Should Water Be Used as a Refrigerant?
Water is available everywhere on the planet. Why then do we not use it as a refrigerant but instead use other substances like ammonia, carbon dioxide, and sulfur dioxide. To understand why we pick these substances over water, we first need to understand the refrigeration cycle. Secondly, we need to know some properties that a good refrigerant should have. Finally, we need to take a look at some desirable and undesirable properties of water as a refrigerant. We can then decide based on all these factors whether it is practical to use water as a refrigerant or not.
The Refrigeration Cycle
Understanding the refrigeration cycle is very important in helping us choose whether to use water as a refrigerant or not. So how does a refrigerator work? The basic principle governing its operation is to pass a colder liquid (refrigerant) continuously around the object which is to be cooled, which may be your food in the refrigerator. The colder liquid (refrigerant) thus takes heat out from the object and makes it cold. The colder liquid (refrigerant) on the other hand gains heat. However, we need to make the liquid (refrigerant) cold again so that it can continuously absorb heat from the object. This is the whole idea of how a refrigerator works and is based on the ability to produce cold liquid continuously around the object which is to be cooled. To achieve this, the refrigerant goes through four stages. The first stage occurs in the evaporator where the liquid refrigerant is converted to a high-temperature and low-pressure gas after heat transfer from the inside air (in the refrigerator) to the refrigerant. The second stage occurs in the compressor where the gas is compressed. This changes the low-pressure gas to a high-pressure gas with a further increase in temperature. The third stage takes place in the condenser where the high-pressure gas is converted to a high-pressure liquid after heat is transferred from the refrigerant to the outside air. The final stage occurs in the expansion valve where there is an obstruction to the flow of the refrigerant which causes a tremendous pressure drop. The high-pressure liquid is thus converted to a low-pressure and low-temperature liquid. This cold liquid goes to the evaporator and the whole cycle repeats again.
To be able to go through these four stages effectively and efficiently, a refrigerant should possess certain properties. We would have a look at these properties below.
Properties of Refrigerants
Here are some of the properties a good refrigerant should have and a detailed explanation of why it should have these properties.
Low freezing point: When the refrigerant goes through the expansion valve in the refrigeration cycle, it experiences a tremendous pressure drop and hence a tremendous decrease in temperature as well. It is therefore important that the refrigerant has a low freezing point than the normal operating conditions. This prevents the possibility of blockage of passages during flow of fluid through evaporator.
Low boiling point: In the evaporator, heat is transferred to the refrigerant and this causes it to turn into gas. It is very important that the refrigerant has a low boiling point, that is it should have the ability to turn into gas easily when it absorbs heat. If it has a high boiling point, the compressor will have to create too much of a vacuum in order to lower the pressure to make vaporization happen.
Low condensing pressure: The lower the condenser pressure, the lower the power required for compression. Higher condenser pressure will result in high operating costs. Refrigerants with low boiling points will have high condenser pressure and high vapor density. The condenser tubes have to be designed for higher pressures which increases the overall cost of the equipment.
High heat of vaporization: For every kilogram of refrigerant that gets vaporized at the evaporator, it should take away a large amount of heat from the refrigerator. This is important because the higher the value of heat taken by the refrigerant, the greater the cooling effect accomplished.
High critical temperature and pressure: The critical temperature of the refrigerant should be as high as possible above the condensing temperature in order to have a greater heat transfer at a constant temperature. If this is not taken care of, then we will have excessive power consumption by the refrigeration system. The critical pressure should be moderate and positive. A very high pressure will make the system heavy and bulky whereas in case of very low pressures, there is a possibility of air leaking into the refrigerating system
High vapor density: Refrigerants with high vapor density or low specific volume will require a smaller compressors and velocity can be kept small and so the condenser tubes used will also be in smaller diameter. If the vapor produced after vaporization of the liquid at the evaporator coil occupies a minimum volume, pipeline diameter and compressor size can be kept small and compact.
It should also be noted that since boiling point and gas density are affected by pressure, refrigerants may be made more suitable for a particular application by choice of operating pressures.
Some other desirable properties that a refrigerant should have include:
- Non-flammable and Non-explosive
- Compatible with crank case oil, oil seals, gaskets, etc.
- Easy leak detection possible
- Environmental friendly
- Easily available
- Easily stored
Desirable and Undesirable Properties of Water as a Refrigerant
It should first be pointed out that most substances used as refrigerants do not possess all the properties required of a good refrigerant. Most substances which make good refrigerants possess most but not all the properties and hence some compromises have to be made.
Water has several desirable properties that make it a good refrigerant. First and foremost it is cheap and easily available. It is non-toxic, non-corrosive and environmentally friendly. It also has a very high heat of vaporization which makes it absorb a lot of heat when it boils.
There are two major properties of water that make it undesirable as a refrigerant. The first is that it has a high boiling point and the second is that it has a high freezing point. Its freezing temperature and boiling temperature are too high and too distant from each other.
The main problem with using water as a refrigerant lies in the compression stage of the refrigeration cycle. One desirable property of a refrigerant is that it should have a low boiling point. In order to lower the boiling point of water, we need to subject it to extremely low pressures. These pressures cannot be obtained with a conventional compressor and also water creates such a volume of vapor that the compressor needed would be huge. Even if we managed to design such a compressor, it would take a lot of energy to get it down to such low vacuum pressures that the refrigeration system would end up being inefficient. Water is as such an inefficient refrigerant because it requires too much power to make any sort of refrigeration happen.
However, water is actually used as a refrigerant in evaporative coolers that cool air through the evaporation of water. Evaporative coolers differ from typical refrigeration systems which use vapor-compression or absorption refrigeration cycles. Evaporative cooling works by employing water's large enthalpy of vaporization.