GAS TECHNOLOGY INSTITUTE • 1700 South Mount Prospect Road • Des Plaines, Illinois 60018 • www.gastechnology.org Maisotsenko-Cycle Enhanced Cooling Towers By enhancing cooling towers with the Maisotsenko‐Cycle (M‐cycle) one can: • Cool water to dew point temperature • Reduce pressure drop and fan power • Modify existing cooling towers to substantially decrease cooled water temperature I. Introduction The first time the M‐Cycle technology was proven and realized by Coolerado Corporation, which produces several air conditioners (commercial, residential, solar and hybrid). As proven by National Renewable Energy Lab (NREL) Coolerado’s Air Conditioners are 10 times and Coolerado’s Hybrid Air Conditioners are up to 80% more efficient than traditional systems. Also the M‐cycle as an extremely efficient evaporative cooling process can be applied for cooling towers. Cooling towers are heat rejection devices which transfer process waste heat to the atmosphere though the cooling of a water stream to a lower temperature. Common applications for cooling towers are providing cooled water for air conditioning, manufacturing and electric power generation. The cooling tower is a high‐energy utilization device as well as a mass heat transfer device. Quite often, the limiting factor of production is the quality (temperature) of water from the cooling tower. Colder water reduces energy consumption to power turbines and compressors in air conditioning and refrigeration. The colder the water in chemical plant processes, the more efficient condensation process and the greater the volume of saleable product yielded at lower cost. Hot water in power generation stations produces an energy penalty which reduces cost‐effectiveness. Poor vacuum and higher back pressures on turbine inlets, due to insufficient cooling water, require additional energy to production to meet load requirements, reducing profits. Typical cooling tower design uses direct evaporative cooling technology. The temperature of the cold water produced is limited to the outside air wet bulb temperature (Figure 1). As a result the typical cooling towers are most suitable for extremely dry regions. However, a significant amount of the typical cooling towers in operation in the U.S. are in locations that are not optimal for efficient operation (e.g. Gulf Coast). The cooling tower process shown in Figure 1 is estimated for water cooled from 90°F to 72°F which corresponds to 75% efficiency of the cooling tower at desired conditions. The evaporation rate (percent of total hot inlet water evaporated) is 1.5% and air humidity gain is 0.0162 lb of water per lb of dry air. Novel indirect evaporative cooling tower designs have been invented that use the Maisotsenko Cycle (M‐Cycle) (U.S. Patents No 6,854,278 and 6,497,107 ). While an M‐Cycle‐based cooling tower prototype has not yet been reduced to practice, the principles of sub‐wet bulb cooling are expected. These properties make the M‐Cycle ideal for many evaporative cooling applications, including cooling towers, where steady cooled water temperatures can be delivered as the higher inlet temperatures increase the cooling capacity. Unlike many evaporative cooling technologies, heat transfer within the M‐Cycle is driven by a dew point temperature gradient, not wet bulb temperature. As such, only the absolute outside ambient humidity is important, not the temperature of incoming air or water.
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GAS TECHNOLOGY INSTITUTE • 1700 South Mount Prospect Road • Des Plaines, Illinois 60018 • www.gastechnology.org
Maisotsenko-Cycle Enhanced Cooling Towers By enhancing cooling towers with the Maisotsenko‐Cycle (M‐cycle) one can:
• Cool water to dew point temperature • Reduce pressure drop and fan power • Modify existing cooling towers to substantially decrease cooled water temperature
I. Introduction
The first time the M‐Cycle technology was proven and realized by Coolerado Corporation, which produces several air conditioners (commercial, residential, solar and hybrid). As proven by National Renewable Energy Lab (NREL) Coolerado’s Air Conditioners are 10 times and Coolerado’s Hybrid Air Conditioners are up to 80% more efficient than traditional systems. Also the M‐cycle as an extremely efficient evaporative cooling process can be applied for cooling towers.
