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  • Scholenberg

HVAC - Principles & Functions

Air conditioning is the process whereby the condition of Air, as defined by its temperature and moisture content, is changed. In practice, other factors must also be taken into account especially cleanliness; odour; velocity & distribution pattern.

Principles of Air- Conditioning:

Human comfort

Inevitably 'comfort' is a very subjective matter. The Engineer aims to ensure

'comfort' for most people found from statistical surveys. Most people (90%) are comfortable when the air temperature is between 18-22°C and the %sat is between 40-65%. This zone can be shown on the psychometric chart. And is known as the comfort zone.

Outside air is quite likely to be in a different condition from the required comfort zone condition. In order to bring its condition to within the comfort zone, we may

need to do one or more of the following:-heat it; cool it; dehumidify it; humidify

it; or mix it.

Dry air mass flow

In order to use the psychometric chart for air-conditioning work, we need to find & use dry air mass flows. However, in practice, air-flows are frequently measured in terms of volume flow. In order to find dry air mass flow, we need to use the specific volume of the air.

Specific volume = volume/mass

The specific volume of the air is given from the

The psychrometric chart in m³/kg of dry air, therefore the Mass flow will be in terms of dry air mass flow.

Obviously, the condition of the air must be known (Typically d.b. temp. & %sat) in order to find the specific Volume.

Heat Transfer:

Heat is a form of energy. Every object on earth has some heat energy. The less heat an object has, the colder we say it is. Cooling is the process of transferring heat from one object to another. When an air-conditioning system cools, it is actually removing heat and transferring it somewhere else. This can be demonstrated by turning on a Spot Cooler and placing one hand in front of the cold air nozzle and the other over the warm air exhaust. You will feel the action of the transfer of heat.

Sensible and Latent Heat

There are two forms of heat energy: sensible heat and latent heat.

Sensible heat is the form of heat energy that is most commonly understood because it is sensed by touch or measured directly with a thermometer. When weather reporters say it will be 90 degrees, they are referring to sensible heat. Latent heat cannot be sensed by touch or measured with a thermometer. Latent heat causes an object to change its properties. For example, when enough latent heat is removed from water vapour (steam or humidity), it condenses into water (liquid).

If enough latent heat is removed from water (liquid), it will eventually freeze. This process is reversed when latent heat is added.

Change of State

An object that changes from a solid to a liquid or liquid to vapour is referred to as a change of state. When an object changes its state, it transfers heat rapidly.



Humidity

Moisture in the air is called humidity. The ability of air to hold moisture directly relates to its temperature.

The warmer air is, the more moisture it is capable of holding. The relative humidity is the percentage of moisture in the air compared to the amount of moisture it can hold. The moisture content of 70°F air with 50% relative humidity is lower than 80°F air with 50% relative humidity.

When the humidity is low, sweat evaporates from your body more quickly. This allows you to cool off faster. High humidity conditions do not allow sweat to evaporate as well because the air is at its maximum capacity.

Humidity is also a form of latent heat. When the air contains more humidity, it has more latent heat.

REFRIGERANT

Refrigerants are substances used by air conditioners to transfer heat and create a cooling effect. Air-conditioning systems use specially formulated refrigerants designed to change state at specific temperatures providing optimum cooling. Portables use a refrigerant called R-22 or HCFC-22. HCFC stands for hydrochlorofluorocarbon. This is currently the most common refrigerant used by air-conditioning systems.

REFRIGERANT PHASE-OUT

Many of the current forms of refrigerants used today are being phased out based on concern for the depletion of the ozone layer. Portables use R-22, which has been deemed acceptable for use by the EPA until the year 2010. By that time, an ozone-friendly refrigerant that can be easily substituted for R-22 will be readily available.

Environmental control in buildings

Until recently the use of air cycle has been largely restricted to aircraft cabin air conditioning systems. A recent trial has demonstrated the advantages that air cycle technologies can offer to passenger train air conditioning systems. An important conclusion of this trial was that air cycle train air conditioning systems will have lower overall life cycle ownership costs than comparable vapour compression systems. The successful demonstration of these units in Germany’s ICE2.2 high-speed trains by Normalair-Garrett Ltd. led to the company receiving the Engineering Council’s Environmental Award for Engineers in 1996.

