Module (1.2) psychometrics-air parameters-hvac_by ss-eng. juma
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Transcript of Module (1.2) psychometrics-air parameters-hvac_by ss-eng. juma
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Psychometrics: AIR PARAMETERS
Module 02
Fundamentals of HVAC
By: Start Smart–Your Skills Solution
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THE IMPORTANCE OF HVAC:
1.4 AIR PARAMETERS
•
Figure (1.4) HVAC Process
Psychometrics: is the study of the thermodynamic properties of moist air. It is used extensively to illustrate and analyze the characteristics of various air conditioning processes and cycles.
Atmospheric Air: It makes up the environment in almost every type of air conditioning system. Hence a thorough understanding of the properties of atmospheric air and the ability to analyze various processes involving air is fundamental to air conditioning design.
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THE IMPORTANCE OF HVAC:1.4 AIR PARAMETERS
•
Moist Air:The surface of the earth is surrounded by a layer of air called the atmosphere, or atmospheric air.From the point of view of psychrometrics, the lower atmosphere, or homosphere, is a mixture of dry air (including various contaminants) and water vapor, often known as moist air.
Figure (1.4) HVAC Process
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1.4 AIR PARAMETERS •
The composition of dry air is comparatively stable. It varies slightly according to geographic location and from time to time. The approximate composition of dry air by volume percent is the following:The moist air can be thought of as a mixture of dry air and moisture. For all practical purposes, the composition of dry air can be considered as constant. In 1949, a standard composition of dry air was fixed by the International Joint Committee on Psychrometric data. It is given in Table 1.1 and Figure (1.5).
Figure (1.5) HVAC Process
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• 1.4 AIR PARAMETERS
Figure (1.5) HVAC Process
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1.5 TEMPERATURE AND SCALES•
The temperature of a substance is a measure of how hot or cold it is. Two systems are said to have equal temperatures only if there is no change in any of their observable thermal characteristics when they are brought into contact with each other. Various temperature scales commonly used to measure the temperature of various substances are illustrated in Figures
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1.5 TEMPERATURE AND TEMPERATURE SCALES
•
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1.5 TEMPERATURE AND SCALES•
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1.5 TEMPERATURE AND SCALES•
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1.5 TEMPERATURE AND SCALES•
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1.5 TEMPERATURE AND SCALES•
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1.5 TEMPERATURE AND SCALES•
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Figure (1.6) Commonly Used Temperature Scales
1.5 TEMPERATURE AND SCALES
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THE IMPORTANCE OF HVAC:Dry bulb temperature (DBT)
•
Figure (1.7) Thermometer/Relative Humidity Meters
is the temperature of the moist air as measured by a standard thermometer or other temperature measuring instruments. Figure (1.7)
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THE IMPORTANCE OF HVAC:Wet-Bulb Temperature
•
Figure (1.8) Schematic of a Wet-Bulb Thermometer
When unsaturated moist air flows over the wet bulb of the psychrometer, liquid water on the surface of the cotton wick evaporates, and as a result, the temperature of the cotton wick and the wet bulb. Figure (1.8)
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THE IMPORTANCE OF HVAC:
1.6 PSYCHROMETER
•
Figure (1.10) A psychrmeter
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THE IMPORTANCE OF HVAC:
1.6 PSYCHROMETER
•
Figure (1.11) A psychrmeter
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THE IMPORTANCE OF HVAC:Humidity Ratio
•
Figure (1.8) Schematic of a Wet-Bulb Thermometer
The humidity ratio of moist air w is the ratio of the mass of water vapor mw to the mass of dry air ma contained in the mixture of the moist air, in lb / lb (kg/kg). The humidity ratio can be calculated as:
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THE IMPORTANCE OF HVAC:Relative Humidity
•
Figure (1.9) Thermometer/Relative Humidity Meters
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Relative Humidity
•
Figure (1.9.1) Thermometer/Relative Humidity Curves
THE IMPORTANCE OF HVAC:
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Relative Humidity
•
Figure (1.9.1) Thermometer/Relative Humidity Curves
THE IMPORTANCE OF HVAC:
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THE IMPORTANCE OF HVAC:
Dew-point temperature:
•
If unsaturated moist air is cooled at constant pressure, then the temperature at which the moisture in the air begins to condense is known as dew-point temperature (DPT) of air.
