The Different Types of Air Conditioning Systems and How They Work to Control Temperature and Humidity within a Room or Building

Summary: In this project guide we look at air conditioning system and how each different air conditioning system works. You will learn about relative humidity and the moisture content of air and how air conditioners can be used to control this, what differences there are between domestic air conditioning and industrial, and why these systems are measured as a Ton and what 1 Ton equals in the more familiar Kilowatts. You will also find out how to calculate the required BTU output of an air conditioner to make sure that it will be adequate for the space you need to cool and also about the different setups such as split air conditioning systems, packaged air conditioning systems, wall fitted units, central conditioning systems and also the more familiar UK portable air conditioner.

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Although air conditioning units (also known as HVAC systems) are not commonplace in many UK homes they do feature in many office buildings, places of work and entertainment venues. If an air conditioning unit does feature in the home, it tends to be one of the portable units that doubles up as a dehumidifier.

Despite most common misconceptions that conditioners only cool the air, some of the more integrated models can provide heat and other features.

Though mostly found in and associated with hotter climates such as north and south America, middle east etc… where the climate warrants such a device, they can certainly also have benefits in a climate such as ours. The UK often suffers from high humidity which can make working conditions difficult and a conditioning unit can certainly be used to control this.

A common industry term associated with such cooling and heating systems is “HVAC”. This stands for “Heating, Ventilation and Air Conditioning” as this acronym covers the three core features of most common types of setup.

What is Air Conditioning?

The purpose of air conditioning is to control the filtration, air movement, temperature and humidity of an atmospheric environment. Air conditioning is always associated with the cooling and dehumidification process of air and is always therefore identified with refrigeration equipment.

The full control over relative humidity by the addition of moisture by means of a humidifier and the use of a humidifier constitutes full air conditioning, but this control is not always exercised. However, the more often used partial or comfort air conditioning which uses refrigeration equipment only and is therefore capable of cooling as well as dehumidifying is still referred to as air conditioning.

In respect to the latter, the term 'ventilation' should not be confused with air conditioning as refrigeration equipment is not necessarily provided with ventilation equipment.

Common portable air conditioner

Common portable air conditioner

How Does an Air Conditioning System Work?

Air conditioning units work in a similar manner to fridges and freezers but with one critical difference. Whereas the fridge or freezer works to cool or freeze items within itself, and air conditioner works to cool the air around it.

They do this by moving all the warm air from inside an area to the outside and then circulating cooled air back into the space. This is achieved in the following manner:

  • This section houses a compressor, fins for cooling and a fan (including also required pipework). Within the fins there is pipework that contains a special coolant fluid. Fans suck air across the fins and pipework, cooling the coolant drastically
  • The coolant is then pumped by the compressor to the evaporator, through copper piping and the coolant drastically cools the pipes and fins that are featured on the evaporator unit
  • A fan system then sucks the warm air from the room space through the evaporator, which is cooled drastically. The evaporator also dehumidifies the air and removes any moisture
  • The cold air is then pumped back into the room space, the hot air taken from the room is vented (depending on the type of system) and any moisture taken from the air is drained away to an appropriate source

Some systems also feature a thermostat. In this instance the above process continues until the thermostat sense that the air is at the required temperature and would then shut off. When the thermostat senses that the temperature has risen past the point it is required to be at, the system springs into life and starts cooling the air once more.

How air conditioning works

Basics of how an air conditioning system works

What is Relative Humidity?

The air and the atmosphere constantly contains water and moisture that resides as water vapour. The volume of vapour that can be contained at any one time is down to the air temperature - The greater the air temperature, the more water it can contain.

When both the temperature and humidity are at high levels, this can become very uncomfortable, even for those that are used to it. The ability for the body to cool itself through sweat becomes limited due to the fact that perspiration is limited.

This is the point that an air conditioner with humidity controls really comes into its own as the moisture levels can be reduced along with the fact that cooling of the air occurs at the same time.

Coming back to relative humidity, this is the ratio between the actual moisture levels in the air, measured against the actual moisture content that would be needed for saturation to take place (the saturation point or 100% relative humidity) at the same temperature.

