Make your own free website on Tripod.com

                                                            EXPERIMENT NUMBER 1

Objective:

Determination of the psychrometric properties of ambient air such as vapour pressure, mixing ratio, relative humidity, density, apparent density & enthalpy.

 

Introduction:

Psychrometry is the science concerning the thermodynamic properties of air- water mixture. Most of the air conditioning problems require the determination of the psychrometric properties of the air. For a given atmospheric condition known as “state point”, at the known barometric pressure, any two-psychrometric properties of air fix the state point. The easiest to measure & employ are the dry & wet bulb temperatures. Following this. Other properties such as vapour pressure, mixing ratio, relative humidity, air density, & enthalpy etc. are computed.

 

Instruments:

 Aneroid (no liquid) barometer is commonly used for the atmosphere pressure measurement in the mines owing to its portable & rugged nature. It consists of an airtight chamber with flexible diaphragm retained under partial vacuum conditions. The collapse and motion of the diaphragm is restricted by a spring element. Any increase or decrease in the atmospheric pressure results in the diaphragm moving inward or outward respectively from its normal position. The movement is magnified to 200 times through a system of levers connected to a pointer moving over dial over which graduations are in pressure units.

                        Assmann psychrometer, or a similar instrument, whirling hygrometer is designed to obtain dry & wet bulb temperatures of the ambient air. The device has two thermometers mounted side by side in a rigid frame. One thermometer whose bulb is exposed reads the “dry” bulb temperature. The bulb of the other thermometer is encased in muslin or a cotton sleeve. It reads the “wet” bulb temperature. As the sleeve is dampened & the instrument is exposed to a current of air. Evaporation takes place from the damp sleeve thereby cooling the thermometer bulb. One the evaporation rate reaches a steady state value the temperature read by the thermometer becomes constant which is known as the wet bulb temperature. In order to facilitate the dispersion of the vapour formed around the bulb, air with certain velocity is to be passed around the bulb. This is achieved by whirling the thermometer as in the case of whirling hygrometer, or by forcing air over the bulbs by using a small spring operated fan as in the case of Assmann psychrometer.

 

Procedure:

(1)   Dampen the sleeve on the wet bulb thermometer with distilled water.

(2)   Tap gently on the outer casing of the aneroid barometer to cause a slight deflection of the needle to indicate that the barometer is in the working condition.

(3)   Position the Barometer properly at the observation station & note the atmospheric value, after waiting for about half an hour in which time creep related error in the needle deflection may be minimized.

(4)   Whirl the hygrometer (in a shade if the observation is outdoors) at about 3 revolutions per minute for at least a minute. Read the temperatures rapidly, immediately after whirling is stopped starting with the wet bulb temperature first because it has a tendency to climb up quickly. Several readings may be taken until consistency in the observations is attained.

(5)   When using the Assmann psychrometer, wind the clockwork with the key provided until the fan operates at full speed. Hold the instrument away from the body, & read the thermometer after a minute. Repeat the readings at short time intervals till they are consistent.

 

Calculations:

 Let B, TD, and TW denote the barometric pressure in kPa, Dry bulb temperature in°C, & wet bulb temperature in °C respectively. Based on those three known psychrometric properties certain other important properties may be computed as follows:

·        Vapour pressure : The saturation vapour pressure (PS) at TW may be obtained from psychrometric tables, or from the relationship

 

            PS = 0.6105 exp{(17.27 * TW)/(TW+ 237.3)}   kPa.

 

The actual (partial) vapour pressure (PV) for the atmospheric conditions is given by:

           

            PV = PS – 0.000644* B* (TD – TW)                   kPa.

            W = 0.622 * PV / (B – PV)                                kg/kg  ----------(Mixing Ratio)

 

·        Air density: Using the PV value, the air density (r = mass of the moist air per unit volume) is computed as :

            RH = (PV/PS) * 100   %

 

·        Enthalpy :The heat content of  air(h) is given by

H = 1.005 * TD + W( 2501.6 + 1.884 * m)        kg/kg.

 

       Also read the values of vapour pressure, mixing ratio, relative humidity, specific volume (reciprocal of apparent density) & enthalpy from the psychrometric chart provided & compare with calculated values.

 

Remarks:

(1)    A rapid assessment of the psychrometric properties can be facilitated by reading the values from a psychrometric chart. The procedure may, however, provide somewhat erroneous results, partly because the charts are designed for certain standard barometric conditions.

(2)    The bulbs of the hygrometer shall be protected from direct radiation falling on them from intense light sources, human body, & from surrounding objects, in order to obtain correct temperature estimation.

(3)    Students must, on their own, learn to compute the other psychrometric properties such as specific humidity, dew point temperature, and saturation ratio.