Studies carried out by the Buildings Research Establishment (BRE) and for perc have demonstrated that air cycle systems in buildings would have a number of advantages. These include -

• Lamination of the need to use environmentally damaging CFC, HCFC or other alternative refrigerants in building air conditioning systems

• Use of high-grade heat recovery from air cycle cooling systems resulting in lower energy consumption

• Improved reliability and reduced maintenance compared with conventional systems

• Maintenance of near full load efficiency at part load conditions

• No susceptibility to refrigerant leakage

COMFORT COOLING SYSTEMS

The need for heating and cooling in buildings:

The

prime requirement in respect of the indoor climate in a building is that room temperature should be at a comfortable level, regardless of the weather conditions outside. In addition, the indoor air must be acceptably clean, lighting and acoustic conditions must be good etc. Nevertheless, the first and foremost condition for a building to be usable at allies is that the indoor temperature is unacceptable. As soon as the ambient temperature is lower than the Indoor temperature, heat flows out from the building through its boundary surfaces (the building envelope). At the same time, the building also loses heat through air infiltration, i.e. the inward leakage of outdoor air into the building through gaps and cavities in walls, roofs, doors and windows. Bearing in mind the fact that the indoor temperature in most buildings is maintained at a little over20 °C, this means, throughout most of the year, the building is losing heat to its surroundings.

The internal heat generation in commercial premises and some industrial buildings, on the other hand, is often relatively great. In combination with the fact that construction standards have been developed and improved so that buildings are nowadays well insulated and airtight, this means that the heat losses through the building envelope are small. If we consider new office buildings, department stores, hospitals and similar buildings within the commercial premises and industrial sector, we find that heat deficit usually occur only during the night and at weekends, while there is nearly always a ahead surplus during working hours. Such buildings require only simple heating systems to meet the modest heat deficits, as opposed to the considerably more extensive systems needed in order to deal with the substantial heat surpluses and to prevent the indoor temperature from becoming unacceptably high during working hours.

In general terms, the greater the heat surplus, and therefore the greater the capacity of the cooling system, the more difficult it is to produce an indoor climate that is good in all respects. It is therefore always important to attempt to design the building in general so that there will be only a low heat surplus.

Constant air volume systems (CAV systems)

In such systems, the temperature of the air supplied to the building can vary, but the airflow rate is kept constant

. Such systems are referred to as Constant Air Volume (CAV) systems. It is the rooms having the greatest cooling requirement that normally determine the supply air temperature delivered by the central air conditioning unit: the air may, if necessary, be heated before supplied to other rooms. Although a CAV system supplies air at a constant flow rate, the fans are sometimes powered by two-speed motors, running at a slower speed when the building cooling requirement falls. The airflow rate is then reduced in proportion to the fan speed.

Variable air volume systems (VAV systems)

The airflow rate to each room is varied as necessary, but with the maintenance of a constant supply temperature, i.e. the supply temperature does not change even if the load changes. However, the supply air temperature is normally changed in step with the time of year, as a function of the ambient temperature.

Systems of this type are referred to as Variable Air Volume (VAV) systems.

The airflow to each individual room is controlled by dampers in some form of a box (VAV-box) in the immediate vicinity of the supply point to the room, while the central supply and exhaust air fans are controlled by variable inlet vanes or by adjustable speed drive controlled motors, usually of the frequency-inverter type. The control system normally maintains a constant static pressure in the supply air duct. The flow rate varies from a maximum, during the hottest days, down to perhaps 20 % of maximum flow rate during the coldest days of the year, when the purpose of the air is only to maintain the air quality.

Cooling supply devices

Cooling can be supplied to a room in a number of different ways. The following are brief descriptions of how chilled beams, cooling panels, fan coil units and induction units operate. Fan coil units and induction units are normally positioned below windows in the outside walls.

Chilled beams

These are units which, by natural convection from a finned heat exchanger, cool the air in the room. They may also be combined with the supply air terminal device in order to provide both functions and, in many cases, to increase the cooling capacity of the baffle. Some chilled beams can also incorporate a heating function.

Cooling panels

Cooling panels can be hung from the ceiling. Coldwater flows through an aluminium plate, which transfers heat from the air to the cold water. The panel cools the warm room air and also cools the room surfaces by low-temperature radiation. These panels are produced in a number of versions, e.g. for mounting flat against the ceiling, hanging, or for integration in a false ceiling. Most of their cooling capacity is provided by radiation.

Fan coil units

These are units by which both heating and cooling can be supplied to a room (although not at the same time).

A fan coil unit incorporates a fan that circulates the room air through the unit, in which the air is either heated or cooled as required. The two heat exchangers (heating and cooling) are supplied with hot or cold water from a central unit in the building. This type of room cooler unit can meet the highest cooling requirements, but it also has the highest noise level.

Induction units

These are units by which both heating and cooling can be supplied to a room . When in use, the ventilation air for the room is supplied through the induction unit. It flows through a nozzle with high velocity, which therefore has the effect of inducing air from the room through the heating or cooling heat exchangers.

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