves only with its application in warming up our dwellings in wintertime.
The idea of using a refrigeration plant, serving in summer to cool the air in spaces either occupied by human beings or used for other purposes, to produce heating in winter in the same space is not new,
About one hundred years ago Lord Kelvin came to the following conclusion: "Why not set up a refrigeration machine and carry heat into a house and thus heat it, instead of burning fuel inside the house to produce heat?"
The shortest definition of mechanical refrigeration can be put as follows: Extraction of heat at lower tempera- tures and dissipation of heat at higher temperatures. It looks like a conjurer's trick, but that is what happens in any refrigeration equipment. Every refrigerator or air- conditioner produces a lower temperature in the refrigera- tor or air-conditioned room than the prevailing outdoor temperature, yet the heat extracted is thrown into the surrounding air or other medium, which is at a higher temperature than that of the cooled space. The underlying idea of Lord Kelvin's statement is this: Why not use the heat so wasted for heating purposes? The logical con- clusion is that with the same refrigeration plant we can extract heat from an outside medium, air, water etc. and transfer this heat gain to our house for distribution. To do so only certain parts of the equipment have to change their roles. The following two very elementary drawings give an idea of this.
WINTER
23
heating effect is increased by the heat generated by the compressor in doing work on the refrigerant gas as it goes through the compressor. This is called the "heat of com- pression." This additional heat is in summer dissipated to the outside air, cooling water etc., but in winter is a useful increase for our heating purposes.
Suppose we use to drive the compressor in our system a motor of one horse power, besides some smaller motors for fans etc., with a total consumption of electricity of about 1500 Watts. If we convert this electricity into heat through a conventional resistance heater we obtain 5100 B.T.U's (1500W X 3.4 B.T.U's). In using our cooling plant for heating we obtain first the 12,000 B.T.U's plus the heat of the compression. Suppose our motor works on 60% efficierey, this means that 40% goes into frictional and other losses. Sixty per cent of the 1500 watt con- sumption is converted into the heat of compression, which means 60% of the 1500 watts, or 900 watts, are converted into useful heat. This gives us an additional 3060 B.T.U's (900 W X 3.4 B.T.U's). So the resulting heating effect will be 15,060 B.T.U's (12,000 plus 3060), against 5100 B.T.U's through an ordinary type of con- ventional resistance heater.
This seems to be a very attractive proposition. Look- ing into further details we will see that there are certain restrictions to the application of our cooling system for heating purposes.
Referring to the principle of mechanical refrigeration "extract heat at lower and discharge at higher tempera- tures", we have to consider the following. The lower the temperature of the medium, air, water etc. from which we extract a given quantity of heat, accordingly the size of our equipment becomes bigger. Imagine separating a mixture of gold-dust and sand. If we have two mixtures
SUMMER.
HEAT
HEAT OUT
IN
HEAT
HEAT IN
N
ROOM
(WARM)
OUTSIDE
(COLD)
ROOM (COOL)
A
OUTSIDE (WARM)
In practice the plant has of course various other parts, but for the understanding of the application we are not concerned with these.
The fact remains that when we change the roles of the two main parts, the evaporator and the condenser. instead of cooling we obtain heating.
Is this kind of heating proposition efficient enough to make it worth our while to apply it instead of the conven- tional types of heating, L.e. different kinds of fuel burners. electricity etc.? The answer to this question can only be found by looking at the problem from different angles.
Having installed one cooling system for summer use. few, if any, additions are required to use the same system for winter heating. Suppose we have # small air- conditioner which extracts 12,000 B.T.U's per hour during the cooling period. Obviously, by using it in winter for heating we will get the 12,000 B.T.U's for heating. The
containing respectively ten per cent gold and five per cent gold we must separate twice as much five per cent gold- sand mixture to obtain the same quantity of gold as in the case of the ten per cent gold-sand mixture.
It is obvious, therefore, that there will be a great difference between extracting heat for our heating purposes in winter when the outdoor temperature is 50 degrees F. and when the outdoor temperature is 0 degrees F. The higher the average winter outdoor temperature is the more convenient and efficient will be our system for heating purposes. This applies whether we use air, water or the earth itself as the medium for our purpose.
To balance out the system, i.e. to have easy exchange- ability of the two main purts, condenser and evaporator, it is also important that temperature differences in sum- mer and winter are not very different from the average working temperature. The measure for this is the "degree day", which is the difference between 65 degrees F. and the
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