Relative humidity against air temperature

Chart showing relative humidity against air temperature

What are the Differences Between Domestic and Industrial Air Conditioning?

When it comes to the domestic side, the main goal here is achieve and maintain a “comfortable” environment for any person to be in. This has quite a bit to do with the humidity levels within a given space.

In light of this, most people are at comfort somewhere between 20% and 55% relative humidity.

As humidity levels tend to be higher during the summer months, these are the main times of concern, while the colder months (i.e. the winter) do not necessarily need to be controlled.

A typical summer time temperature and humidity would be something like 22°C with 50% relative humidity.

When it comes to industrial conditioning, the main goal is normally to achieve a very controlled atmosphere for either the comfort of employees or due to a given industrial process(s) that need a controlled temperature.

Due to the vast array of potential requirements it's very hard to give any suggested figures. If dealing with working conditions for employees, temperatures between 20°C and 26°C should suffice, with humidity levels at around 50%.

In light of the above, many workplaces now survey their employees to find the ideal levels, although everyone is different and responds to temperatures in a different way, so it can sometimes be quite hard to find a level that everyone agrees with.

What is Meant by a "Ton" of Refrigeration?

As you may imagine, this has very little to do with an actual measurement of weight. In actual fact the term reflects the standard measurement for the cooling ability of a given refrigeration or cooling appliance.

The measurement itself refers to the volume of heat that is absorbed by a single ton of ice while melting from a solid into a liquid at 32°F. This equates to around 288,000 BTU's over a period of 24 hours, or when broken down, 12,000 BTU's an hour.

When used in a real world situation, an appliance that produces a capacity of 10 tons would equal 120,000 BTU's per hour.

In terms of BTU's, this measurement amount is most commonly used in Canada, North America and some parts of Asia whereas most European countries tend to use “Watts” as a measure of cooling capacity. When comparing Watts to BTU's:

  • 1 Ton = 3.5kW (approx)
  • 1kW = 0.28 Tons (approx)

If you need to convert kW's to Ton's or vice-versa then a handy converter can be found here.

BTU's - Measuring the Required Power for an Air Conditioning System

BTU stands for British Thermal Unit and this measurement is generally used as a heat flow measurement for things such as radiators (estimating number and size of radiators required to heat a room to required temperature) and also for calculating the power required for cooling a given room/building space.

In respect to the measurement scale itself, it is calculated on the basis of how much heat power is needed to increase the temperature of a single pound in weight of water (as a liquid) by a single degree at a constant pressure - heating from 60°C to 61°C.

As you might imagine, when it comes to measuring and estimating the output power of an air conditioning system there are a huge range of factors that can impact this:

  • The overall size of the room or space to be cooled
  • What objects in the room are heat producing e.g. people, computers, photocopiers, servers etc....
  • Where the air conditioning unit is to be installed e.g. is it shaded, in direct sun light etc....
  • The number and size of windows in the space and the directions they face in e.g. if south facing will catch sun light for large parts of the day, generating heat
  • Is the room(s) to be cooled in the centre of the building or on an edge subject to sun light for large parts of the day

As you can see from the above, there are a huge number of factors that can affect the overall effectiveness of an air conditioning setup and this makes it very hard to produce any guidelines on the general efficiency of a given system. However, the following "rules of thumb" give a very rough estimate in terms of what you should look for:

  • A large space 35 - 40 square metres will need at least 15,000 BTU's to maintain cooler temperatures
  • A medium area of 25 - 30 square metres will need at least 10,000 BTU's
  • A small space of around 15 - 20 square metres will need at least 7000 BTU's

What is Direct Expansion Equipment?

This is a bit of a generic term used to describe refrigeration units and some air conditioning systems. Direct Expansion (or “DX” as it is sometimes known) utilises a refrigerant or cooling vapour that is used to cool the air that is supplied to a given room or space using compression or expansion. This covers both split and packaged air con systems.

The expansion part of the system features a valve to reduce both pressure and temperature before the refrigerant enters the evaporator. The system works through the evaporator absorbing heat when the refrigerant that flows within it expands.

The refrigerant is then transferred to the compressor unit where (as you might guess) it is compressed. The act of compressing it causes it to condense in the condenser where any heat is then released and exhausted accordingly.

