Differences between Refrigeration & Air Conditioning • Temperature • Humidity • Fresh Air requirement • Air purification • Air movement • For Human comfort & safety
Human body temperature • The human body is warm-blooded and that it must maintain a body temperature within very close limits. It uses food as a fuel, converting it into energy. Some of this energy may leave the body in the form of external work done; the balance represents heat production within the body and is available for the maintenance of body temperature. As the body must produce heat continuously, it must also lose it at a rate that provides control of body temperature.
The human body maintains a basic minimum rate of heat production at about 75 watts during sleep and about 120 watts when awake but sedentary. As bodily activity increases, the rate of oxidation of food, with its attendant release of energy, must increase. The level of heat production for light work will be about 190 watts, the extreme value for heavy work, about 700 watts.
It is possible to recognize the main ways in which the body will lose heat and to relate them in a simple equation as follows: • H = S + E + R + C • whereH = rate of internal heat productionS = rate of heat storage in the bodyE = rate of loss by evaporationR = rate of radiant energy exchange with surroundingsC = rate of loss by convection
Most of the heat, however, must be rejected from the body surfaces through convection losses to the surrounding air, by radiation exchanges with surrounding surfaces, and by the evaporation of perspiration from the skin when required. The body involuntarily makes adjustments that influence these processes by increasing or decreasing the rate of heat loss as required. It can attempt to induce evaporation by pouring out perspiration on the skin when it is too warm.
Whether or not these measures are effective depends on the temperature, moisture content and motion of the air, and the temperature of the surrounding surfaces. The amount of clothing becomes a major factor also since it is interposed between the skin and the influence of the surroundings and becomes involved in the convective, evaporative, and radiative losses for those areas of the body that are covered.
Increasing the air temperature tends to reduce convective and radiative losses and to increase evaporative losses under sweating conditions. If air temperature rises above skin or clothing surface temperature there will be a heat gain rather than a heat loss by convection, and this must be offset by increased losses in other ways.
Perspiration and respiration • Perspiration and respiration components of the evaporative loss are dependent upon the rate at which water is actually evaporated. This depends in turn upon the degree of saturation of the air, which may be measured in terms of relative humidity. Thus, there can be no evaporative loss with air saturated at 37°C, regardless of the rate of perspiration unless the body temperature rises above normal. On the other hand, under conditions that lead to comfort, with only light activity the main evaporative loss is that from the lungs, with little or no contribution from perspiration, at least until the upper limits of comfort are reached.
Air motion • Air motion is another factor that can have a marked effect. Increased air speed over the body and clothing surfaces can increase convective losses and, when there is perspiration, the evaporative losses as well. Thus, under conditions of high temperature and high humidity, discomfort can often be greatly reduced by increasing the air flow. It is of more than passing interest to note that under these conditions even high air speeds can be pleasant. With cooler conditions, however, even small localized air circulation may give rise to complaints of chilliness.
Effective Temperature • It is the temperature of still, saturated air which would produce the same feeling of warmth.
Effect of Relative humidity • A marked influence of relative humidity was found. The value in summer at which 97 per cent of the subjects were comfortable was 21.50C Effective Temperature, which corresponds to conditions of 27.50C degrees at 10 per cent, 24.50C at 50 per cent and 21.50C at 100 per cent relative humidity. Correspondingly, the value for winter conditions at which the greatest proportion of subjects was comfortable was about 200C, which can be obtained with 260C at 10 per cent and 25.50C at 50 per cent relative humidity.
Air purification • Outside air must be introduced to all living spaces, although the amount of fresh air necessary to sustain life is very small indeed. They are governed by factors such as body odours and smoking, which may require a fresh air supply of 12 litre/s per person or more.
Air conditioning • Air conditioning is the process of treating air so as to control simultaneously its temperature, humidity, cleanliness and distribution to meet the requirements of the conditioned space. Action involved: • Temperature control • Humidity control • Air filtering, cleaning and purification • Air movement and circulation
Winter conditioning relates to increasing temperature and humidity whilst summer conditioning relates to decreasing temperature and humidity. Basically the practical difference is dependent upon whether the air fluid is passed over a hot grid (steam) or cold grid (brine or direct expansion refrigerant).