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THE IMPORTANCE OF HVAC:Dew-point temperature:
•
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THE IMPORTANCE OF HVAC:
1.6 PSYCHROMETER
•
A psychrometer is an instrument that permits one to determine the relative humidity of a moist air sample by measuring its dry-bulb and wet-bulb temperatures. Figure 2.4 shows a psychrometer, which consists of two thermometers. The sensing bulb of one of the thermometers is always kept dry. The temperature reading of the dry bulb is called the dry-bulb temperature. The sensing bulb of the other thermometer is wrapped with a piece of cotton wick, one end of which dips into a cup of distilled water. The surface of this bulb is always wet; therefore, the temperature that this bulb measures is called the wet-bulb temperature. The dry bulb is separated from the wet bulb by a radiation- shielding plate. Both dry and wet bulbs are cylindrical. See Figure (1.10), Figure (1.11)
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THE IMPORTANCE OF HVAC:
1.6 PSYCHROMETER
•
Sling psychrometer(becoming obsolete)Sling psychrometerOther types of psychrometric instruments: 1. Dunmore Electric Hygrometer 2. DPT meter 3. Hygrometer (Using horse’s or human hair)
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THE IMPORTANCE OF HVAC:HUMIDITY MEASUREMENTS
Humidity sensors used in HVAC&R for direct humidity indication or operating controls are separated into the following categories: mechanical hygrometers and electronic hygrometers.
Mechanical HygrometersMechanical hygrometers operate on the principle that hygroscopic materials expand when they absorb water vapor or moisture from the ambient air. They contract when they release moisture to the surrounding air.
Electronic HygrometersThere are three types of electronic hygrometers: Dunmore resistance hygrometers, ion-exchange resistance hygrometers, and capacitance hygrometers.
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THE IMPORTANCE OF HVAC:HUMIDITY MEASUREMENTS
•
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• Relative Humidity and DEW Point
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THE IMPORTANCE OF HVAC:Psychrometric chart
A Psychrometric chart graphically represents the thermodynamic properties of moist air. Standard psychrometric charts are bounded by the dry-bulb temperature line (abscissa) and the vapour pressure or humidity ratio (ordinate). The Left Hand Side of the psychrometric chart is bounded by the saturation line. Figure 27.2 shows the schematic of a psychrometric chart. Psychrometric charts are readily available for standard barometric pressure of 101.325 kPa at sea level and for normal temperatures (0-50oC). ASHRAE has also developed psychrometric charts for other temperatures and barometric pressures (for low temperatures: -40 to 10oC, high temperatures 10 to 120oC and very high temperatures 100 to 120oC). See Figure (1.13)
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Temp. & Humidity Loggers
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Curve
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Wireless Logging-Weather station
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THE IMPORTANCE OF HVAC:•
Psychrometric chart
Figure (1.13) Schematic of a psychometric chart for a given barometric pressure
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DB-Dry Bulb Temperature
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DB-SENSIBLE COOLING
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SENSIBLE COOLING (Process O-A): • During this process, the moisture
content of air remains constant but its temperature decreases as it flows over a cooling coil. For moisture content to remain constant, the surface of the cooling coil should be dry and its surface temperature should be greater than the dew point temperature of air. If the cooling coil is 100% effective, then the exit temperature of air will be equal to the coil temperature. However, in practice, the exit air temperature will be higher than the cooling coil temperature. Figure (2.1) shows the sensible cooling process O-A on a psychrometric chart. The heat transfer rate during this process is given by:
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SENSIBLE HEATING (PROCESS O-B)
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SENSIBLE HEATING (PROCESS O-B)
During this process, the moisture content of air remains constant and its temperature increases as it flows over a heating coil. The heat transfer rate during this process is given by:
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WB-Wet Bulb Temp.