Cooling towers are heat rejection devices which transfer process waste heat to the atmosphere though the cooling of a water stream to a lower temperature. Common applications for cooling towers are providing cooled water for air conditioning, manufacturing and electric power generation.
The cooling tower is a high‐energy utilization device as well as a mass heat transfer device. Quite often, the limiting factor of production is the quality (temperature) of water from the cooling tower. Colder water reduces energy consumption to power turbines and compressors in air conditioning and refrigeration. The colder the water in chemical plant processes, the more efficient condensation process and the greater the volume of saleable product yielded at lower cost. Hot water in power generation stations produces an energy penalty which reduces cost‐effectiveness. Poor vacuum and higher back pressures on turbine inlets, due to insufficient cooling water, require additional energy to production to meet load requirements, reducing profits.
Typical cooling tower design uses direct evaporative cooling technology. The temperature of the cold water produced is limited to the outside air wet bulb temperature (Figure 1). As a result the typical cooling towers are most suitable for extremely dry regions. However, a significant amount of the typical cooling towers in operation in the U.S. are in locations that are not optimal for efficient operation (e.g. Gulf Coast).
The cooling tower process shown in Figure 1 is estimated for water cooled from 90°F to 72°F which corresponds to 75% efficiency of the cooling tower at desired conditions. The evaporation rate (percent of total hot inlet water evaporated) is 1.5% and air humidity gain is 0.0162 lb of water per lb of dry air.
Novel indirect evaporative cooling tower designs have been invented that use the Maisotsenko Cycle (M‐Cycle) (U.S. Patents No 6,854,278 and 6,497,107). While an M‐Cycle‐based cooling tower prototype has not yet been reduced to practice, the principles of sub‐wet bulb cooling are expected. These properties make the M‐Cycle ideal for many evaporative cooling applications, including cooling towers, where steady cooled water temperatures can be delivered as the higher inlet temperatures increase the cooling capacity. Unlike many evaporative cooling technologies, heat transfer within the M‐Cycle is driven by a dew point temperature gradient, not wet bulb temperature. As such, only the absolute outside ambient humidity is important, not the temperature of incoming air or water.
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a b c
Figure 1. Forced draft conventional cooling tower: flow diagram (a), packing (b), and psychrometric chart (c) Subscripts: DB – dry bulb, WB – wet bulb, DP – dew point
Film‐type fill (packing) has gained prominence in the cooling tower industry because of its ability to expose greater water surface within a given packed volume. Its efficient heat transfer can give greater cooling capacity, requiring less fan energy. Using the conventional evaporative cooling process with any existing type of fill for cooling towers, it is possible to cool down fluid (air or water) to near the wet bulb temperature. However, with the M‐Cycle, using the existing film fill and a change of air and water distributions for the opened cooling towers, cooled water temperatures nearing the dew point temperature of the working gas is possible.
The M‐cycle is a unique thermodynamic cycle which has the potential for cost‐effective Dew Point Evaporative Cooling, to be combined with existing technologies to produce higher efficiency (both energy and water) thermal transfer equipment (cooling systems, cooling towers, air/water chillers, evaporative condensers, etc.), and the M‐Cycle can be designed as a retrofit to the more than 500,000 cooling towers in operation today (Parker 1995).
II. M-Cycle Enhanced Open Circuit Cooling Tower
The M‐cycle open cooling tower (Figure 2) is an enclosed structure with internal means to distribute the warm water fed to it over packing or fill (U.S. Patent No 6,854,278). The fill provides an expanded air‐water interface for heating of the air and evaporation to take place. The heated and humidified air rejected to the atmosphere at Point 2 will be done so to prevent its being drawn back into the cooling tower inlet at Point 1.
The cooling tower process shown in Figure 2 is estimated at the same initial conditions (air inlet temperature and humidity, water inlet temperature) as typical cooling towers originally discussed in Figure 1. In Figure 2, water is cooled from 90°F to 55.6°F compared to 72°F. The water outlet temperature is lower than the wet bulb temperature of incoming air. Evaporation rate is 3.1% as opposed to 1.5% and air humidity gain is 0.0219 lb of water per lb of dry air in contrast to 0.0162 lb of water per lb of dry air. The rejected air at Point 3 is saturated (100% relative humidity) at approximately 90°F.
