Tuesday, April 22, 2014

Some Technically Wise Considerations For Buying and Installing an AC For Your Home or Office!

This article is for the guidance of those thinking of buying and air conditioner for their home of small one room office located in the cities and town of power deficient India.

Being a very populous country, India needs electric power in huge quantities even when its average domestic consumption is much lower than the developed nations. As per the 2012 data, the average domestic power consumption per year was about 750 units (KWh). This translates into about 3500-4000 units per family on the average per year or about 290-330 units per month per family. Since this is an average value, the higher income families must be consuming much higher as the a good percentage of lower income group families might be consuming zero to 30 units per month. It is also interesting to note the wide disparity of domestic electric power usage across the various regions and states of India.

Electricity requirement for lighting, fans, TV, etc are much less as compared to its requirements for the purpose of water pumping, cooking, air conditioning etc. In reality, those homes which use air conditioners consume much more electricity than those who do not have air conditioners.
Again, air conditioners which are of old designs and wrongly selected draw much more electricity. Hence, it is very essential that air conditioners are selected and used with some care. There is ample scope for reducing your electricity bills if you are a bit careful with regard to your air conditioners.

Most people are not very clear about the working principle of a common air conditioner unit. Any one who is thinking of purchasing and installing an air conditioner for home or small office purpose should try to understand the essential principle of working of an air conditioner.

An air conditioner is essentially used to cool air and remove humidity. Humidity in air is due to the presence of water vapor in air. Normal room air conditioner units also remove micro fine dust particles in air using an air filter. Some air conditioners also have electric heating coils in built which could be used for heating the air in winter time. However, cooling happens to be the primary function of a room air conditioner that we use today.

Cooling in air conditioners take place using the Joule-Thomson effect. When a gas is compressed suddenly (the scientists call it adiabatic compression) it gets heated up to expel the heat which it had. If the compressed hot gas in a pipe or container is kept for some time it cools and some gases even become liquids. When this compressed and cooled gas (which might have become a liquid) is suddenly expanded its temperature goes down much below the surrounding temperature. This cold gas can absorb heat from the surroundings which is at a higher temperature. 

This principle is made use in the air conditioners that we use for our comforts in our homes. An air conditioner essentially has a refrigerant gas ( a gas which is easy to compress and has some other desired qualities) contained within a hermetically sealed system of copper tubes attached to a compressor unit. It also has one or two fans for circulating air externally through the two tube coils kept separately. The principle of working of an air conditioner is shown in the figure below:

The refrigerant now commonly used in domestic air conditioning systems is a chemical substance called Hydro-chloro-fluro-carbon (HCFC) which is commonly now known as R-22. This substance when leaked to the atmosphere causes ozone layer depletion in the atmosphere and hence considered as not so environment friendly. Hence, alternatives are being experimented.

The air conditioner unit has two pipe coils, called the vaporizer and the condenser interconnected using and incoming and out going copper tubes passing through an electric motor operated gas compressor. The coils and pipes are filled with the refrigerant gas such as R-22. When the compressor starts, it continuously compresses the gas coming from the vaporizer coil placed inside the room to be cooled. The gas inside the vaporizer expands and cools and the room air is blown through it by a fan becomes cool. The expanded gas from the vaporizer moves to the compressor to get compressed and the refrigerant gas at the outlet of the compressor becomes hot due to compression . The hot compressed gas then moves to the condenser coil kept outside the room and its heat is dissipated to the air outside from the condenser coil. The compressed thus gets cooled in the condenser coil to become a liquid. The liquid refrigerant moves to a liquid gas separator from which only the liquid refrigerant is sent back to the vaporizer coil to repeat the process.

So long as the compressor keep working the refrigerant takes out the room heat and expel it to the outside through the condenser. The condenser is sometimes called the radiator as it radiates or discharge heat to the outside air. 

In a window air conditioner all these systems are suitably kept inside one packing. The vaporize and the fan or blower that circulates the room air is kept inside the room and the condenser is placed outside through the opening made in the wall. 

Now-a-days, split air conditioners are common. In this, the compressor, condenser and its fan unit are placed in one pack while the cooling unit (vaporizer) with its fan is placed in another pack. The cooling unit houses the electrical and electronic controls and connects to the compressor unit using electric power cables and refrigerant piping which are connected skillfully after installation without the gas getting leaked. The window air conditioners come as factory fitted and tested with initial refrigerant filling.