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RH%-Relative Humidity Lines
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COOLING AND DEHUMIDIFICATION (PROCESS O-C):
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P-Point will indicate All 7-Properities of Air
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What is the Process each Direction indicate?
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COOLING AND DEHUMIDIFICATION (PROCESS O-C):
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COOLING AND DEHUMIDIFICATION (PROCESS O-C):
When moist air is cooled below its dew-point by bringing it in contact with a cold surface as shown in Figure (2.3), some of the water vapor in the air condenses and leaves the air stream as liquid, as a result both the temperature and humidity ratio of air decreases as shown. This is the process air undergoes in a typical air conditioning system. Although the actual process path will vary depending upon the type of cold surface, the surface temperature, and flow conditions, for simplicity the process line is assumed to be a straight line. The heat and mass transfer rates can be expressed in terms of the initial and final conditions by applying the conservation of mass and conservation of energy equations as given below: • By applying mass • balance for the water:
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COOLING AND DEHUMIDIFICATION (PROCESS O-C):
• It can be observed that the cooling and de-humidification process involves both latent and sensible heat transfer processes, hence, the total, latent and sensible heat transfer rates (Qt, Ql and Qs) can be written as:
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COOLING AND DEHUMIDIFICATION (PROCESS O-C):
By separating the total heat transfer rate from the cooling coil into sensible and latent heat transfer rates, a useful parameter called Sensible Heat Factor (SHF) is defined. SHF is defined as the ratio of sensible to total heat transfer rate, i.e.,
From the above equation, one can deduce that a SHF of 1.0 corresponds to no latent heat transfer and a SHF of 0 corresponds to no sensible heat transfer. A SHF of 0.75 to 0.80 is quite common in air conditioning systems in a normal dry-climate. Lower value of SHF, say 0.6, implies a high latent heat load such as that occurs in a humid climate.
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COOLING AND DEHUMIDIFICATION (PROCESS O-C):
• The amount of moisture that is removed depends on several factors including:
•The temperature of the cooling fluid
•The depth of the coil•Whether the fins are flat or embossed
•The air velocity across the coil.
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2.4HEATING AND HUMIDIFICATION (PROCESS O-D):
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2.4HEATING AND HUMIDIFICATION (PROCESS O-D):
During winter it is essential to heat and humidify the room air for comfort. As shown in Figure (2.4), this is normally done by first sensibly heating the air and then adding water vapor to the air stream through steam nozzles as shown in the figure.
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2.5 COOLING & HUMIDIFICATION (PROCESS O-E):
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2.5 COOLING & HUMIDIFICATION (PROCESS O-E):
As the name implies, during this process, the air temperature drops and its humidity increases. This process is shown in Figure (2.5). As shown in the figure, this can be achieved by spraying cool water in the air stream. The temperature of water should be lower than the dry-bulb temperature of air but higher than its dew-point temperature to avoid condensation (TDPT < Tw < TO).
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2.5 COOLING & HUMIDIFICATION (PROCESS O-E):
It can be seen that during this process there is sensible heat transfer from air to water and latent heat transfer from water to air. Hence, the total heat transfer depends upon the water temperature. If the temperature of the water sprayed is equal to the wet-bulb temperature of air, then the net transfer rate will be zero as the sensible heat transfer from air to water will be equal to latent heat transfer from water to air. If the water temperature is greater than WBT, then there will be a net heat transfer from water to air.
If the water temperature is less than WBT, then the net heat transfer will be from air to water. Under a special case when the spray water is entirely re-circulated and is neither heated nor cooled, the system is perfectly insulated and the make-up water is supplied at WBT, then at steady-state, the air undergoes an adiabatic saturation process, during which its WBT remains constant
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2.6 HEATING AND DE-HUMIDIFICATION (PROCESS O-F):
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2.6 HEATING AND DE-HUMIDIFICATION (PROCESS O-F):
This process can be achieved by using a hygroscopic material, which absorbs or adsorbs the water vapor from the moisture. If this process is thermally isolated, then the enthalpy of air remains constant, as a result the temperature of air increases as its moisture content decreases as shown in Figure (2.6). This hygroscopic material can be a solid or a liquid. In general, the absorption of water by the hygroscopic material is an exothermic reaction, as a result heat is released during this process, which is transferred to air and the enthalpy of air ……………….increases.
