Psychorometrics

AIR CONDITIONING AND PSYCHROMETRICS
Air conditioner sizing is a very important as pect of maintaining a desirable environment in
buildings. Get the sizing wrong, and you’ll be facing problems such as insufficient cooling,
insufficient moisture removal, and unnecessarily high electricity bills. Use of psychrometry and
psychrometric chart is very important to determine the volume flow rates ofair to be pushed
into the ducting system and the sizing of the major system components.
The goal of air conditioning processes is to provide a sufficient amount of fresh air at desirable
temperature and humidity. Outside air is taken in and, depending on its original temperature
and humidity, must undergo one or several of the following processes:

Air conditioning system allows performingall of these processes and the psychrometric
analysis illustrates the change in properties of the air conditioning processes.
Approaches to Temperature Control
Temperature control in an air conditioning system is achieved by passing the air through the
cooling or heating coil, which may use any of the following approaches:
1. Vary the temperature of air supplied to the space while keeping theairflow rate constant.
This is the basic constant volume, variable temperature approach.
2. Vary the airflow rate while keeping the temperature constant for air supplied to the space.
This is the variable volume, constant temperature approach.
3. Vary the airflow rate and change the temperature for air supplied to the space. This is the
variable volume and temperature approach.
4. Vary boththe supply air temperature and flow ra te where the airflow rate is varied down to
a minimum value, then energy input to reheat the coil is controlled to vary the supply air
temperature. This is the variable volume reheat approach.
Approaches to Humidit y Control

Humidity control in a conditioned space is done by controlling the amount of water vapor
present in the air in the space. Whenrelative humidity at the desired temperature set-point is
too high, dehumidification is required to reduce the amount of water vapor in the air for
humidity control. Similarly, when relative humidity at the desired temperature set point is too
low, humidification is required to increase the amount of water vapor in the air for humidity
control.
Commonly used dehumidification methods include:1. Surface dehumidification on cooling coils simultaneous with sensible cooling.
2. Direct dehumidification with desiccant-based dehumidifiers
Humidification is not always required in an HVAC system but, when required, it is provided by a
humidifier. Commonly used humidification methods include:
1. Water spray humidifier
2. Steam pan humidifier

AIR CONDITIONING SYSTEM DESIGN
In designingair conditioning systems, the first challenge is to understand the components that
affect the building heat gain or heat loss - this process is called heating or cooling load
estimation. The reactive challenge is to "design" controlled processes to maintain the desired
condition or state-point within the occupied space - these are usually called the system
processes that use psychrometrics.Estimating Cooling & Heating Load
Load estimates are the summation of heat transfer elements into (gains) or out of (losses) the
spaces of a building. Each heat transfer element is called load components, which can be
assembled into one of three basic groups, external space loads, internal space loads and
system loads. To properly understand the workings of the various external, internal andsystem
load components, the following items need to be gathered from a set of plans, existing building
surveys or occupant interviews:
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Building square-footage and volume

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Orientation of the building (sun effects on surfaces)

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Year round weather data (design conditions, heat transfer)

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Use of the spaces within the building (offices, conference room, lab, data center)...