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UNIT 4 PSYCHOMETRICS AND COMFORT AIR CONDITIONING SYSTEMS
Psychometry is the study of the properties of mixtures of air and water vapour. A mixture of various gases that constitute air and water vapour. This mixture is known as moist air.
When the moisture content is maximum, then the air is known as saturated air, which is established by a neutral equilibrium between the moist air and the liquid or solid phases of water.
The temperature of the moist air as measured by a standard thermometer or other temperature measuring instruments.
The saturated partial pressure of water vapour at the dry bulb temperature. This is readily available in thermodynamic tables and charts. ASHRAE suggests the following regression equation for saturated vapour pressure of water, which is valid for 0 to 100o C
The ratio of the mole fraction of water vapour in moist air to mole fraction of water vapour in saturated air at the same s ame temperature and pressure
The humidity ratio (or specific humidity) W is the mass of water ass ociated with each kilogram of dry air1. Assuming both water vapour and dry air to be perfect gases. gases .
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-pointtemperature(DPT)of air. The temperature indicated by the thermometer immersed in the water is the thermodynamic wet-bulbtemperature.
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. Above figure shows the schematic of a psychrometric chart.
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. Shows the sensible cooling process O-A on a psychrometric chart. The heat transfer rate during this process is given by:
During this process, the moisture content of air remains const ant and its temperature increases As it flows over a heating coil. The heat transfer rate during this process is given by: Where c
is the humid specific heat (≈1.0216 kJ/kg dry air) and m is the mass flow rate of dry a
air (kg/s). Figure 28.2 shows the sensible heating process on a ps ychrometric chart.
When moist air is cooled below its dew-point by bringing it in contact with a cold surface as shown in Fig. 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.
(SHF) is defined as the ratio of sensible to total heat transfer rate
The slope of the cooling and de-humidification line is purely a function of the s ensible heat factor, SHF. Hence, we can draw the cooling and de-humidification line on psychrometric chart if the initial state and the SHF are known. In some standard psychrometric charts , a protractor with different values of SHF is provided. The process line is drawn through the initial state point and in parallel to the given SHF line from the protractor as shown in Fig
We can define a
A contact factor (CF) can be defined which is given by: CF = 1 – BPF
Mass balance of water vapor for the control volume yields the rate at which steam has to be added, i.e., m : w
Where Q is the heat supplied through the heating coil and h is the enthalpy of steam. h
As the name implies, during this process, the air temperature drops and its humidity increases. This process is shown in fig.28.6. 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 drybulb temperature of air but higher than its dew-point temperature to avoid condensation (t < dpt
t < t ). w
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 recirculated 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. This is the process of adiabatic saturation, the process of cooling and humidification is encountered in a wide variety of devices such as evaporative coolers, cooling towers etc.
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 Fig 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 t he enthalpy of air increases.
Adiabatic saturation temperature is defined as that temperature at which water, by evaporating into air, can bring the air to saturation at the same temperature adiabatically. An adiabatic saturator is a device using which one can measure theoretically the adiabatic saturation temperature of air. As shown in Fig. an adiabatic saturator is a device in which air flows through an infinitely long duct containing water. As the air comes in contact with water in the duct, there will be heat and mass transfer between water and air. If the duct is infinitely long, then at the exit, there would exist perfect equilibrium between air and water at steady state. Air at the exit would be fully saturated and its temperature is equal to that of water temperature. The device is adiabatic as the walls of the chamber are thermally insulated. In order to continue the process, make-up water has to be provided to compensate for the amount of water evaporated into the air. The temperature of the
make-up water is controlled so that it is the same as that in the duct. After the adiabatic saturator has achieved a steady-state condition, the temperature indicated by the thermometer immersed in the water is the thermodynamic wet-bulb temperature. The thermodynamic wet bulb temperature will be less than the entering air DBT but greater than the dew point temperature. Certain combinations of air conditions will result in a given sump temperature, and this can be defined by writing the energy balance equation for the adiabatic saturator. A line passing through all these points is a constant wet bulb temperature line. Thus all inlet conditions that result in the same sump temperature, for example point 1’ have the same wet bulb temperature. The line is a straight line according to the straight-line law. The straight-line joining 1 and 2 represents the path of the air as it passes through the adiabatic saturator. Normally lines of constant wet bulb temperature are shown on the psychrometric chart. The difference between actual enthalpy and the enthalpy obtained by following constant wet-bulb temperature is equal to (w -w )h . 2
It consists of
Spray type humidifier,
Heating coil and a Fan. Atmospheric air flows through the dampers.
