Air Conditioning Psychrometrics
Course No: M05-005
Credit: 5 PDH
A. Bhatia
Continuing Education and Development, Inc.
9 Greyridge Farm Court
Stony Point, NY 10980
P: (877) 322-5800
F: (877) 322-4774
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Air Conditioning Psychrometrics
Course No: M05-
Credit: 5 PDH
A. Bhatia
Continuing Education and Development, Inc.
9 Greyridge Farm Court
Stony Point, NY 10980
P: (877) 322-
F: (877) 322-
[email protected]
AIRCONDITIONING PSYCHROMETRICS
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:
- Surface dehumidification on cooling coils simultaneous with sensible cooling.
- 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:
- Water spray humidifier
- Steam pan humidifier
AIR CONDITIONING SYSTEM DESIGN
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 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 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 center)
- 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 psychrometrics, load estimating establishes the foundation upon which HVAC system design and operation occur.
*One ton is equivalent to heat extraction rate of 12000 Btu’s/hr and 1 kW is equivalent to 3414 Btu’s/hr.
Determine Design Supply Airflow Rate
HVAC engineers use psychrometrics to translate the knowledge of heating or cooling loads (which are in kW or tons) into volume flow rates (in m 3 /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 (foot print) 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. We will study this in detail but before that let’s first refresh some elementary psychrometrics.
Uses of Psychrometric Chart
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 or not 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. A rule of thumb is that, a 10°F rise in air temperature can decrease relative humidity 20 percent. Use of a psychrometric chart will show that this is true.
A psychrometric chart also helps in calculating and analyzing 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 have to perform this calculation to prove that their equipment is working satisfactorily.
READING PSYCHROMETRIC CHART
To the novice, a psychrometric chart seems a dizzying maze of lines and curves going every which way, but once a few fundamental things are understood about the psych chart, it is not really that difficult to understand. If we dissect the components piece by piece, the usefulness of the chart will be clearer.
Temperature: Dry Bulb
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. In the inch-pound (I-P) system of units, at standard atmosphere, the Fahrenheit scale has a water freezing point of 32°F and a boiling point of 212°F. In the International System (SI) of units, the Celsius scale has a water freezing point of 0°C and a boiling point of 100°C. On the Kelvin scale, 0ºK equals -273°C.
Dry-bulb temperature is located on the X-axis, of the psychrometric chart and lines of constant temperature are represented by vertical chart lines.
Lines of constant wet-bulb temperature on the Psychrometric Chart
Enthalpy
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).
The enthalpy scale is located above the saturation, upper boundary of the chart. Lines of constant enthalpy run diagonally downward from left to right across the chart; follow almost exactly the line of constant wet bulb temperature.
Enthalpy lines shown on Psychrometric Chart
Relative Humidity (RH)
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.
Lines of constant relative humidity are represented by the curved lines running from the bottom left and sweeping up through to the top right of the chart. The line for 100 percent relative humidity, or saturation, is the upper, left boundary of the chart.
Humidity ratio is represented on the chart by lines that run horizontally and the values are on the right hand side (Y-axis) of the chart increasing from bottom to top.
Humidity Ratio lines shown on Psychrometric Chart
Dew Point
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.
Dew point is represented along the 100% relative humidity line on the psychrometric chart. Dew point temperature is determined by moving from a state point horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation temperature boundary is reached. At dew point, dry bulb temperature and wet bulb temperature are exactly the same.
Dew Point lines shown on Psychrometric Chart
The dew point is closely related to the nighttime low temperature on still nights. When the air temperature drops to the dew point, energy is added back to the air as frost or dew forms and the temperature stabilizes at 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. So the dew point temperature measured during daytime hours can be used as an estimate of the nighttime low temperature.
Specific Air Volume
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 (as per Boyle’s law). Thus the specific volume will increase with increasing temperature.
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. The more the moisture vapour present in the air, the greater shall be the specific volume. With increased atmospheric pressure, the greater the density of the
When water changes state from a liquid to a gas, as it does when it evaporates into the air, the water molecules in the vapour expand. Just as air pressure is directly related to the number of gas molecules per cubic meter of space, so vapour pressure results from the number of water vapour molecules per cubic meter. The greater the moisture vapour content of air, the greater the vapour pressure. Thus vapour pressure is linearly related to absolute humidity and is represented on Psychrometric Chart by lines that run horizontally and the values are on the far right hand side of the chart increasing from bottom to top. The unit of measure for vapour pressure is inches-w.g. or PASCAL.
Vapour Pressure shown on Psychrometric Chart
Vapour pressure directly affects evaporation rate. If the vapour pressure in the air is already very high, it is more difficult for water molecules to break free from a liquid surface and enter the air as vapour. That is why there is very little evaporation in humid environments. The point at which absolutely no more evaporation will occur because the air is already saturated is called, interestingly enough, saturation pressure and coincides with the saturation point.
