dition far to the right of the cooling and dehumidification coil process line in the lower crosshatched area of Figure 19-2. An all-too-frequent exam- ple of this is a humid or mixed climate outside air makeup air cooling unit providing conditioned outside air to the corridors of a hotel or school. The designer desires to supply the outside air a few degrees cooler than the room temperature with a humidity ratio slightly below the room humidity ratio. This process is not physically attainable in a cooling and dehumid- ification coil process. No amount of words or legal language will over- come mother nature and make the process possible. The result is a coil Figure 19-5—Deviations between scheduled and actual coil performance.
having a significant surplus of sensible cooling capacity and a significant deficiency in water vapour removal (latent cooling) capacity. This leads to high humidity, clammy spaces, and the possibility of mold and bacte- riological growth.
Evaporative Cooling Process (Adiabatic). Ideal evaporative cool-
ing is an adiabatic process (no heat is added or removed), which follows the entering air wet-bulb line up and to the left toward the saturation curve. Air is brought into intimate contact with recirculating water at the wet- bulb temperature. If there is insignificant heat gain through the apparatus casing and water piping and the heat gain from pump energy is minor, then the process will be close to a true adiabatic process. The dry-bulb tem- perature decreases as the humidity ratio increases. The energy transferred from the air to the water in the sensible cooling of the air is equal to the latent heating energy required to vaporize the water. No energy is trans- ferred to or from the surrounding environment except for a small amount of energy transferred to the air by the enthalpy of the mass of makeup water required to sustain the process. Vector E in Figure 19-2 shows this process.
Evaporative coolers are rated by percentage saturation effectiveness, which expresses the actual dry-bulb temperature decrease relative to the maximum possible dry-bulb temperature decrease in a perfect saturator. The somewhat theoretical adiabatic saturator discussed in the section on wet-bulb temperature is an ideal evaporative cooler operating at 100% sat- uration effectiveness. Industrial air washers use multiple opposing spray banks to achieve intimate mixing of the air and water and achieve satura- tion effectiveness of 95% to 98%. Rigid media (Munter’s fill) evaporative coolers with 12 in. fill can achieve saturation effectiveness of 88% to 91%. Some residential evaporative coolers use aspen wood excelsior media resembling coarse steel wool. Other residential evaporative coolers use a plastic mesh referred to as a “hogs hair filter media.” The saturation effec- tiveness of residential evaporative media may be in the 50% to 60% range.
Water Spray Processes. These processes utilize water sprays or wet-
ted media to achieve intimate contact between air and water. Equipment includes cooling towers, dense-water-spray air washers, wetted fluted cel- lulose or fiber glass paper-like media, and sprayed coil equipment. Cool- ing towers are available with both counterflow of air and water and crossflow. Air washers are generally parallel flow. The process condition lines are dependent on the flow arrangement, the mass flow and state of entering air, the mass flow and temperature of the entering water, and the heat and mass transfer coefficients. A thorough discussion of the basic heat and mass transfer and the plotting of process condition lines for all options could be the subject of a separate book.
Understanding Psychrometrics, Third Edition
Figure 19-6 shows the relationship between water and air as they pass through a parallel flow apparatus for a cold water and a warm water pro- cess. Figure 19-7 shows similar information for a counterflow apparatus. The process lines in Figures 19-6 and 19-7 are curved because the water changes temperature as it passes through the apparatus. Two air washer processes have straight condition lines: (1) the adiabatic evapora- tive cooling process in the preceding section and (2) a process such as a sprayed coil unit in which heat is either added or removed during the pro- cess to maintain the spray water at constant temperature.
Desiccant Dehumidification Process (Adiabatic) (Chemical Dehumidification). This process vector is the opposite of the adiabatic
evaporative cooling process. Water vapour is adsorbed from the airstream by the desiccant and the water vapour is condensed to its liquid form within the pores of the desiccant. The latent heat of condensation released
Figure 19-6—Parallel flow water spray process.
as the water vapour condenses within the desiccant causes the desiccant and the air in contact with the desiccant to warm. The ideal adiabatic pro- cess follows the entering wet-bulb line down and to the right on the psy- chrometric chart. The latent heat released in condensing the water vapour becomes a sensible heat gain to the air. The dry-bulb temperature of the leaving air is warmer and the humidity ratio is lower. In actual practice, the dry-bulb temperature gain in the desiccant process is 20% to 30% greater than that predicted by a pure conversion of latent heat to sensible heat due to (1) the residual heat carried over or retained by the desiccant and its supporting structure from the hot regeneration process and (2) a lessor phenomenon called heat of wetting. Vector F of Figure 19-2 illus- trates this process.