Dry and Wet bulb temperature • Dry bulb temperature - Air temperature indicated by a thermometer in a dry condition. • Wet bulb temperature – If a moist wick is placed over a thermometer bulb, the evaporation of the moisture from the wick will lower the temperature reading. This temperature is known as ‘Wet bulb temperature’. It is an indication of the wetness or amount of moisture in the air.
Dew Point • When a mixture of dry air and water vapour has a saturation temperature corresponding to the partial pressure of the water vapour it is said to be saturated. Any further reduction of temperature (at constant pressure) will result in some vapour condensing. This temperature is called the dew point. Air at dew point contains all the moisture it can hold at that temperature, as the amount of water vapour varies in air then the partial pressure varies, so the dew point varies. • The dew point temperature is the temperature at which the ambient air must cool to reach 100% relative humidity where condensate and rain form; and conversely, the wet bulb temperature rises to converge on the dry bulb temperature. • At 100% relative humidity, the dew point temperature will coincide with wetbulb temperature.
Relative Humidity • Is the mass of water vapour per m3 of air compared to the mass of water vapour per m3 of saturated air at the same temperature. This also equals the ratio of the partial pressure of the actual air compared to the partial pressure of the air if it was saturated at the same temperature • It indicates the amount of moisture carried by the air at a particular temperature as a percentage of the maximum amount that could be carried by air at that temperature. • The capacity of air to hold water vapour is dependent upon the temperature. At higher temperatures this is greater than at lower temperatures. When the maximum is reached at a given temperature, air is said to be saturated and at this point it has 100% relative humidity.
Humidity Ratio • Humidity ratio can be expressed as the ratio between the actual mass of water vapor present in moist air - to the mass of the dry air. Humidity ratio is normally expressed in kilogram or pounds of water vapor per kilogram or pounds of dry air. • It is another form of representation of Moisture content of the air. • Also referred to as absolute humidity or moisture content or saturation humidity.
Psychrometric Chart • Psychrometry is the study of moist air and of the changes in its conditions. It is essentially the understanding of the properties of mixtures of air and water vapour. • This subject is important to air-conditioning because the systems handle air-water vapor mixtures, not dry air. Some air-conditioning processes involve the removal of water from the air-water vapor mixture (dehumidification) while some involve the addition of water (humidification). A convenient way to represent the properties of air-water vapor mixtures is the psychrometric chart. On the chart, such properties as dry bulb temperature, wet bulb temperature, dew point, relative humidity, humidity ratio, specific volume, and enthalpy are presented in graphical form.
Reading the psychrometric chart • In order to locate any condition of air on the chart, two independent properties must be determined. The air condition can then be plotted on the chart, and all other properties can be read from the chart. Dry bulb temperatures are read along the horizontal scale at the bottom of the chart, humidity ratio is read along the right-hand vertical scale, and the wet bulb temperature, dew point temperature, and enthalpy are read along the diagonal scale at the upper left. Lines of constant relative humidity and specific volume are labeled in the body of the chart.
From wet and dry bulb readings the various properties of the air-vapour mixture can be estimated. Enthalpy is a function of the wet bulb temperature, and moisture content and vapour pressure are functions of dew point. The chart gives a quick performance check on the air entering and leaving the cooling coil, dew point, temperature, humidity, enthalpy, etc.
Dalton’s Laws of Partial Pressures • The pressure exerted by a mixtures of gases and vapours is the sum of the pressure which each would exert if it occupied the same space alone, assuming no interaction of constituents. • Barometer pressure = partial pressure of N2+p.p. O2 +p.p. of H2O • from Dalton’s Laws, viz: • Pressure exerted by and the quantity of, the vapour required to saturate a given space (i.e. exist as saturated steam) at any given temperature, are the same whether that space is filled by a gas or is a vacuum.
