What is Air Conditioning and the role of modern unitary systems

[fusion_builder_container hundred_percent=”no” equal_height_columns=”no” menu_anchor=”” hide_on_mobile=”small-visibility,medium-visibility,large-visibility” class=”” id=”” background_color=”” background_image=”” background_position=”center center” background_repeat=”no-repeat” fade=”no” background_parallax=”none” parallax_speed=”0.3″ video_mp4=”” video_webm=”” video_ogv=”” video_url=”” video_aspect_ratio=”16:9″ video_loop=”yes” video_mute=”yes” overlay_color=”” video_preview_image=”” border_size=”” border_color=”” border_style=”solid” padding_top=”” padding_bottom=”” padding_left=”” padding_right=””][fusion_builder_row][fusion_builder_column type=”1_1″ layout=”1_1″ background_position=”left top” background_color=”” border_size=”” border_color=”” border_style=”solid” border_position=”all” spacing=”yes” background_image=”” background_repeat=”no-repeat” padding_top=”” padding_right=”” padding_bottom=”” padding_left=”” margin_top=”0px” margin_bottom=”0px” class=”” id=”” animation_type=”” animation_speed=”0.3″ animation_direction=”left” hide_on_mobile=”small-visibility,medium-visibility,large-visibility” center_content=”no” last=”no” min_height=”” hover_type=”none” link=””][fusion_text columns=”” column_min_width=”” column_spacing=”” rule_style=”default” rule_size=”” rule_color=”” class=”” id=””]So, it’s back to basics.

Unitary systems are very popular and these units do offer some advantages, perhaps the most obvious are ease of installation and lower initial cost.

But let’s look into what air conditioning is.

Air-conditioning is the simultaneous control of:

  • The temperature of the air i.e. by cooling or heating
  • The humidity ratio (moisture content) of the air i.e. by humidification or de-humidification
  • The movement of the air i.e. by the supply and distribution of the air
  • The cleanliness of the air i.e. by filtration
  • The freshness of the air i.e. by the introduction of outside air

There are, of course, many air-conditioning systems that do not do all of the above. That does not mean that it is not really an air-conditioning system, but that all the requirements are not necessary met in that particular case, or have not been included in the plant.

What is air-conditioning, depends on the point of view that is being considered. A man working in an office would probably say “keeping cool”. A man in charge of a computer room would probably say that it means closely controlling the temperature, the humidity and the dust content of the air so that the computers will operate correctly. The one statement is from the standpoint of human comfort, the other is about a commercial consideration.

So, let’s look at all the aspects that involve “Air Conditioning”…

Human Comfort Control

The human body is a heat generating device. Its normal temperature is 37°C. This temperature is very closely controlled by the body. It does this by four methods – convection, radiation, conduction and evaporation. Controlling the space temperature ensures that the body can maintain its temperature.

The moisture content of the air is indicated by a feeling of dryness in winter or that sticky feeling in summer. Sometimes a person can be uncomfortable due to the relative humidity being too high or too low in spite of the temperature being correct. The relative humidity in an air-conditioned space must be in a range so that the air will be dry enough in summer to absorb body perspiration. In winter, the air should not be so dry that skin, nose and throat have that dry sensation.

High humidity can also cause mould, mildew and rust, whilst low humidity can dry out furniture and food.

The body also requires “fresh air” to renew its oxygen supply and/or to dilute undesirable odours. (I often encounter problems here).

Stated simply, the body needs a comfortable and healthy atmosphere and for this the temperature, humidity, movement and cleanliness of the air must be treated.

Comfort Zone

It is logical to ask what the desired temperature/humidity relationship for comfort conditions is.

The answer is that there is no one specific condition. People react differently to different conditions. ASHRAE (the American Society of Heating, Refrigeration and Air-conditioning Engineers) conducted a research study over many years, checking the reaction of large numbers of people to establish a range of combined temperatures, humidities and air velocities that provide the most comfort. This is known as the “comfort zone”. Each combination is known as an “effective temperature” (ET). It was found, for example, that with a given air velocity, a number of different combinations of dry bulb temperatures and relative humidity readings would give the same feeling of comfort to over 90% of the people involved.

From the comfort zone it can be determined what combination of dry bulb temperature and relative humidity will produce that sensation.

Basic Winter and Summer Comfort Zones

Basic winter and summer comfort zones

ASHRAE offers an effective temperature chart and the comfort range.

This chart is useful since it explains at what level temperature and humidity must be controlled. It is representative of the conditions in spaces where long occupancy occurs. However, it is not completely accurate for situations where short duration of occupancy occurs. Therefore, when designing a system, the client must always be consulted for his requirements.