The condensed refrigerant liquid is then transferred to a thermal expansion controller, which in turn then controls its pressure and flow as it returns back to the evaporator.

As discussed, DX systems can be both packaged and split systems, but what does this mean? Well, we will go into more detail about these systems below, but as a brief summary:

  • Split: The majority of the core components such as compressor, condenser, condenser coil etc… are located externally (normally) and the remaining components such as ducting, fans and evaporator are located within the given structure it is installed in and the two sites are connected and refrigerant liquid is moved around through pipework connecting the two locations
  • Packaged: All the main components mentioned above are contained within the same unit, in one location

Depending on the size and the type of air conditioner and the area that it is serving will in many cases depend on how the system runs. If it is a relatively small system serving quite a large area then when under heavy load it may struggle slightly in terms of issues with humidity, varying temperature output, issues with the compressor and so on.

There are techniques that can be employed on certain systems (possibly not all) to help with these issues called Hot Gas Bypass.

This technique involves maintaining and stabilising the temperature of the evaporator coil regardless of what load it is under by taking the hot gases from the compressor output and adding them to those gases on the output side of the expansion valve.

As mentioned, employing this technique and featuring it within a system very much depends on where it is to be used, the type of system it is and many other factors.

Types of Air Conditioner System Setup

Split Air Conditioning Systems

As the name suggests, this type of unit is split into two different components, one inside and the other outside. This has several benefits in that they can be made to look a little more appealing (unlike some window fitted units) and also as the components are split over two locations, also a little smaller and slim line.

In respect to the separate units - the outside section tends to house the main compressor and condenser unit while the indoor section contains the cooling fan and coil.

In terms of its cooling capacity, this type of unit can really only handle a maximum of two rooms without having to install additional systems. Despite this they really do offer some excellent cooling power and are about the closest you can get to a full blown central system.

In terms of installation, if you opt for a ductless system, these can be relatively easy to install. Despite their name, you will still need to install some pipework to connect the outside section to the inner section, but if floorboards are reasonably accessible then you can fit with limited hassle and without the need to start hacking great holes in your walls. Also, they do tend to be very quiet when running as the noisy parts (compressor etc…) are normally located outside.

Despite the several advantages - reasonably easy on the eye, great cooling output, straightforward installation, but they can be rather expensive.

Split air conditioning system

Split air conditioning system

Packaged Air Conditioning Systems

Where split systems are good for smaller needs (several rooms in a small house/office) and large scale central systems are good for large space, packaged systems tend to sit somewhere in-between e.g. larger houses, smaller entertainment venues, restaurants etc…

As we have established briefly above, all the core components of the system are contained in one single unit (very similar to the window units we will talk about below, but a bit larger).

There are several different cooling methods for this type of system:

  • Those that feature a water cooled condenser
  • Those with an air cooled condenser

With the water-cooled systems, the water has to flow continually for the system to operate correctly and cool the condenser effectively. The units are constructed so that the compressor is located at the base with the condenser, the evaporator is situated above this followed by the air handling objects such as main blower, air filter and cooling coil. Depending on the size of the main blower will depend on how many rooms can be handled.

In an air-cooled system, the core components are located in a single outdoor unit (e.g. compressor, condenser) in a place where it can receive a constant flow of air. The large fan located in the unit sucks air in and over the condensing coil producing the desired cooling effect.

The second unit (commonly called the cooling unit) features the evaporator, the filter and blower and normally (but not always) the expansion valve. All ducting carrying the cooled air then runs from the cooling unit to any rooms requiring it.

The air cooled types do tend to be the most common simply due to the fact that air is freely available pretty much anywhere, whereas establishing a constant flow of water can be much trickier.

Packaged air conditioning system

Packaged air conditioning system

Types of Air Conditioning Units and Systems

In terms of the different types of units, there are several options available depending on your requirements:

Window/Wall Fitted Air Conditioning Units (Packaged System)

You may be familiar with this type of unit from the many American films they are seen in and in terms of the states, these are arguably the most commonly used.

All of the main components are contained within one box that is normally fitted in place of a pane of glass in a window (usually at the bottom, resting on the window sill). The exterior of the unit vents warm air to the outside, while pumping cool air into the interior of the room it is installed into.