In Figure 2, the plate is not limited to a flat plane. Such a plate can be shaped for a particular retrofit installation and include curved, angled, spiraled, corrugated or otherwise contoured configuration to better function as a heat transfer surface in a given application. In all cases, one side of the plate is one wall of the wet channel and the other side is a wall of the dry channel. In certain designs, such a plate can be made from a single layer of
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hard, smooth, waterproof or low permeability material such as a sheet of plastic, metal or ceramic material. Additionally, the M‐Cycle cooling tower is not limited to rejecting heat to ambient air. Nitrogen, carbon dioxide, or industrial waste gas, may be used to cool the process liquid.
a b
Figure 2. M‐cycle open cooling tower: flow diagram (a) and psychrometric chart (b)
Dew point evaporative cooling processes, including the M‐Cycle, can be used to remove undesired heat from industrial, commercial, and residential facilities, including electrical power plants, oil refineries, chemical production plants, and air conditioning systems. Dew point evaporative cooling processes take advantage of the fact that when a parcel of air is sensibly cooled, the saturated water vapor pressure decreases, reducing its wet bulb temperature, thus increasing its evaporative cooling potential. Consequently, as the working fluid is humidified, the temperature of the evaporative cooling liquid that it is in contact with is also cooled to theoretically as low as the incoming air dew point temperature1. This is accomplished in Figure 2 through the sensible pre‐cooling of the air at Point 1, lowering its wet bulb temperature. This wet bulb depression is limited to the incoming air dew point temperature.
The operating efficiency of this concept cooling tower can be enhanced by various methods of reducing the dew point temperature (thus the wet bulb temperature) of the incoming air or other gas. For example, reduced dew point temperatures can be achieved by dehumidifying an incoming stream of air with solid and/or liquid desiccants. This dehumidification can take place before and/or after the air actually enters the dry channel. This desiccant system would be continually regenerated through the utilization of fuel combustion, solar thermal, or waste heat sources. Additionally heat sources can be utilized to increase the system cooling capacity, in preheating the incoming working gas (e.g. ambient air) improves heat transfer within the M‐Cycle. This is especially true in high absolute humidity climates where the dew point temperature of ambient air is higher than about 60°F to 70°F (depending upon the elevation of the site). Above these dew point temperatures, the effect of changing absolute humidity, with changing temperature, will become even larger. Similar principles apply to the use of a warmer incoming evaporative liquid (e.g., water) as such heating of an incoming evaporative liquid stream will have the same effect as pre‐heating the incoming gas (e.g. air). 1 A detailed discussion of psychrometrics and how the M‐Cycle can achieve dew point evaporative cooling are in Appendices A and B respectively.
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At low ambient relative humidity, the M‐cycle for cooling towers requires reduced fan power due to favorable density gradients. In a vertical orientation (Figure 2) a downdraft in a dry channel and the updraft in a wet channel are created. As air is cooled in the dry channel, it becomes heavier, or denser, and sinks. Conversely, as air heats and increases its humidity in the wet channel, this makes the air lighter, or less dense, so it rises. As these buoyancy effects are coincident with the mean flow, this affords an opportunity to reduce fan power.
III. M-Cycle Enhanced Closed Circuit Cooling Tower
The closed cooling tower shown in Figure 3 involves no direct contact of the air and the process fluid, usually water or a glycol/water mixture. Unlike the open cooling tower, the closed cooling tower has two separate fluid circuits, with the process fluid circuit exchanging heat sensibly from working wet channel, in which the working gas is humidified from the second fluid circuit. In operation the heat flows from the internal fluid circuit to the external circuit, through the heating and humidification of the working gas. The only difference between the closed and open cooling tower designs is that the process fluid being cooled is in a closed circuit, not exposed to the atmosphere or the recirculated external evaporative fluid. Like the open design, the closed cooling tower can deliver a process fluid cooled to temperatures nearing the working gas dew point.
