Air Conditioning Key Concepts
Questions & answers
- What is air conditioning?
- What is relative humidity?
- Is there a difference between comfort conditioning and industrial conditioning?
- What is meant by a ‘ton’ of refrigeration?
- What is direct expansion equipment?
- What is mixed flow ventilation / air conditioning?
- …in view of this is it not possible to use direct expansion equipment in a low level displacement application?
- If it were possible to vary the off coil temperature during the cooling cycle of direct expansion equipment wouldn’t this have the same advantage as using a chilled water system?
- When would a system be designed with chilled water as the cooling medium rather than direct expansion equipment?
- What about heating?
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 humidistat 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.
From the foregoing therefore the term ‘ventilation’ should not be confused with air conditioning as refrigeration equipment is not necessarily provided with ventilation equipment.
What is relative humidity?
The atmosphere always contains moisture in the form of water vapour. The maximum amount of water vapour that may be contained in the air depends on the temperature of the air and the higher the temperature of the air, the more water vapour may be contained. At high temperatures and high moisture contents extreme discomfort is experienced as the evaporation of moisture from the body into the atmosphere by the process of perspiration becomes difficult.
In the air conditioning process the moisture content of the air may be reduced by the use of a cooling coil or added by the use of a humidifier.
The term relative humidity is simply a ratio between the actual moisture content of the air compared with the moisture content of the air required for saturation at the same temperature, ie at 100% relative humidity (also known as saturation point).
The air conditioning engineer uses the psychrometric chart to analyse how the state of moist air alters as an air conditioning process takes place.
Is there a difference between comfort conditioning and industrial conditioning?
Yes, the object of comfort conditioning as the name implies is solely to provide a comfortable environment for the majority of occupants. Humans are reasonably tolerant to humidity and may be comfortable from a range of between 55% and 20% relative humidity at normal comfort temperatures. It is therefore common when specifying to limit the humidity in summer and not specify a limit in winter. Typically therefore a specification would state an internal condition of 22°C / 50% relative humidity being maintained at 30°C / 20°C wet bulb external conditions in summer. In winter the specification may typically be 21°C internal temperature at -3°C saturated outside air temperature.
Industrial conditioning is provided generally for a process which requires a closely controlled atmosphere. A typical specification may be that an internal environment is required of say 21°C ±0.5°C and 50% relative humidity ±2.5% at all external conditions. It will be seen therefore that the industrial conditions for clearly defined limits rather than comfort conditioning which is based on statistical surveys of occupants feelings.
What is meant by a ‘ton’ of refrigeration?
Confusingly the unit has little to do with weight, as used in common parlance. One ton of refrigeration is the term used to refer to 12,000 B.T.U.s/hour (British Thermal Units/Hour) of cooling effect. Thus a chiller or condensing unit with a cooling capacity of 60,000 B.T.U.s/hour is said to have a capacity of 5 tons. It should be noted that the unit B.T.U./hour is a unit of heat flow still widely used in North America, Canada and parts of Asia whereas Europe uses the ‘watt’. One ton of refrigeration approximates to 3.5kW of cooling.
The origin of the term is the amount of heat absorbed by one ton of ice when melting from solid to liquid state at 32°F and assuming a latent heat of ice of 144 B.T.U.s/lb. The heat absorbed is found to be 288,000 B.T.U.s over 24 hours, or 12,000 B.T.U.s/hour (in reality the latent heat of ice is slightly less than 144 B.T.U.s/lb.)
What is direct expansion equipment?
‘Direct expansion,’ ‘DX,’ ‘refrigeration’ or ‘split’ units are all generic terms used to identify the same equipment. The terms are in fact rather loose but in any event it has become accepted that the terms refer to two or more units, one usually positioned externally and one or more usually positioned internally. The units are connected together by site installed refrigeration pipework which is charged with a refrigerant. The external unit may take one of three forms:
- The heat pump - which consists of a fan, compressor, coil and reversing valve, and rejects unwanted heat to atmosphere during the cooling cycle and extracts heat from the atmosphere during the heating cycle.
- The condensing unit - which is as described above but does not have a reversing valve and therefore cools only.
- The condenser which consists of a fan and coil (as the compressor is contained in the indoor unit); the condenser is used less often than (a) and (b).
The indoor units consist of fan coil units or air handling units which may be located in the atmosphere being air conditioned or remotely in a plant room.
Some manufacturers produce ‘external’ units that may be located internally and in the case of these units ductwork is usually connected to atmosphere to reject heat or extract heat.
DX systems are in direct contrast to hydraulic systems or chilled water systems. With these systems cooling is achieved by circulating chilled water with a hydraulic pump
Generally speaking with direct expansion equipment the manufacturers match the indoor and the outdoor units and many well not sell the units individually for fear of ‘mismatching’ occurring.
What is mixed flow ventilation / air conditioning?
Mixed flow describes a method of air distribution from an air conditioning or ventilation system.
It has been and still is the most widely used method of supplying air into an atmosphere being air conditioned. This form of air distribution commonly uses ceiling diffusers or wall grilles at high level. As most air conditioning units are manufactured for the mixed flow air distribution market the associated air handling equipment has also been developed for the mixed flow market.This air handling equipment is therefore widely available and at very competitive costs as there are many manufacturers of this equipment throughout the world.
Recently displacement air distribution has gained in popularity, mainly due to comfort and cleanliness considerations. (With displacement ventilation air is introduced into the air conditioned space at low level and at low velocity). As the displacement market is still much smaller than the the size of the mixed flow market the complimentary air handling equipment has lagged behind and has generally to be made to order and is thus more expensive.
In view of this is it not possible to use direct expansion equipment in a low level displacement application?
When using displacement ventilation it is necessary to design the system so that the during the cooling cycle the air temperature being supplied to the space is at a higher temperature than is used for mixed flow ventilation - this is because of comfort considerations. However, if it is accepted that the air temperature adjacent to low level outlets is uncomfortably low and the layout of the space can be made to accommodate this then it may be a cost effective solution - particularly for high spaces such as large studios where it is inevitable a vertical temperature gradient arises but is of little consequence how high the air temperature is above the occupied zone. This system breaks all the rules of accepted displacement design but may be an acceptable compromise.
If it were possible to vary the off coil temperature during the cooling cycle of direct expansion equipment wouldn’t this have the same advantage as using a chilled water system?
It is possible to vary the off coil temperature using the direct expansion equipment by the use of ‘hot gas bypass’ and this would make it acceptable for displacement. There are a few manufacturers that modify their equipment for ‘hot gas bypass’ using larger compressors, and this equipment may be applied to displacement ventilation.
When would a system be designed with chilled water as the cooling medium rather than direct expansion equipment?
There are several considerations:
- If the air conditioning system, no matter how small, was to be designed as a conventional displacement system then chilled water, with its inherent infinitely variable off coil temperatures, would generally be required.
- Direct expansion systems have considerably less capital costs on small to medium sized installations. However, the capital costs of both types of systems become comparative after about a cooling load of about 1000kW. (This figure varies greatly depending on the site layout.)
- Chilled water should be used for systems where flexibility is required i.e. where partitions and room layouts are to be altered regularly.
What about heating?
Chilled water systems require a separate source of heating. This would normally be in the form of a boiler and a hot water flow and return similar to the chilled water system. It is possible to provide chilled water and hot water through the same set of pipes, (known as a manual or automatic changeover system) but these systems are more appropriate in offices when there is a clearly defined heating and cooling changeover season.
An alternative is to use electrical resistance heating although the running costs are usually prohibitively high.
Direct expansion equipment may utilise a heat pump which heats and cools through the same set of pipes.