MEASURING PSYCHROMETRIC VARIABLES
All psychrometric properties of air are determined by measuring two psychrometric variables: for example, if wet- and dry-bulb temperatures are measured, then relative humidity, vapor pressure and dew point. Many variables can be measured to determine the psychrometric state of air, but dry-bulb temperature, wet-bulb temperature, dew point temperature, and relative humidity are most commonly measured.
Measuring Dry Bulb Temperature
Dry-bulb temperature can be simply measured by a mercury-in-glass thermometer, which is freely exposed to the air but is shielded from moisture and radiation heat sources. A thermometer can be made from any substance whose property changes predictably with temperature for instance the volume of mercury increases with increasing temperature. Beside mercury-in-glass, other liquids can also be used in this way. Other forms of volume-based thermometers use the differential expansion of metals in the form of a wound spring.
More recently, materials have been found whose electrical resistance change with temperature. These are usually manufactured from the oxides of transition metals such as manganese, cobalt, copper or nickel. They are called thermistors and are usually found in data-loggers and as computer-based sensors. When the temperature increases, the resistance of the thermistor decreases. Conversely, if the temperature decreases, the thermistor's resistance increases.
In a weather station, the temperature and relative humidity equipment is usually housed in a white box shelter called Stevenson Screen. It shields the instruments from sunshine and precipitations and has louvered sides to permit the free movement of air. Ideally the shelter is placed over grass, mounted at 1 meter above the ground and as far from any buildings as circumstances permit.
Measuring Wet Bulb Temperature
Wet bulb temperature is easily measured with a standard thermometer which has its sensing bulb encased in a wetted wick that is subjected to rapid air motion across its surface. Such devices, called sling or whirled psychrometers, have a frame that can be whirled in the air by hand. When the sling psychrometer whirls through the air, water from the wetted sack evaporates, causing it to cool to the wet-bulb temperature. The amount of cooling that occurs depends on the relative humidity. The lower the humidity, the faster will be the evaporation, and the more the bulb will cool. High humidity will cause less evaporation, slowing the cooling process.
An accurate wet-bulb temperature reading depends on 1) sensitivity and accuracy of the thermometer, 2) maintenance of an adequate air speed past the wick, 3) shielding
the same frame that can be whirled in the air by hand. This is described in detail in the wet bulb temperature measurement above.
The wet- and dry-bulb temperatures together determine the state point of the air on the psychrometric chart, allowing all other variables to be determined. In air that has less than 100 per cent relative humidity, the wet bulb will record a lower temperature than the dry bulb. This difference in temperature is known as wet-bulb depression. A special chart is used to convert the wet bulb depression to relative humidity.
The relationship between dry-bulb, wet-bulb and RH is tabulated below:
Dry Bulb (°F)
Wet Bulb (°F)
Relative Humidity (%)
68 67 95
68 66 90
68 63 76
68 58 55
68 48 17
Higher the differential between the dry bulb and wet bulb temperature, lower shall be the relative humidity. A 10°F differential represents the Relative humidity of 55%.
- If the wet bulb temperature is lower than dry bulb the air-vapor mixture is un- saturated
- If the wet bulb temperature is the same as the dry bulb the air vapor is saturated
Other humidity measurement devices include an electric sensing element or a mechanical system.
The mechanical hygrometer uses a single human hair as a sensing element. The hair is attached to a spring and a dial. When the relative humidity increases, the cells in the hair swell and contract, pulling the spring and the dial hand. At a lower relative humidity, the hair cells relax and it lengthens, releasing the tension on the spring and
allowing the dial hand to move in the opposite direction. The response to changes in relative humidity is slow and is not dependable at very high relative humidity. These devices are acceptable as an indicator of a general range of humidity but are not suitable for accurate measurements.
A more accurate measurement is achieved with an electronic hygrometer. It measures the change in the electrical resistance of a thin layer of lithium chloride, or of a semiconductor device, as the relative humidity changes. Other hygrometers sense changes in weight, volume, or the transparency of various substances that respond to relative humidity.
Measuring Dew Point
Two types of dew point sensors are commonly used today: a condensation dew point method and a saturated salt system.
The condensation dew-point hygrometer is a polished metal mirror that is cooled until moisture just begins to condense onto it. This occurs when the surrounding air reaches its dew point. The temperature of the metal is then the same as the dew point temperature. Knowing the atmospheric temperature and the dew-point temperature, the relative humidity can be determined by means of a table.
The saturated salt system operates at dew points between 10° to 100°F with an error of less than ± 2°F. The system costs less than the condensation system, is not significantly affected by contaminating ions, and has a response time of about 4 minutes. The condensation type is very accurate over a wide range of dew point temperatures (less than ± 1°F from -100° to 212°F). A condensation dew point hygrometer can be expensive.
Measuring Vapour Pressure
Measuring vapour pressure is very difficult without complex laboratory instrumentation. However, it can be relatively easily derived from more measurable properties. The ratio between the weight of water vapour actually present in the air and the weight it can contain when saturated at the same temperature is called the relative humidity of the air. It is usually expressed as a percentage. As the vapour pressures are set by the quantities of vapour in the air, the relative humidity is also given by the ratio between the actual vapour pressure and the saturation vapour pressure at the same