If a window is not available to be used for fitting then they can also be installed on a wall.

This type of unit is really only suitable for conditioning and cooling the air within a single room, but if that is all you need to do then they make a fantastic, cost effective solution due to the fact that all the components are in one small self-contained area, making installation reasonable cheap.

Window or wall mounted air conditioner

Window or wall mounted air conditioner

Central Air Conditioning Systems (Split System)

These are the real “top-end” systems and are most commonly used where large areas need to be cooled e.g. large office buildings, shopping centres, leisure centres, entertainment venues etc…. Although more common in large structures, smaller scale setups can also be incorporated into the home.

This type of system normally incorporates several features such as the ability to supply heat, fans for simply moving air around and also dehumidifiers for controlling the humidity of the air.

As their name suggests, all the core elements of the system are normally centrally located (externally, in a plant room etc…) such as the compressor, condenser, evaporator, coils and fans and any cooled air is then blown through a ducting system to the desired locations. Any air return is also carried out via separate ducting. Any extracted warm air is then normally exhausted out of the building from the roof or another location.

The “Split Setup” that we have already discussed above would incorporate this form of air conditioning system.

Zone controls also commonly accompany these systems. This type of setup allows different areas of the building (sometimes down to an individual room level) to be individually set at different temperatures, giving much greater control over the individual elements of the building environment.

To provide heating, the system normally incorporates some form of heat pump or heat exchanger that, instead of transferring the warm air outside when cooling is needed, takes warmth from the outside air and transfers it inside.

Normally the heating element of the setup will be supported by a backup so that if more heat is required then it can be supplied with ease, instead of the pump trying to extract warm air that is just not present.

Despite the costs involved in building and installing systems of this type they are highly economical and efficient and if you are looking to cool large houses or structures then a central air conditioner is certainly one to consider as, regardless of the initial outlay, they will save you overtime when it comes to running costs.

Large central air conditioning system

Large industrial central air conditioning system - Image courtesy of

Portable Air Conditioning Units (Packaged System)

These units are certainly more common in UK domestic situations as opposed to most of the alternatives mentioned in this project.

As you may imagine, they are portable and can be moved from location to location as required. As they are essentially a self contained unit, they do not need any form of professional installation or setup, just plug it in and off you go.

There are several different types of portable conditioner:

  • Single Hose Unit: The unit features one hose that is used to exhaust hot air from the room you are cooling to the outside or other location
  • Single hose portable air conditioner

    Single hose portable air conditioner

  • Dual Hose Unit: The unit features two hoses - one for exhausting the hot air from the room you are trying to cool and the other to help keep the unit itself cool
  • Twin hose portable air conditioner

    Twin hose portable air conditioner

Although ideal for cooling small rooms and other smaller spaces, these units are not well suited to larger spaces and can be a little inefficient.

One of the main issues (especially with the single hose systems) is that when you remove the hot air from a space it causes a drop in air pressure which in turn allows more hot air in through doors, windows etc.... In this situation, the system starts to work against, causing it to work harder and also produce heat within the room itself, making it rather inefficient

Despite the above, if you only need to cool a small room e.g. bedroom, home office etc.... for a few hundred pounds, these units make a great solution.

The majority of modern portable air conditioners will feature an in-built thermostat allowing the unit to shut itself down and turn itself on depending on the air temperature and temperature setting it's been set at.

In terms of settings - most will also feature a digital display that shows the current temperature within he room and also an area that allows the user increase/decrease their desired temperature for the room space.

As mentioned above, most modern units will also feature an included dehumidifier that will extract unwanted moisture from the air while it is being cooled and deposit it in a water tank, normally located at the base of the unit. If this is the case, make sure you check this regularly, especially in areas of high humidity as once the tank is full, the unit will normally shut down and not turn on again until the tank has been emptied.

When it comes to air conditioning systems and domestic situations, the UK is not somewhere you would immediately think of. This thought would probably go to some of the warmer locations in the world.

Despite this, a properly spec'd out system incorporating a heat supply could be something to consider, especially when coupled with a dehumidifier to help deal with high humidity levels during the warmer months.

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