The M‐Cycle Enhanced Closed Circuit Cooling Tower shown in Figure 3, has performance estimations at the same initial conditions (air inlet temperature and humidity, water inlet temperature) as the process shown in Figures 1 and 2. For the closed cooling tower the water is cooled from 90°F to 55.6°F, evaporation rate is 3.1%, and air humidity gain is 0.0219 lb of water per 1 lb of dry air, equal to that of the open cooling tower design.
Interestingly, the Heat and Mass Exchanger (HMX) based on the M‐Cycle that is currently used by Coolerado Air Conditioners is conceptually analogous to the closed cooling tower or closed water cooler. Each HMX produces about 1 ton of cooling capacity. The M‐Cycle has been proven to cool air to below the wet bulb and approaching the dew point temperature of outside air (Kozubal 2009). In addition, the M‐Cycle’s cooling capacity increases when the temperature of the incoming product fluid increases, in which the greater temperature difference increases the rate of internal heat exchange. The modular design of the HMX facilitates scaling in size. Each HMX contains the product channels for cold air and dry and wet channels for working air. Water cooling is achieved through flooding the product channels of the HMX with water instead of air. After passing through the product channels, the process water will be close to the dew point temperature of outside air. In this case, the HMX acts as the fill of the closed cooling tower.
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References:
ASHRAE. 2009. ASHRAE Handbook: Fundamentals.
Kozubal, E. and Slayzak, S. “Technical Report: Coolerado 5 Ton RTU Performance – Western Cooling Challenge Results.” National Renewable Energy Laboratory (8/09).
Maisotsenko V., Gillan L., “Evaporative cooling fundamentals. “Idalex” coolers advantages and achievements”, Proceedings of the Second International Workshop: Non‐Compression Refrigeration & Cooling, Odessa State Academy of Refrigeration, Ukraine, p. 50‐55, (1/01).
Parker, S. Ozone Treatment for Cooling Towers. Pacific Northwest National Laboratory (12/95)
US Patents 7,228,699; 7,197,887; 6,776,001; 6,705,096; 6,581,402; 6,497,107
For Further Information Paul Glanville, PE Principal Engineer End Use Solutions, GTI Phone: 847-768-0782 Mobile: 847-222-3278 Fax: 847-768-0916 [email protected]
Dr. Aleksandr Kozlov Senior Engineer End Use Solutions, GTI Phone: 847-768-0736 Mobile: 847-281-6963 Fax: 847-768-0916 [email protected]
Lee Gillan, PE Chief Engineer Coolerado Corporation Phone: 720-974-9614 Mobile: 303-870-7122 [email protected]
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Appendix A: Psychrometric Primer
Dry bulb temperature (commonly referred to as “temperature”) and Relative Humidity (RH) are familiar to us through meteorology. Absolute Humidity is a mass ratio of water vapor to dry air, and as mass is conserved regardless of heat input or output, it is independent of dry bulb temperature.
Wet bulb temperature is the thermodynamic limit of cooling a given parcel of air through adiabatic humidification (constant enthalpy). Once fully humidified the air is saturated, where any additional moisture will condense out and the parcel is at its Dew Point Temperature. When studying the M‐cycle, it is important to know that the wet bulb temperature is a function of both dry bulb temperature and absolute humidity. Wet bulb temperatures are dependent on the local water vapor pressure, which changes with the local dry bulb temperatures.
Psychrometric Charts
The chart is set up with ranges of dry bulb temperature and absolute humidity on the horizontal and vertical axes respectively (see chart below). The chart terminates in the upper left corner along the saturation line. The corresponding dry bulb temperatures along this line are the dew point temperatures.
Wet bulb temperature lines are diagonal with respect to the graph. Lines of constant enthalpy are quite close to parallel with the lines of constant wet bulb. This makes sense because the wet bulb temperature is an expression of an energy potential3.
