HUMIDITY AND AIR CONDITIONING

Humidity refers to the amount of moisture (water vapor) in the surrounding air. Relative Humidity is a measure of the amount of moisture in the air compared with the amount of moisture the air can hold. Relative humidity is expressed as a percentage of how much moisture the air could possibly hold at the temperature it happens when you measure it.

Humidity: It is also known as humidity ratio Humidity,H = mass of vapour present in the air(kg) mass of dry air(kg) Amount of vapour is measured in terms of pressure ,then humidity is expressed as partial vapour pressure of water in air Humidity,H = partial vapour pressure of dry air = PA ( 1- PA) Since molecular weights of and air are 18 and 29,respectively,then humidity at the temperature and pressure can also be written as: H =MA PA = 18 PA MB ( 1- PA) 29(1- PA )

Vnet = volume of total air and water vapor mixture ,  Absolute humidity Absolute humidity is an amount of water vapor per unit volume. Mw= the mass of water vapor,  Vnet = volume of total air and water vapor mixture Enthalpy :enthalpy of air-water vapour is the heat content of 1kg of dry air and its associated moisture(kj/kg)

Relative humidity(HR) :Relative humidity is expressed as a %. PERCENT HUMIDITY: percentage of humidity may be expressed at certain temperature and pressure as percent humidity (HA) = actual humidity(H ) Х 100 saturation humidity(H s) Relative humidity(HR) :Relative humidity is expressed as a %. actual partial pressure of water vapour in the air-water mixture(p) Х 100 Relative humidity(HR)= partial pressure of water vapour at saturation (PS)

HUMID VOLUME (V): It is the volume occupied by a unit mass of dry gas and its associated vapour HUMID HEAT: it is the heat required to raise unit mass of dry gas and its associated vapour through one unit difference in temperature at constant pressure. s=ca +HCw Where s= humid heat ,kj/kg.k ca = specific heat capacity of gases kj/kg.k Cw= specific heat capacity of vapour, kj/kg.k H =humidity

There are various devices used to measure and regulate humidity There are various devices used to measure and regulate humidity. A device used to measure humidity is called a psychrometer or hygrometer.

How relative humidity (RH) is measured? Wet- and Dry Bulb Psychrometer

Wet- and Dry Bulb Psychrometer It consists of two identical mercury thermometers, one of which has a wet cotton or linen wick around its bulb. Evaporating water from the wick absorbs heat from the thermometer bulb, causing the thermometer reading to drop. The difference between dry-bulb and wet-bulb temperatures is compared on psychrometric charts.

Dry bulb thermometer and a wet bulb thermometer mounted together Dry bulb tells actual temperature Wet bulb shows how much water can be evaporated – temperature lowers as water is evaporated The difference in temperature on the 2 thermometers is an indication of the amount of water vapor in the air.

Wet- and Dry Bulb Psychrometer Psychrometric Chart. If the difference between wet and dry bulb is 6º F and the temperature is 72º F (dry bulb), then the RH is 54%.

DEW POINT The dewpoint is the temperature of air which is needed for condensation or dew (at that particular temperature). If you take a glass of ice water and it develops condensation on the glass surface, the air on the glass has condensed to its dewpoint and created dew. Dewpoint actually measures how much water vapor is in the air.

HUMIDITY CHART

Humidity charts Humidity vs temperature: Humidity is plotted on y-axis and temperature is plotted on x-axis at atmospheric pressure . The plots are curved and each represents a definite humidity value. (a): any point on the chart represents the temperature and humidity of the sample of the air . (b) : the curved line marked 100% gives the humidity of saturated air at various temperatures. (C):to the left of the saturation curve ,mixtures of air and water vapour can’t exist . (d): curved lines below the saturated air curve represent various percents of humidity .

Humid heat vs humidity : this line is plotted by taking humidity on y-axis and humid heat on x-axis . specific volume vs temperature: specific volume of dry air is plotted with temperature on x-axis and meter cube per kg dry air along y-axis . The humid volume of sample of air at a given temperature and humidity can be found by linear interpolation to the humidity-temperature . adiabatic cooling lines : the groups of lines which are at right side of the saturation curve, are the adiabatic

AIR CONDITIONING Types of equipments Air conditioning is the process of treating air so as to control its temperature, humidity, cleanliness and distribution simultaneously to meet the requirements of the conditioned space. Types of equipments Self contained air conditioner Central air conditioner.

1.Self contained air conditioners : Room air enters the casing at the front pannel . It is mixed with outdoor air. This mixture is forced over cooling coils by centrifugal fan . Moisture in air is condensed by propeller type of condenser 2. Central air condition system: These systems air supplied through duct network . Air is cleaned by filters Cooling is obtained by water Steam are hot water coils are used for heating Humidification system have surface type water nozzles.

Principle: The air is heated to higher temperature and is cooled along an adiabatic cooling line to reach the desired humidity . It is then reheated to the desired temperature at constant humidity

Working Out door air is drawn in to the equipment using a fan Air is passed through filters Air is heated Hot air is passed through water spray The pump draws water from the reservoir below and sprays on the air stream. Air enters eliminator baffles and entrained water is removed A second set of coils performs reheating A fan draws the air and discharges it to point of use

APPLICATIONS Processing and preserving of material: Compression of tablets : conditions are 45% RH and 22˚c .in tabletting section conditions are 15%RH and 22˚c . The relative humidity should not exceed 10 to 15% Manufacture of soft gelatin capsules : temperature is usually in range of 20 ˚c to 22˚c . Manufacture of sterile of products : in case of parenterals and ophthalmic products.

REFERENCES Pharmaceutical engineering, C.V.S. Subrahmanyam WWW.Wikipedia.Com WWW.google.com