EVAPORATIVE COOLING SYSTEM

Evaporative Cooling System



CHAPTER: PSYCHOMETRIC CHART

Introduction

The psychrometric chart is a graphical representation that describes the relationships between the air temperature and relative humidity. Although complicated in appearance, this chart can be used to establish state points and is used to calculate specific humidity, dew point and vapor pressure (Puckorius 1983, 79-81).

On the Psychrometric chart, the dry-bulb temperature is the horizontal axis of the chart. Lines of constant wet bulb temperatures run diagonally from the upper left to the lower right in the Psychrometric chart. The vertical axis represents humidity ratio, the moisture content of the air mixture. In this chart, humidity ratio is expressed as grains of moisture per pound of dry air. Some charts show humidity ratio as pounds of moisture per pound of dry air. Note that 7000 grains of moisture is equivalent to 1 pound of water. The curved lines represent relative humidity expressed as %ages. The 100% relative humidity line is the left most upward line shown as “Saturation Curve”. This represents the maximum amount of water vapor that air can hold at a specific temperature (Wijeysundera 2011, . 25-29).

A psychrometric chart presents physical and thermal properties of moist air in a graphical form. It can be very helpful in troubleshooting greenhouse or livestock building environmental problems and in determining solutions. Understanding psychrometric charts helps visualization of environmental control concepts such as why heated air can hold more moisture, and conversely, how allowing moist air to cool will result in condensation. A psychrometric chart contains a lot of information packed into an odd-shaped graph. If we dissect the components piece by piece, the usefulness of the chart will be clearer. Boundaries of the psychrometric chart are a dry-bulb temperature scale on the horizontal axis, a humidity ratio (moisture content) scale on the vertical axis, and an upper curved boundary which represents saturated air or 100 percent moisture holding capacity (Puckorius 1983, 79-81).

The Cooling Processes on Psychrometric Chart

The cooling processes in Psychrometric Chart are illustrated below: Sensible Cooling - In sensible cooling process, the temperature of air changes from a point 'A' to point 'C', maintaining constant humidity ratio. The temperature is reduced by [T (A) - T (C) ] and the wet bulb temperature is also reduced. The humidity ratio remains same since there is no addition or loss of moisture. Evaporative Cooling - In evaporative cooling process, both temperature and humidity of air changes along the lines of constant wet bulb temperature (shown as line AB). There is no change in heat content and the energy is merely converted from sensible energy to latent energy. In evaporative cooling process, changes occur in dry bulb temperature, specific volume, relative humidity, humidity ratio, dewpoint temperature, and vapor pressure of the moist air. No change occurs in wet bulb temperature and enthalpy. The evaporative cooling is constant enthalpy process (technically termed as adiabatic process) (El-Refaie & Kaseb 2009, 826-838).

Key Evaporative Cooling Terms

Wet Bulb Depression (WBD): The difference between the dry bulb ...