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2.7 MIXING OF AIR STREAMS:
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2.7.1 MIXING OF AIR STREAMS:
• Mixing of air streams at different states is commonly encountered in many processes, including in air conditioning. Depending upon the state of the individual streams, the mixing process can take place with or without condensation of moisture.
• Without condensation: Figure (2.7.1), (2.7.2) shows an adiabatic mixing of two moist air streams during which no condensation of moisture takes place. As shown in the figure, when two air streams at state points 1 and 2 mix, the resulting mixture condition 3 can be obtained from mass and energy balance.
• From the mass balance of dry air and water vapor:
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2.7.2 MIXING OF AIR STREAMS:
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2.8 AIR WASHERS:
• An air washer is a device for conditioning air. As shown in Figure (2.8), in an air washer air comes in direct contact with a spray of water and there will be an exchange of heat and mass (water vapor) between air and water. The outlet condition of air depends upon the temperature of water sprayed in the air washer. Hence, by controlling the water temperature externally, it is possible to control the outlet conditions of air, which then can be used for air conditioning purposes.:
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2.8 AIR WASHERS:
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2.8 AIR WASHERS:
• In the air washer, the mean temperature of water droplets in contact with air decides the direction of heat and mass transfer. As a consequence of the 2nd law, the heat transfer between air and water droplets will be in the direction of decreasing temperature gradient. Similarly, the mass transfer will be in the direction of decreasing vapor pressure gradient. For example,
A. Cooling and dehumidification: tw < tDPT. • Since the exit enthalpy of air is less than its inlet value,
from energy balance it can be shown that there is a transfer of total energy from air to water. Hence to continue the process, water has to be externally cooled. Here both latent and sensible heat transfers are from air to water. This is shown by Process O-A in Figure (2.11).
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2.8 AIR WASHERS:
• In the air washer, the mean temperature of water droplets in contact with air decides the direction of heat and mass transfer. As a consequence of the 2nd law, the heat transfer between air and water droplets will be in the direction of decreasing temperature gradient. Similarly, the mass transfer will be in the direction of decreasing vapor pressure gradient. For example,
B. Adiabatic saturation: tw = tWBT. • Here the sensible heat transfer from air to water is
exactly equal to latent heat transfer from water to air. Hence, no external cooling or heating of water is required. That is this is a case of pure water recirculation. This is
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2.8 AIR WASHERS:
• In the air washer, the mean temperature of water droplets in contact with air decides the direction of heat and mass transfer. As a consequence of the 2nd law, the heat transfer between air and water droplets will be in the direction of decreasing temperature gradient. Similarly, the mass transfer will be in the direction of decreasing vapor pressure gradient. For example,
C. Cooling and humidification: tDPT < tw < tWBT. • Here the sensible heat transfer is from air to water and
latent heat transfer is from water to air, but the total heat transfer is from air to water, hence, water has to be cooled externally. This is shown by Process O-C in Figure (2.11).
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2.8 AIR WASHERS:
• In the air washer, the mean temperature of water droplets in contact with air decides the direction of heat and mass transfer. As a consequence of the 2nd law, the heat transfer between air and water droplets will be in the direction of decreasing temperature gradient. Similarly, the mass transfer will be in the direction of decreasing vapor pressure gradient. For example,
• D. Cooling and humidification: tWBT < tw < tDBT.• Here the sensible heat transfer is from air to water and latent
heat transfer is from water to air, but the total heat transfer is from water to air, hence, water has to be heated externally. This is shown by Process O-D in Figure (2.11. This is the process that takes place in a cooling tower. The air stream extracts heat from the hot water coming from the condenser, and the cooled water is sent back to the condenser.