Lines of constant RH are curved similar to the saturation line (100 % RH). Absolute humidity is preferred over RH in discussion due to its precision.
3 The lines of constant enthalpy and wet bulb diverge due to the slight dependence of heat capacity of air and water on dry bulb temperature over the given range.
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Appendix B: Detailed M-Cycle Discussion
Discussion of M-Cycle Flow Arrangements (Figure 4)
Figure 4 (A) illustrates a flow diagram through the M‐cycle with a product channel used for cooling a product air (or any fluid). This arrangement is used in Coolerado’s air conditioners. The product air is fed along the product channel of the heat exchange unit. Working air (for example, ambient air) is fed along the working dry channel and the working wet channel is arranged in heat transfer contact with working dry channel via a plate. The reverse side of the plate is wetted with a moving film of evaporative liquid (for example, water) using any available method. The plate can be made of wick, plastic, metal, solid desiccants, micro sieve, etc. materials or composition of these materials. It is understood that the term “plate” is used, but any structure that performs the function of separating the working dry channel from the working wet channel or separating working air from the product channel is suitable.
Satur
ation
line
Abso
lute
Hum
idity
Figure 4. Variations of M‐cycle with (A) Product Air Cooling and B) Partial Extraction of Air
Working air can either be drawn to the induced draft fan mounted downstream of the working wet channel or pushed through by forced draft fan upstream of the working air dry channel. Product air is directed along the product channel where it is cooled down without changing its moisture content. At the same time, the working air is directed concurrently with respect to the working dry channel in contact with a heat exchange surface of a plate. In so doing, the working air is cooled down without changing its moisture content due to evaporative cooling taking place in the working wet channel. This cooled air is turned to the working wet channel where it flows counter currently in contact with the moist surface (wick or capillary‐porous material being wetted by water). As the working air passes along the working wet channel, it is heated, humidified, and is drawn by the induced draft fan to the atmosphere.
As the working air passes along the heat exchange surface in dry channel it is cooled as a result of the heat exchange by the same flow passing along the surface of the working wet channel, that is wetted by vaporized water. In the working wet channel the latent heat of evaporation is transferred via the plate to precool the working air in the dry channel, which results in cooling of working air on the wet surface. Should outside air taken directly from the atmosphere be used as the working air, then by the time it has passed through the dry channel and contacts the water vapor in wet channel, it will cool down to near the dew point temperature of the
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working air. In so doing, ideally the product air can be cooled to the dew point temperature by evaporative action in the wet channel, transferring 100% of the latent heat from the heat exchange plate between the product channel and working wet channel. In reality this temperature is higher due to the product channel plate thermal resistance and other irreversibilities.
Comparing the embodiments according to Figure 4 (A) and (B) with the associated psychometric chart represented in Figure 4 (C), differ from the conventional indirect evaporative cooling (Figure 2) by the whole working air, which after its passing through the working dry channel is caused initially to pass through the working wet channel.
Through the M‐cycle, this results in that the product air (as the working air inside the working dry channel) to be conditioned will be cooled down to a lower value than according to the conventional indirect evaporative cooling (Figure 2) as the intake temperature of the working air stream and more importantly, its wet bulb temperature, is lower than initial temperature of ambient air. In the embodiments in consideration, it has been assumed that cooling for the product air (as the working air inside working dry channel) takes place from point 1 on horizontal line to point 2, which corresponds to approach to the dew point temperature (Figure 4, C). The change of the state of the working air during its passing through the wet channel follows the saturation line from point 2 up to the point 3 located higher in psychrometric chart. Thereafter the humidified working air stream returns to the atmosphere.
Thus in the M‐cycle, the Working Air stream cools itself and conditions the product air.
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The M-Cycle on Psychrometric chart (Figure 5)
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Discussion of M-Cycle Theory
Moving from Indirect‐Direct Evaporative Cooling, the M‐Cycle differs through integration of the working wet and dry channels, in which the working fluid participates in a latent to sensible energy swap with itself. This appendix will go into detail in substantiating the two key claims of the M‐Cycle (Figure 6):
1. At steady state, the coolest point throughout the M‐Cycle is near the transition from working dry to wet channels, at state 2 in the working dry channel.