The quantity of air depends upon the "load and the dampers control it. Air then passes through the Air filter. The filter removes dirt, dust and other impurities. The air now passes over a coolin g coil. So when air is cooled below its dew point temperature, the water vapour is removed from the air in the form of water droplets. The surface temperature of the cooling coil has to be maintained below the dew-point temperature of the atmospheric air to accomplish dehumidification. The quantity of water removed from air is collected in the sump and is drained. The temperature of air leaving the cooling coil is lower than the ambient temperature for comfort. During the dry weather the spray type humidifier is used to increase the humidity of the conditioned air. During wet weather condition the relative humidity of the air is high, is controlled by the heating coil. For the comfort condition required is DBT around 23OC and relative humidity 60%. So the air is to be cooled and humidified to the comfort condition. Now the conditioned air is supplied to the conditioned space by a fan and ducts.
The working of the window air conditioner shown in Figure is described as under: The refrigerant vapour leaving the compressor is at high pressure and temperature. It then passes through the condenser. Outside air is drawn in by the fan and it cools the refrigerant in the condenser, the refrigerant then becomes liquid. The high pressure, low temperature liquid refrigerant enters the expansion valve. The pressure and temperature of the refrigerant falls when it leaves the valve. The cold refrigerant from the valve passes through the evaporator (the evaporator side of the air conditioner faces the room to be cooled). The warm air from the room
is drawn in by blower. The evaporator cools this air and the liquid inside the evaporator tube gets vaporized by absorbing the heat from the warm air. The cool air is again sent to the room through the opening at the top of the air conditioning unit. The liquid and vapour refrigerant from the evaporator passes to the compressor and is compressed to high-pressure, high temperature liquid. The operation hereafter is carried out in cycle as the same manner as explained.
A Streamlined and light-weight air handler is mounted on the inside wall. Refrigerant and condensate lines run through a small hole in the wall to the outside unit. Initial power is to the outside unit and then relayed to the air handler. Extremely quiet as the compressor and condenser coil are outside. Full electronic and remote control. The compressor (6) in the exterior unit compresses the refrigerant into a high-temperature, high-pressure gas. When this gas flows along the cooling fins of the condenser (7), heat is exuded and the gas is led to the evaporator (1) in the interior unit. The liquid expands into a gas at a low temperature and low pressure. This gas absorbs the warmth of the air in the room, the cooled air is blown back into the room and the heat is led to the compressor along with the gas.
A fan (3) draws the air (a) over the filter (2) and blows the cooled air (b) back into the room. A fan (8) draws air over the condenser and blows warm air (d) away. As with cooling, the moisture in the air condenses on the cold evaporator at room temperature.