The air conditioners also has a thermo-stat switch which keeps keep the compressor unit on or off depending upon the temperature of the cold air in the room.

The capacity of air conditioners to cool is designated in terms of ton refrigeration (TR). One TR is equivalent to a cooling rate of 3024 Kilocalories/hour (kcal/hr). Standard sizes of room air conditioning units used for home and small offices are 0.75 TR, 1 TR, 1.5 TR, 2 TR etc. Commonly, TR is simply called ton. So we hear about one ton, 1.5 ton, 2 ton and higher rated AC s. 

The compressor is the key moving machinery of an air conditioner. The compressor technology has advanced much in recent years. In old air conditioner, the compressors used to be reciprocating type. These used to be big and heavy and used to draw much electricity. In modern compressors, rotary compressor technology is used. The power consumption for rotary units are much lower for the same TR.

Air conditioner traders usually try to sell higher tonnage machines which are costlier and draws much higher electric power. As a simple thumb rule, a room of size 10 ft x 10 ft x 10 ft high which is only exposed to direct sunlight only on one or two sides can easily be managed with a 0.75 TR AC. However, people might be tempted to go for an AC of 1.5 TR quite often. Of course, the cooling rate of the higher ton machine is faster. But ACs are designed to work on-off using the thermostatic temperature control. A lower ton AC works a few minutes more than a higher capacity one under similar conditions. 

Electric Efficiency rating (EER) with star ratings are used for designating energy efficiency of electric appliances. The highest rating is 5 star and it has the lowest power consumption. But between a 2 or 3 star and a five star rating, the practical net savings on electricity is not much for an average user because the cost of the machine with higher star ratings usually much higher than the lower star rated machines. When the cost difference is in thousands of rupees, it is not prudent to go for the highest star rating unless the machine is intended for continuous working throughout the year which seldom happens.

Window ACs are to be preferred in place of split ACs if your room layout allows installation of window AC. Split AC should be the choice only if Window AC cannot be installed suitably with its condenser radiator in the open. Again, use a split AC when there is a risk of some one intruding through the window AC opening.
In split ACs since the compressor unit could be located quite at a distance (such as the roof top) the likelihood of room noise could be much lower. The only noise you hear is of the fan of the cooling unit. However, split ACs are not a well suited choice if the unit is required to be shifted quite often. 

In my opinion rotary type window air conditioners should be the first choice for small and medium homes and offices having rooms which are not so big. It is also better to limit its size to 0.75 TR or 1 TR. This is because, the lower TR machine draws much lower current from the house hold electric circuit. For example, a 0.75 TR window a/c consumes less than 1000 W of electric power if it is a rotary compressor type with at least 2 star rating. It may cost around Rs.15000/- while a 1.5 TR machine of similar rating would cost around Rs 26000/- and consumes about 1800 W of electric power. Obviously the lower rated ones are better both on account of initial costs and running costs. The higher rated ones would start and stop more making your home's electric wiring prone to higher sparking and burn outs. Again, the machines with reciprocating compressors for the same TR rating draws much higher electric power. Such second hand and reconditioned a/c machines are available in the grey markets in India at throw-away prices. However, their running costs would be very high if they are regularly to be used.

In places where humidity is low and water supply is not a big issue, the evaporation type coolers are much preferable both on account of initial and running costs. As the summer heat goes up with lowering of humidity in the air, these coolers give very comfortable cooling. However, they are not useful when humidity goes up during the onset of rains after summer. They are also not very effective in the coastal areas of India due to high humidity.

It is also not a very prudent decision to spent much money for fancy features of air conditioners. Of course, such fancy features are for those for whom money is not a big issue.

The best brands in air conditioners in India are Voltas, LG, Carrier Aircon, Samsung, etc. Voltas is totally an Indian brand and the current market leader.

Though I have advocated for window a/c of one or 0.75 TR, such machines are apparently in short supply as the traders normally do not seem to be keen in selling these for obvious reasons. So, if you are thinking of low tonnage AC, you have to be a little persuasive with the dealers. 


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