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2.8 AIR WASHERS:
• In the air washer, the mean temperature of water droplets in contact with air decides the direction of heat and mass transfer. As a consequence of the 2nd law, the heat transfer between air and water droplets will be in the direction of decreasing temperature gradient. Similarly, the mass transfer will be in the direction of decreasing vapor pressure gradient. For example,
• E. Heating and humidification: tw > tDBT.• Here both sensible and latent heat transfers are from water to air,
hence, water has to be heated externally. This is shown by Process O-E in Figure (2.11).
• Thus, it can be seen that an air washer works as a year-round air conditioning system. Though air washer is a and extremely useful simple device, it is not commonly used for comfort air conditioning applications due to concerns about health resulting from bacterial or fungal growth on the wetted surfaces. However, it can be used in industrial applications.
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2.8 AIR WASHERS:
Various Psychrometric Processes that can take place in an air washer
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2.9 SUMMER AIR CONDITIONING SYSTEMS:
Figure (2.9) A Simple 100 % re-circulation Type air Condoning System
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2.9 SUMMER AIR CONDITIONING SYSTEMS:
• 2.9.1 Simple system with 100 % re-circulated air: • In this simple system, there is no outside air and the same air is
recirculated as shown in Figure (2.9), also shows the process on a psychrometric chart. It can be seen that cold and dry air is supplied to the room and the air that leaves the condition space is assumed to be at the same conditions as that of the conditioned space. The supply air condition should be such that as it flows through the conditioned space it can counteract the sensible and latent heat transfers taking place from the outside to the conditioned space, so that the space can be maintained at required low temperature and humidity. Assuming no heat gains in the supply and return ducts and no energy addition due to fans, and applying energy balance across the room; the Room Sensible Cooling load (Qs,r), Room Latent Cooling Load (Ql,r) and Room Total Cooling load (Qt,r) are given by:
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2.9 SUMMER AIR CONDITIONING SYS.:
2.9.1Simple system with 100 % re-circulated air:
The sensible and latent heat transfer rates at the cooling coil are exactly equal to the sensible and latent heat transfer rates to the conditioned space:
Assuming no heat gains in the supply and return ducts and no energy addition due to fans, and applying energy balance across the room; the Room Sensible Cooling load (Qs,r), Room Latent Cooling Load (Ql,r) and Room Total Cooling load (Qt,r) are given by:
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2.9 SUMMER AIR CONDITIONING SYSTEMS:
2.9.2 System with outdoor air for ventilation:
Figure (2.9.2A) A Summer Air Conditioning System with Outdoor
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2.9 SUMMER AIR CONDITIONING SYSTEMS:
• In actual air conditioning systems, some amount of outdoor (fresh) air is added to take care of the ventilation requirements. Normally, the required outdoor air for ventilation purposes is known from the occupancy data and the type of the building (e.g. operation theatres require 100% outdoor air). Normally either the quantity of outdoor air required is specified in absolute values or it is specified as a fraction of the re-circulated air. See Figure (2.9.2A), (2.9.2B)
2.9.2 System with outdoor air for ventilation:
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2.9 SUMMER AIR CONDITIONING SYSTEMS:
Air for Ventilation and a Zero by –Pass factor
Figure (2.9.2B) A Summer Air Conditioning System with outdoor -Air for Ventilation and a non-zero by-pass factor
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2.9 SUMMER AIR CONDITIONING SYSTEMS:
• Advantages and disadvantages of reheat coil: • Refrigeration system can be operated at reasonably
high evaporator temperatures leading to high COP and low running cost.
• However, mass flow rate of supply air increases due to reduced temperature rise (ti-ts) across the conditioned space
• Wasteful use of energy as air is first cooled to a lower temperature and then heated. Energy is required for both cooling as well as reheat coils. However, this can be partially offset by using waste heat such as heat rejected at the condenser for reheating of air.
• Thus the actual benefit of reheat coil depends may vary from system.
