Ashok A Khedkar
In air-conditioning system, the air must undergo cooling, heating, humidification and dehumidification.
Atmospheric air is the mixture of both dry air and water vapor. It is this mixture that is referred to as moist air. Psychometrics is the name given to the study of the air and water vapor mixture. Seven properties of atmospheric air are shown on the psychrometric chart. These properties are: dry bulb temperature, wet bulb temperature, dew point, humidity ratio, relative humidity, specific volume, and specific enthalpy.
If the thermometer is measuring the air temperature and the bulb of the thermometer is dry and open to the air, the temperature rating is called the dry bulb temperature. If the bulb is surrounded by a water soaked wick and moving through the air, then the temperature will be different than the dry bulb temperature and that is what is called wet bulb temperature.
Psychometrics can be used to predict changes in the environment when the amount of heat and/or moisture in the air changes. Use of psychrometric analysis is also important to determine the volume flow rates of air to be pushed into the ducting system and the sizing of the major system components.
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 the airflow 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 both the supply air temperature and flow rate 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.
Humidity control in a conditioned space is done by controlling the amount of water vapor present in the air in the space. When relative 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
In designing air 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 psychometrics.
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 and system load components, the following items need to be gathered from a set of plans, existing building surveys or occupant interviews: • Building square-footage and volume • Orientation of the building (sun effects on surfaces) • Year round weather data (design conditions, heat transfer) • Use of the spaces within the building (offices, conference room, lab, data centre)
• Hours of operation (occupied and unoccupied) • Thermostat set points (main comfort parameter) • Dimensions of walls, roofs, windows and doors • Construction materials (gather densities, external color and U-factors or describe material type layer by layer (R-values) • Stairways and elevators (floor-to-floor openings) • People occupancy and activity, and when they are present • Lighting intensity and hours used • Motor and appliance sizes or kW and times they are used • Ventilation needs (IAQ and exhaust makeup)
The total cooling load is than determined in kW or tons by the summation of all of the calculated heat gains. Along with psychometrics, load estimating establishes the foundation upon which HVAC system design and operation occur.
HVAC engineers use psychometrics to translate the knowledge of heating or cooling loads (which are in kW or tons) into volume flow rates (in m3 /s or CFM) for the air to be circulated into the duct system. The volume flow rate is used to determine the size of fans, grills, outlets, air-handling units, and packaged units. This in turn affects the physical size (footprint) of air handling units and package units and is the single most important factor in conceptualizing the space requirements for mechanical rooms and also the air-distribution ducts. The main function of the psychrometric analysis of an air-conditioning system is to determine the volume flow rates of air to be pushed into the ducting system and the sizing of the major system components.
Psychometrics is the science of studying the thermodynamic properties of moist air. The amount of moisture vapour in the air varies quite significantly under different conditions. When the air is hot, it can contain a large amount of moisture vapour, sometimes as much as 5% by volume. When it is cold, its capacity to hold the moisture is reduced. When the temperature of warm air begins to fall, the vapour also cools and, if cooling continues, it will condense into tiny moisture droplets. In the atmosphere this results in the formation of clouds and eventually rain.
· Atmospheric air - contains nitrogen, oxygen, carbon dioxide, water vapor, other gases, and miscellaneous contaminants such as dust, pollen, and smoke. This is the air we breathe and use for ventilation.
· Dry air - exists when all the contaminants and water vapor have been removed from atmospheric air. By volume, dry air contains about 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases. Dry air is used as the reference in psychometrics.
· Moist air - is a mixture of dry air and water vapor. For practical purposes, moist air and atmospheric air can be considered equal under the range of conditions normally encountered.
The Psychrometric Chart provides a graphic relationship of the state or condition of the air at any particular time. It displays the properties of air: dry bulb temperature, wet bulb temperature , dew point temperature, and relative humidity. Given any two of these properties, the other two can be determined using the chart. The chart’s usefulness lies beyond the mere representation of these elementary properties – it also describes the air’s moisture content, energy content, specific volume, and more.
The psychrometric chart conveys an amazing amount of information about air. It provides an invaluable aid in illustrating and diagnosing environmental problems such as why heated air can hold more moisture, and conversely, how allowing moist air to cool will result in condensation. To predict whether moisture condensation will occur on a given surface you need three pieces of information; the temperature of the air, the relative humidity of the air, and the surface temperature. The psychrometric chart explains that by raising the surface temperature or by lowering the moisture content of the air or employ some combination of both can avoid surface condensation. Use of a psychrometric chart will show that this is true. A psychrometric chart also helps in calculating and analysing the work and energy transfer of various air-conditioning processes. In practical applications, the most common psychrometric analysis made by HVAC contractors involves measuring the dry and wet bulb temperatures of air entering and leaving a cooling coil. If these temperatures are known along with the volumetric air flow rate (CFM) through the coil, the cooling capacity of a unit can be verified. Using the dry and wet bulb temperature information, two points can be located on a psych chart and the corresponding enthalpy values read for them. The total BTUH cooling capacity can then be determined by multiplying 4.5 times the CFM value times the enthalpy difference of the two air state points [i.e. 4.5 CFM ∆h]. Contractors often must perform this calculation to prove that their equipment is working satisfactorily.
Dry Bulb Temperature (DBT) is the temperature that we measure with a standard thermometer that has no water on its surface. When people refer to the temperature of the air, they are commonly referring to its dry bulb temperature. Several temperature scales commonly are used in measuring the temperature.
