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Air Conditioner Basics Tutorial: Introduction To Air Conditioners

Posted By Nicholas Brown.

How Air Conditioners Work

Chapter 1

I will start this air conditioning basics tutorial on a very simple and non-technical level with layman’s terms, and then discuss technical details later.

Table Of Contents

  1. Understanding Cooling
  2. Basics Of Residential Air Conditioner Operation

Snowy Air Conditioner Condensing Unit. Image obtained with thanks from siette on Flickr.
Snowy air conditioner condensing unit. Image obtained with thanks from siette on Flickr.

Understanding Cooling

First, you must understand what cooling actually is and how it’s done. Cooling is the removal of heat from a substance. There are many different cooling methods, but most of them are based on the same principle, which is expansion.

Alcohol feels cool in your hand because it is evaporating. Evaporation is when the particles of  alcohol move further away from each other (this is called expansion) until they turn into a gas (vapour). This is not to be confused with the fact that cooling a substance with a refrigerator, for example actually does the opposite, which causes particles to move closer to each other.

If you don’t understand particles, then all substances and objects are made of millions of tiny particles that you cannot see which appear to be one large solid object to you.

As the alcohol gradually turns into a gas, it absorbs heat from your hand. Cold is simply a lack of heat, cold is not “produced”.

All substances contain at least a little heat, even if they are cold. They only feel cold to you because they are colder than your hands. Ice feels cold in your hand because it is absorbing heat from it.

A physics rule is that heat moves to cooler places. If you put your finger which is 98 °F on a light post which is 8 °F cooler than your hand, then some heat from your hand will automatically move into the light post. This tip will be helpful later in the tutorial.

Air Conditioner Operation

How The Vapour Compression Cycle Works/How Phase Change Refrigeration Technology Works

You probably realized by now that A/C cools air by removing heat from it.  This is why air conditioners are sometimes referred to as heat pumps. Their evaporators (indoor unit) remove heat from the air, and that heat is then transferred to the condensing unit outside. A picture of a condensing unit is at the very top of this page.

Phase-change air conditioners contain many parts, but I will discuss the three parts that actually remove heat from the air, and the fans: They are the evaporator, compressor, and condensing unit. Please visualize only those three parts until I start discussing the others, for the sake of simplicity.

Coloured diagram of the vapour compression cycle that conventional air conditioners use.
Coloured diagram of the vapour compression cycle that conventional air conditioners use. Reuse of this image is permitted provided that you don’t crop out the copyright.

Step 1: A compressor continually forces more and more of the refrigerant in gas form into the condenser, and as it does that, the particles of the gas are forced to move closer to each other until it condenses into a liquid. This is called a phase change, hence the use of the term ‘phase change’ to describe the technology.

During the compression process, heat is radiated from the refrigerant by the condenser coil, and the fins attached to it which make it easier for the air to absorb heat from the refrigerant. That helps to condense the refrigerant, because cooler gases condense more easily.

The condenser fan accelerates the cooling process by passing cool (room temperature) air over the condenser coils, because cooler air is capable of absorbing more heat than hot air. As the particles of the refrigerant gas move closer to each other, that causes the refrigerant to release heat.

Step 2: After being condensed in the condenser, the compressor still forces refrigerant into it continuously and that forces it out of the condenser through what is called the capillary tube (not all designs are like this, some air conditioners use expansion valves). The capillary tube is a very narrow tube which leads into the evaporator and restricts the flow of refrigerant.

As refrigerant enters the evaporator, which is under low pressure (refrigerant evaporates faster in low pressure areas) due to the fact that the compressor is sucking from it, the refrigerant evaporates in it very quickly, and that evaporation process causes the refrigerant to absorb heat. The faster it evaporates, the colder it becomes. This is the second phase change that takes place in air conditioners and refrigerators utilizing the vapour compression cycle.

The cold refrigerant gas then absorbs heat from the evaporator coils (because the gas is cold) and it enters the compressor afterwards, still cold, and absorbs heat from the compressor, cooling it and extending its life. The evaporator or blower fan draws air from the room and blows it over the coil evaporator, which cools it, and then step 1 is repeated.

Understanding Air Conditioner Capacity

A conventional air conditioner is an appliance that removes heat from your room, and radiates it outside. The cooling capacity ratings (normally rated in BTU or kW) are a measure of how much heat the unit can remove from your room per hour. This means that a 24,000 BTU air conditioner would pump 24,000 BTU of heat outside for every hour (assuming it’s turned all the way up) that it’s turned on. The BTU rating of the unit needs to exceed the heat gain of your house (the amount of heat entering the house) in order to operate reliably, efficiently, and to keep your room at the desired temperature.

Air conditioner sizing formulas have been created to size units efficiently. For example, in certain megathermal climates, the required unit size is calculated by multiplying the square footage of the room by 80, as the size of the unit required is somewhat proportional to room size.

BTU means British Thermal Unit. 

Air conditioner capacity is often measured in kW or tons.

  • 0.5 tons = 6,000 BTU.
  • 1 ton = 12,000 BTU.
  • 2 ton = 24,000 BTU.
  • 3 tons = 36,000 BTU.
  • 4 tons = 48,000 BTU.
  • 1.75 kW = 6,000 BTU.
  • 3.5 kW = 12,000 BTU.
  • 7 kW = 24,000 BTU.
  • 10.5 kW = 36,000 BTU.
  • 14 kw = 48,000 BTU.

Many air conditioner manufacturers and sellers offer air conditioners in capacities incremented by 12,000 BTU (or 1 ton) like above.

Further Reading: Properties Of Common Refrigerants

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