When the desiccant approaches a condition of saturation, it must be removed (or rotated) from the process airstream and regenerated. Regen- eration is a separate sequence of processes in which heated air (usually
Understanding Psychrometrics, Third Edition
heated outside air) flows across the water-saturated desiccant. The first process in regeneration is sensible heating of the air, which is followed by the second process in which the heated air takes up the adsorbed water in an evaporative cooling process. The regenerative airstream is heated to a temperature exceeding 120qC for silica gel desiccants and to as low as about 65qC for some molecular sieve desiccants. The coefficient of per- formance (COP) of the regeneration process varies from 0.3 for inefficient regenerators to 0.5 for industrial regenerators to approximately 0.7 for regenerators that incorporate heat pipes or rotary heat wheels to transfer a portion of the carry-over-heat from the leaving process airstream to the entering recuperative airstream.
Mixing of Two Airstreams Process (Figure 19-8). This is an adia-
batic process involving the mixing of air at state 1 with a second stream of air at state 2. The resultant mixture is at state 3. The most common example is the mixing of a mass flow of outdoor air with a mass flow of return air in the mixing box of an air-handling unit. Other examples cov- ered later include the (ordinary) face and bypass damper process and the return air face and bypass damper process. Other examples of the mixing process include dual-duct mixing boxes and cold deck-warm deck or cold deck-neutral deck-warm deck multizone air-handling units.
On a psychrometric chart using enthalpy-humidity ratio plotting coordinates, the final state of the mixture process lies on a straight line connecting the two original states. The enthalpy of the resultant mixture
Figure 19-8—Mixing of two airstreams process.
is determined by summing the product of each mass flow times its enthalpy and dividing by the total mass flow. The humidity ratio of the mixture is determined by summing the product of each mass flow times its humidity ratio and dividing by the total mass flow. An approximation of the final dry-bulb temperature (suitable for most air-conditioning work) can be obtained by summing the product of each volumetric flow times its dry-bulb temperature and dividing by the total volumetric flow. The small error introduced in this approximation is caused by the differences in spe- cific volumes of the two airstreams and, to a minor extent, by differences in the air specific heat capacities. For maximum accuracy, the final dry- bulb temperature should be determined using mass flows and the calcu- lated enthalpy and humidity ratio as input into psychrometric algorithms.
Room Effect Process (Figure 19-9). This process vector shows the
progression of states as the parcel of supply air passes through and becomes intimately blended with the air in the room(s) or space(s). The vector in Figure 19-9 shows the parcel being heated and humidified by the room air (in a cooling application). The room effect process vector varies throughout the day and seasonally as the space sensible and latent loads vary. The room effect process vector can proceed from the supply air state- point in 360 (in reality an infinite number of) different directions depend- ing on the time of day, the season, the climate, interior loads, exterior loads, and the air leakage characteristics of the exterior walls, windows, roof, and floor. The possible changes in both space sensible and latent load have a major impact on the ability of the HVAC system to maintain space
Figure 19-9—Room effect process.
Understanding Psychrometrics, Third Edition
comfort conditions as well as the energy requirements of the HVAC sys- tem at partial loads. These issues are addressed in Chapter 20, “Process Calculations and Definitions of Sensible and Latent Enthalpy Change,” under the topic part-load operating conditions.
In the case of summer air conditioning, sensible heat from solar effects, envelope conduction, lights, business machines, infiltration, and people is transferred to the supply air. Water vapour gain (latent heat) from people, infiltration, and other water vapour sources in the conditioned space is also transferred to the supply air. The summer air-conditioning peak design room effect process vector moves from the supply air state upward and to the right on the psychrometric chart as the supply air takes up sensible heat and water vapour (latent heat) from the room. In effect, this is a combination sensible heating and humidification process. The dif- ference is that the room effect process originates at the statepoint of the supply air and ends at the statepoint of the room air, whereas the heating and humidifying process originates at the room air and ends at the state- point of the supply air. The final state, which should be the desired room temperature and humidity ratio, has a higher dry-bulb temperature and a higher humidity ratio. The peak summer load process vector is shown in Figure 19-9.
In cold weather, the room effect process includes sensible cooling from the envelope due to infiltration and conduction (offset to some extent by heat gains from solar effects, lights, business machines, and people) and a reduction in water vapour due to infiltration of cold dry air leaking into the conditioned space (offset by minor water vapour gains from peo- ple). The cold weather room effect vector proceeds from the state of the supply air entering the room toward a lower dry-bulb temperature and a lower humidity ratio. In effect, this is a combination cooling and dehu- midification process.