In direct evaporative cooling, the process or conditioned air comes in direct contact with the wetted surface, and gets cooled and humidified. FIGURE shows the schematic
of an elementary direct, evaporative cooling system and the process on a psychrometric chart. As shown in the figure, hot and dry outdoor air is first filtered and then is brought in contact with the wetted surface or spray of water droplets in the air washer. The air gets cooled and dehumidified due to simultaneous transfer of sensible and latent heats between air and water (process o-s). The cooled and humidified air is supplied to the conditioned space, where it extracts the sensible and latent heat from the conditioned space (process s-i). Finally the air is exhausted at state i. In an ideal case when the air washer is perfectly insulated and an infinite amount of contact area is available between air and the wetted s urface, then the cooling
and humidification process follows the constant wet bulb temperature line and the temperature at the exit of the air washer is equal to the wet bulb temperature of the entering air (to,wbt), i.e., the process becomes an adiabatic saturation process. However, in an actual system the temperature at the exit of the air washer will be higher than the inlet wet bulb temperature due to heat leaks from the surroundings and also due to finite contact area. One can define the saturation efficiency or effectiveness of the evaporative cooling system ε as:
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 f rom 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 f rom the conditioned space are partially offset by solar and internal heat gains. Thus in a cons ervative design of winter A/C systems, the effects of solar radiation and internal heat gain are not considered. Heating and humidification of air can be achieved by different schemes. Figure shows one s uch scheme along with the cycle on psychrometric chart. As shown in the figure, the mixed air (mixture of return and outdoor air) is first pre-heated (m-1) in the pre-heater, then humidified using a humidifier or an air washer (1-2) and then finally reheated in the re-heater (2-s). The reheated air at state‘s’ is supplied to the conditioned space. The flow rate of supply air should be such that when released into the conditioned space at state ‘s’, it should be able to maintain the conditioned space at state I and offset the sensible and
latent heat losses (Q and Q ). Pre-heating of air is advantageous as it ensures that water in the s
humidifier/air washer does not freeze. In addition, by controlling the heat supplied in the pre heater one can control the moisture content in the conditioned space.
The humidification of air can be achieved in several ways, e.g. by bringing the air in contact with a wetted surface, or with droplets of water as in an air washer, by adding aerosol sized water droplets directly to air or by direct addition of dry saturated or superheated steam. Figure shows another scheme that can also be used for heating and humidification of air as required in a winter air conditioning system. As shown in the figure, this system does not consist of a pre-heater. The mixed air is directly humidified using an air washer (m-1) and is then reheated (1-s) before supplying it to the conditioned space. Though this system is simpler compared to the previous one, it suffers from disadvantages such as possibility of water freezing in the air washer when large amount of cold outdoor air is used and also from health hazards to the occupants if the water used in the air washer is not clean. Hence this system is not recommended for comfort conditioning but can be used in applications where the air temperatures at the inlet to the air washer are above 0o C and the conditioned space is used for products or processes, but not for providing personnel comfort.
A chart which relates effective temperature, dry-bulb temperature, wetbulbtemperature, and air movement to human comfort; comfort zones are indicated on such a chart.
Effective temperature (ET): This factor combines the eff ects of dry bulb temperature and air humidity into a single factor. It is defined as the temperature of the environment at 50% RH which results in the same total loss from the skin as in the actual environment. Since this value depends on other factors such as activity, clothing, air velocity and T , mrt
ASHRAE has defined a comfort chart based on the effective and operative temperatures. Figure 29.3 shows the ASHRAE comfort chart with comfort zones for summer and winter conditions. It can be seen from the chart that the comfort zones are bounded by effective temperature lines, a o
constant RH line of 60% and dew point temperature of 2 C. The upper and lower limits of humidity o
(i.e. 60 % RH and 2 C DPT, respectively) are based on the moisture content related considerations of dry skin, eye irritation, respiratory health and microbial growth. The comfort chart is based on statistical sampling of a large number of occupants with activity levels less than 1.2 met. On the chart, the region where summer and winter comfort zones overlap, people in winter clothing feel slightly warm and people in summer clothing feel slightly cool. Based on the chart ASHARE makes the following recommendations: Inside design conditions for winter: o
T between 20.0 to 23.5 C at a RH of 60% op
T between 20.5 to 24.5 C at a DPT of 2 C op
Inside design conditions for summer: o
T between 22.5 to 26.0 C at a RH of 60% op
T between 23.5 to 27.0 C at a DPT of 2 C TopW Winter zone summer zone DPT=2oC ET=20oC op
The most common type, usually work by drawing moist air over a refrigerated coil with a fan. The cold evaporator coil of the refrigeration device condenses the water, which is removed, and then the air is reheated by the condenser coil. The now dehumidified, re-warmed air is released into the room. This process works most effectively at higher ambient temperatures with a high dew point temperature. In cold climates, the process is less effective. It is most effective at over 45% relative humidity; higher if the air is cold [citationneeded ]. This type of dehumidifier differs from a standard air conditioner in that both the evaporator and the condenser are placed in the same air path. A standard air conditioner transfers heat energy out of the room because its condenser coil releases heat outside. However, since all components of the dehumidifier are in the same room, no heat energy is removed. Instead, the electric power consumed by the dehumidifier remains in the room as heat, so the room is actually heated , just as by an electric heater that draws the same amount of power. In addition, if water is condensed in the room, the amount of heat previously needed to evaporate that water also is re-released in the room (the latent heat of vaporization). The dehumidification process is the inverse of adding water to the ro om with an evaporative cooler, and instead releases heat. Therefore, an in-room dehumidifier always will warm the room and reduce the relative humidity indirectly, as well as reducing the humidity more directly, by condensing and removing water.