The wet bulb temperature (WBT) is a temperature associated with the moisture content of the air. Wet bulb temperature is taken by surrounding the thermometer with a wet wick and measuring the reading as the water evaporates. Because of the evaporative cooling effect, Wet bulb temperatures are always lower than dry bulb temperatures and the only time that they will be the same is at saturation (i.e. 100% relative humidity).
The wet bulb temperature (WBT) relates relative humidity to the dry bulb temperature. If the relative humidity is low and the temperature is high, moisture will evaporate very quickly so its cooling effect will be more significant than if the relative humidity was already high, in which case the evaporation rate would be much lower. Wet bulb temperature on psychrometric chart is represented by lines that slant diagonally from the upper right of the chart (along the line of saturation) down to the lower left of the chart. These follows lines of constant enthalpy, but values are read off at the upper, curved, saturation temperature boundary. The unit of measure used for wet bulb temperature is degrees Celsius (°C) or degrees Farenheight (°F).
Enthalpy is the measure of heat energy in the air due to sensible heat or latent heat. Sensible heat is the heat (energy) in the air due to the temperature of the air and the latent heat is the heat (energy) in the air due to the moisture of the air. The sum of the latent energy and the sensible energy is called the air enthalpy. Enthalpy is expressed in Btu per pound of dry air (Btu/lb of dry air) or kilojoules per kilogram (kJ/kg).
Enthalpy is useful in air heating and cooling applications. Air with same amount of energy may either be dry hot air (high sensible heat) or cool moist air (high latent heat).
Relative humidity (RH) is a measure of the amount of water air can hold at a certain temperature. Air temperature (dry-bulb) is important because warmer air can hold more moisture than cold air. As a rule of thumb, the maximum amount of water that the air can hold doubles for every 20°F increase in temperature
Air at 60 percent relative humidity contains 60 percent of the water it could possibly hold (at that temperature). It could pick up 40 percent more water to reach saturation. Because RH varies significantly with dry bulb temperature, it is important to state dry bulb temperature and relative humidity together, such as 70°F and 50% RH.
RH should not be confused with absolute humidity which is defined as the actual amount of moisture in the air and is measured in pounds of moisture per pound of dry air. Relative humidity (RH) is a measure of how much moisture is present compared to how much moisture the air could hold at that temperature. RH is expressed as a percent.
Absolute Humidity is the vapor content of air. Dew point temperature indicates the temperature at which water will begin to condense out of moist air. When air is cooled, the relative humidity increases until saturation is reached, and condensation occurs. Condensation occurs on surfaces which are at or below the dew point temperature. The dew point temperature is directly related to the actual quantity of moisture in the air and does not change much throughout a day unless a weather front moves through an area and adds or removes large amounts moisture.
Specific Volume is the volume that a certain weight of air occupies at a specific set of conditions. The specific volume of air is basically the reciprocal of air density. As the temperature of the air increases, its density will decrease as its molecules vibrate more and take up more space
Since warm air is less dense than cool air which causes warmed air to rise. This phenomenon is known as thermal buoyancy. By similar reasoning, warmer air has greater specific volume and is hence lighter than cool air. The specific volume of air is also affected by humidity levels and overall atmospheric pressure.
Water vapour is one of several gaseous constituents of air, the other principal ones being nitrogen, oxygen and carbon dioxide. Each exerts its own partial pressure on the surrounding environment in proportion to the amount of gas present, the sum of the pressures making up the total or barometric pressure of the air. When there is a difference in concentration of one of these gases between two points, there will be a corresponding difference in partial pressure. This will cause a flow of that particular gas from the point of higher concentration to the lower.
A tool called a sling psychrometer or sling thermometer is used to measure both dry bulb and wet bulb temperature of an air sample. It consists of two thermometers side by side, where one bulb is covered by a wet cloth and the other is dry. The psychrometer is rotated through the air to get the readings. If air is cooled at a constant pressure, there is a point at which the water vapor in the air will condense. That is, change from a vapor to a liquid state. Evidence of this change is seen by moisture forming on surfaces. This temperature is referred to as the dew point.
Humidity ratio is also known as moisture content. Basically, humidity ratio is simply the weight of the water vapor per pound of dry air. Relative humidity is discussed more often in everyday life than humidity ratio, and is a little easier for most people to understand. Relative humidity is the ratio of the actual water vapor pressure in the air relative to the water vapor present if the air were saturated. Saturated air means that the condition of the air when it contains the maximum amount of water vapor that in can hold. The maximum amount of water vapor that the air can hold is dependent on the air temperature at a given atmospheric pressure. In other words, relative humidity is the amount of moisture the air contains relative to the amount of moisture it could contain, expressed as a percentage.
Specific Volume is the inverse of density, or the volume of air per unit weight of dry air. The enthalpy of the air is actually the sum of the individual enthalpies of the water vapor plus the enthalpy of the dry air. This includes both the sensible heat of the dry air and the latent and sensible heat of the water vapor. Condensation and dew point are important to understand within the built environment. Condensation on window surfaces, pipes, and ductwork must be avoided since this water can result in damage to the building elements.
Wet Bulb Temperature can be measured with a thermometer that has the bulb covered with a water-moistened bandage with air flowing over the thermometer. Enthalpy is the measure of the total thermal energy in air