Comfort Conditions • Comfort under summer conditions is dependent on dry and wet bulb readings i.e. relative humidity as well as air motion. For a given degree of air turbulence (75 mm/s to 127 mm/s), relative humidity between 30% and 70%, average 50%, and thermometer readings 19°C to 250C, average 220C, gives the best degree of summer comfort. Air at low temperature and high humidity can be as comfortable as air at high temperature and low humidity.
6 psychrometric zone classification 6 zones worldwide based on temp and humidity conditions • Hot & Dry: Temp: >33C; RH: 8-40%; AH: 5-13% • Hot & Humid: Temp: >33C; RH: 27-65%; AH: 13-22% • Warm & Dry: Temp: 27-33C;RH:15-57%; AH: 5-13% • Warm & Humid: Temp: 27-33C; RH:40-80%; AH: 13-22% • Moderate: Temp: 20-27C; RH:22-70%; AH: 5-13% • Cool: Temp: 15-20C; RH:35-70%; AH: 5-13%
As the temperature of air is reduced, so too is its capacity for carrying water vapour. Air, with an initial dry bulb temperature of 360C and relative humidity of about 60%, will, when cooled to 27°C dry bulb temperature, have a relative humidity of 100%.The temperature drop reduces the capacity of the air to carry moisture in suspension. Further cooling will cause moisture to be precipitated. Air cooled to a comfortable temperature level of 21°C but having a relative humidity of IOO%,would not be able to take up further moisture andperspiration would notbe evaporated. People in an atmosphere at 21°C with IOO% relative humidity would be uncomfortable.
The remedy of dehumidifying the air is achieved by overcooling to precipitate excess moisture, (removed via the drain) so that when air is brought to the correct temperature, its humidity will be at an acceptable level. Thus the air could be overcooled to about 10°C dry bulb temperature so that warming to about 210C, would bring humidity to about 50%. The air is warmed in the trunking or by contact with warmer air in the space. • An adequate drain is required to remove what can be a considerable flow of water from dehumidification of the air.
Heating • When the temperature of air is increased, so too is its capacity for carrying water vapour. Air, with a very low initial dry bulb temperature of -5°C and relative humidity of about 5O%, will, when heated to 21°C dry bulb temperature, have a relative humidity of about 10%. The temperature rise increases the capacity of the air to carry moisture in suspension. Air heated to a comfortable temperature level of 210C but having a relative humidity of 10% will readily take up moisture whether from perspiration or from the nasal passages and throat. People in an atmosphere at 21°C but 10% relative humidity, would experience discomfort from dryness in their nose and throat and on the skin.
The remedy is to humidify the air with a hot water or steamspray. This action increases humidity towards 100% relative humidity and alsoincreases the temperature from -5°C and 50%relative humidity to say, + 7°C. Straight heating by the zoneheater bringing the air to about 210C, will drop relative humidity to 40%.The humidity will be at an acceptable level but is kept low to minimize condensation on any very cold external bulkheads.
Exercise • Outside air at 35°C and 60% relative humidity is to be conditioned by cooling and heating so as to bring the air to within the "comfort zone". Using the Psychrometric Chart plot the required air conditioning process and estimate (a) the amount of moisture removed (b) the heat removed and (c) the amount of heat added
Cooling to DBT27 RH100% Over cooling to DBT15 RH100% Heating to DBT25 RH50%
Solution • (a) the amount of moisture removed [22-10=11.5g of H20/kg of dry air] • (b) the heat removed [(1)-(2), qcool 88-40= 48kJ/kg-dry-air] • (c) the amount of heat added [(2)-(3), qheat 50-40= 10kJ/kg-dry-air].
Types of air conditioning systems • Air conditioning systems may be divided into two main classes – the central unit type in which the air is distributed to a group of spaces through ducting and the self-contained type, installed in the space it is to serve. • The central unit type is the most widely used, in one or other of a number of alternative systems, characterized by the means provided to meet the varying requirements of each of the spaces being conditioned. The systems in general use are as follows: • Zone control system; • Double duct system;