In the past, it was general practice to design for an indoor summer condition of 22°C with a relative humidity of 50% and a winter condition of 20° with a virtually uncontrolled RH.

Since the energy crisis, design conditions have changed. The United States Government recommended a winter inside condition of 20°C and increasing the summer design temperature to 26°C for government buildings and it encouraged private industry and home owners to adopt similar practices. It must be remembered that the lower the summer inside design temperature, the greater are the capital and running costs of the plant.

Air Motion

Air motion is another factor in comfort considerations.

The comfort chart presented later in this chapter is based on an air movement of below 0,1m/s. The effective temperature drops sharply as the velocity is increased. This would seem desirable for summer air-conditioning, but as the conditioned air is usually supplied at 8 to 11 K below room conditions, higher air velocities create uncomfortable cold draughts. A good rule of thumb is not to exceed 0,25 m/s velocity in the occupied zone.

Too little air circulation should also be avoided because people tend to feel “closed in.”

Air Cleanliness

Cleanliness and ventilation are the last two necessities for proper air-conditioning. The two are, to some extent, related and work against each other.

“Fresh” or “outside” air is often contaminated with ‘impurities such as dust, pollen, smoke, fumes and chemicals. These must be filtered out of the outdoor air that enters the structure. In addition, dust created inside the building must be removed from the recirculated air.

The efficiency of filtration depends on the type of filters used. However, even with the most efficient filters, a degree of ventilation air (forced introduction of outside air) is needed to eliminate that stale, dead air feeling, to dilute odours, to supply oxygen for breathing and to supply make-up air for exhaust fans. The amount of outside air needed depends on what the conditioned space is used for, the number of people present and/or the amount of air exhausted.

ASHRAE recommends a minimum of 2,5 litres per second per person. The highest amount of these two is the amount of fresh air to be supplied. In Southern Africa the recommended value is 7,5 lls per person.

I ran into a situation where a company wished to use office space for a call centre. The call centre was to be constantly manned 24 hours a day, 365 days a year. The occupancy would be at an average of 180 operators. Split type air conditioning was installed to deal with the heat load. The complaint from the call centres management was that the operators working in the call centre kept opening the windows, even though they were instructed not to do so, as the area was air-conditioned. I wonder why? This is often an issue, completely ignoring the need for ventilation.

I also often see board rooms with split type air-conditioning. So – we what to discuss confidential issues in a board room so we close the doors. We have air conditioning so we close the windows. And then? When I last looked we do need some oxygen, very unhealthy.

Medical Considerations

Air-conditioning may contribute to better health as a result of controlling air temperature, humidity, cleanliness, ventilation and motion. Some physicians believe that air-conditioning can provide an environment that is easier to accept for people suffering from ailments such as heart trouble. Just moving around in extreme heat can put an undue strain on vital organs.

An effective air filtering system may relieve the suffering of asthmatic and allergic patients.

It has been proven conclusively, that there is some belief that proper humidity also protects health. Adequate humidity in the air can help the membranes in the nose and the respiratory tract to remain moist. This alleviates the effect of bacteria and viruses. Lack of humidity also promotes dryness of dust sources keeping the dust airborne.

Some people complain that air-conditioning makes them sick. On investigation it is often found that the systems in question are not maintained, produce draughts, or are not properly controlled and produce rapid temperature swings. In shops, banks, etc., where people come from outside to inside for short durations only, “temperature shock” is often experienced if the difference between inside and outside is more than approximately 7 K.

Economic Considerations

The capital and operating costs of an air-conditioning system varies from per square meter of conditioned space.

Higher rentals can also be obtained for air-conditioned spaces.


Humidity control is part of air conditioning. The percentage of relative humidity desired depends on the application. For human comfort we normally work around 50% Relative humidity.

For commercial applications, hospital theatres, computer rooms, clean rooms this may vary and will be stipulated by the user. Unitary systems of course do not control humidity.

Other Benefits

Commerce and industry have used air-conditioning firstly to increase personal productivity and, secondly, to provide space process-cooling for specific needs.

Peoples productivity in air-conditioned areas has improved in terms of less absenteeism, less labour turnover, less noise distraction, more efficient production, fewer mistakes and less time lost due to heat fatigue and accidents.

In general, better morale and better relationships between employee and employer result.

The degree of benefit, of course, is subject to the type of surroundings and the type of work carried out.

So, there you have it, air conditioning.

Grant K Laidlaw

Grant K Laidlaw


References: ASHRAE and ACRA[/fusion_text][/fusion_builder_column][/fusion_builder_row][/fusion_builder_container]

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