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2.10 WINTER AIR CONDITIONING SYSTEMS:
Figure (2.10.A) A winter air conditioning sys with a pre-heater
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2.10 WINTER AIR CONDITIONING SYSTEMS:
• In winter the outside conditions are cold and dry. As a result, there will be a continuous transfer of sensible heat as well as moisture (latent heat) from the buildings to the outside. Hence, in order to maintain required comfort conditions in the occupied space an air conditioning system is required which can offset the sensible and latent heat losses from the building. Air supplied to the conditioned space is heated and humidified in the winter air conditioning system to the required level of temperature and moisture content depending upon the sensible and latent heat losses from the building. In winter the heat losses from the conditioned space are partially offset by solar and internal heat gains. Thus in a conservative design of winter A/C systems, the effects of solar radiation and internal heat gain are not considered.
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2.10 WINTER AIR CONDITIONING SYS:
Figure (2.10.B) A winter air conditioning sys with a pre-heater
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2.11ALL YEAR (COMPLETE) AIR CONDITIONING SYSTEMS:
• Figure (2.11) shows a complete air conditioning system that can be used for providing air conditioning throughout the year, i.e., during summer as well as winter. As shown in the figure, the system consists of a filter, a heating coil, a cooling & dehumidifying coil, a re-heating coil, a humidifier and a blower. In addition to these, actual systems consist of several other accessories such as dampers for controlling flow rates of re-circulated and outdoor (OD) air, control systems for controlling the space conditions, safety devices etc. Large air conditioning systems use blowers in the return air stream also. Generally, during summer the heating and humidifying coils remain inactive, while during winter the cooling and dehumidifying coil remains inactive.
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2.11 ALL YEAR (COMPLETE) AIR CONDITIONING SYSTEMS:
Figure (2.11.A)
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2.11ALL YEAR (COMPLETE) AIR CONDITIONING SYSTEMS:
However, in some applications for precise control of conditions in the conditioned space all the coils may have to be made active. The blowers will remain active throughout the year, as air has to be circulated during summer as well as during winter. When the outdoor conditions are favorable, it is possible to maintain comfort conditions by using filtered outdoor air alone, in which case only the blowers will be running and all the coils will be inactive leading to significant savings in energy consumption. A control system is required which changes-over the system from winter operation to summer operation or vice versa depending upon the outdoor conditions.
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2.11ALL YEAR (COMPLETE) AIR CONDITIONING UNIT:
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2.11 ALL YEAR (COMPLETE) AIR CONDITIONING SYSTEMS:
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2.12 GUIDELINES FOR SELECTION OF SUPPLY STATE AND COOLING COIL:
As much as possible the supply air quantity should be minimized so that smaller ducts and fans can be used leading savings in cost of space, material and power.
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2.12 GUIDELINES FOR SELECTION OF SUPPLY STATE AND COOLING COIL:
However, the minimum amount should be sufficient to prevent the feeling of stagnation. If the required air flow rate through the cooling coil is insufficient, then it is possible to mix some amount of re-circulated air with this air so that amount of air supplied to the conditioned space increases. This merely increases the supply air flow rate, but does not affect sensible and cooling loads on the conditioned space. Generally, the temperature rise (ti-ts) will be in the range of 8 to 15oC.
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2.12 GUIDELINES FOR SELECTION OF SUPPLY STATE AND COOLING COIL:
The cooling coil should have 2 to 6 rows for moderate climate and 6 to 8 rows in hot and humid climate. The by-pass factor of the coil varies from 0.05 to 0.2. The by-pass factor decreases as the number of rows increases and vice versa. The fin pitch and air velocity should be suitable.
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2.12 GUIDELINES FOR SELECTION OF SUPPLY STATE AND COOLING COIL:
• If chilled water is used for cooling and dehumidification, then the coil ADP will be higher than about 4oC.
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Where Our Ambient Location?
O
i
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Comfort Zone as per ASHRAE Recommendation:
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70% 50%
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More Cooling … Means Dehumidification
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Summary of All Procceses:
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By Eng. Juma Yousef Juma [email protected] +971-50-4948385
Start Smart–Your Skills Solution