There are two main kinds of industrial fans:
Centrifugal fans Axial-flow fans
Centrifugal fans Centrifugal fans consist of a wheel or a rotor mounted on a shaft that rotates in a scroll-shaped housing. Air enters at the eye of the rotor, makes a right-angle turn, and is forced through the blades of the rotor by centrifugal force into the scroll-shaped housing. The centrifugal force imparts static pressure to the air. The diverging shape of the scroll also converts a portion of the velocity pressure into static pressure.
There are three main types of centrifugal fans:
Radial-blade fans - Radial-blade fans are used for heavy dust loads. Their straight, radial blades do not get clogged with material, and they withstand considerable abrasion. These fans have medium tip speeds and medium noise factors. Backward-blade fans - Backward-blade fans operate at higher tip speeds and thus are more efficient. Since material may build up on the blades, these fans should be used after a dust collector. Although they are noisier than radial-blade fans, backward-blade fans are commonly used for large-volume dust collection systems because of their higher efficiency. Forward-curved-blade fans - These fans have curved blades that are tipped in the direction of rotation. They have low space requirements, low tip speeds, and a low noise factor. They are usually used against low to moderate static pressures.
Axial-flow fans Axial-flow fans are used in systems that have low resistance levels. These fans move the air parallel to the fan's axis of rotation. The screw-like action of the propellers moves the air in a straight-through parallel path, causing a helical flow pattern. The three main kinds of axial fans are
Propeller fans - These fans are used to move large quantities of air against very low static pressures. They are usually used for general ventilation or dilution ventilation and are good in developing up to 0.5 in. wg (124.4 Pa). Tube-axial fans - Tube-axial fans are similar to propeller fans except they are mo unted in a tube or cylinder. Therefore, they are more efficient than propeller fans and can develop up to 3 to 4 in. wg (743.3 to 995 Pa). They are best suited for moving air containing substances such as condensible fumes or pigments. Vane-axial fans - Vane-axial fans are similar to tube-axial fans except air-straightening vanes are installed on the suction or discharge side of the rotor . They are easily adapted to multistaging and can develop static pressures as high as 14 to 16 in. wg (3.483 to 3.98 kPa). They are normally used for clean air only.
High speed rotating (air) flow is established within a cylindrical or conical container called a . Air flows in a helical pattern, beginning at the top (wide end) of the cyclone and ending at the bottom (narrow) end before exiting the cyclone in a straight stream through the center of the cyclone and out the top. Larger (denser) particles in the rotating stream have too much inertia to follow the tight curve of the stream, and strike the outside wall, then fall to the bottom of the cyclone where they can be removed. In a conical system, as the rotating flow moves towards the narrow end of the cyclone, the rotational radius of the stream is reduced, thus separating smaller and smaller particles. The cyclone geometry, together with volumetric flow rate, defines the cutpoint of the cyclone. This is the size of particle that will be removed from the stream with a 50% efficiency. Particles larger than the cut point will be removed with a greater efficiency, and smaller particles with a lower efficiency as they separate with more difficulty or can be subject to re-entrainment when the air vortex reverses direction to move in direction of the outlet .