2. Saturation is reached rapidly and maintained in the working wet channel.
Satu
ratio
n line
Abs
olut
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umid
ity
Figure 6. M‐cycle with product air cooling
Ignoring the temperatures posted at state (2) in the diagrams (A) and (B) of Figure 4 in the brochure, consider the following thought exercise:
i. In the worst case scenario, let us assume that the wall between the working dry and wet channels would be of infinite insulation and the working dry channel would experience no sensible cooling. This would make working channel states 1 and 2 identical. Based upon our knowledge of Direct Evaporative cooling, state 3 would be saturated and have a dry bulb temperature equal to the incoming wet bulb, 66 °F. Thus, the best cooling offered to the Product Dry Channel would be to slightly above this, at say 68 °F dry bulb. This would be no better than Basic Indirect Evaporative Cooling.
ii. From the discussion of Indirect‐Direct Evaporative Cooling, we know that it is desirable to sensibly cool the working dry channel prior to transitioning to a wet channel. This acts to reduce the wet bulb temperature, thus the evaporative cooling potential. The limit of this potential is to the dew point of the incoming fluid, at state 1.
iii. Now, consider that sensible heat is exchanged across the wall between the working wet and dry channels. For illustrative purposes, let us assume that the working dry channel is sensibly cooled to just above the incoming wet bulb, to 68 °F dry bulb. It is reasonable to expect this, as the wet working channel can easily be reduced to 66 °F as shown in worst case scenario. Now the wet bulb has been decreased from working states 1 to 2, from 68 °F to 60 °F. It is possible now to have the exit state for the working wet channel, state 3, at saturated conditions with a dry bulb temperature equal to the state 2 wet bulb, 60 °F.
iv. What may jump out as strange in the prior statement is the following: o The basis for our assumption of state 2, sensibly cooled from state 1 down to 68 °F dry bulb, was
that the coolest portion of the working wet channel was 66 °F dry bulb. o This assumption of state 2 led to cooler temperatures in the working wet channel, down to 60 °F
dry bulb. This is because the wet bulb temperature was lower at state 2 than state 1 due to sensible cooling of the working dry channel.
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o So if 60 °F dry bulb is now the coolest portion of the working wet channel, couldn’t state 2 be below 68 °F dry bulb? The answer is it could, it would, and a useful feedback mechanism develops.
v. So we have to correct our assumption of state 2 from 68 °F to 60 °F dry bulb, just above the coolest portion of the working wet channel. This further sensible cooling from state 1 to 2 drives the wet bulb temperature at state 2 down further, to 57 °F. As the case before, this reduced wet bulb temperature at state 2 indicates that the working wet channel has more cooling capacity than previously estimated, down to 57 °F dry bulb from 60 °F. This will in turn require a revised assumption of the sensible cooling from state 1 to 2 and the cycle repeats. What is apparent in this thought exercise, is that through each iteration the coolest portion of the working wet channel becomes closer and closer to the dew point of the incoming fluid.
vi. As equilibrium is approached, state 2 will reach dry bulb and wet bulb temperatures near the incoming dew point. As state 2 is sensibly cooled to closer and closer to saturation, reaching the saturation following the transition to the working wet channel will be rapid. Thus, the working fluid is saturated for the remainder of the working wet channel. As heat is accepted by the working wet channel from both the product and working dry channels, its dry bulb temperature will rise once the fluid reaches saturated conditions. Thus, the coolest portion of the working wet channel will be near state 2, at the transition from working dry to wet channels.
As discussed, the working dry channel can be sensibly cooled approaching the incoming dew point at state 1. Into the working wet channel, the fluid is rapidly saturated and exits at saturated condition. A few comments on the conditions at state 3 (Figures 4 and 6):
1. If the product dry channel or partial extraction did not exist, then state 3 would be saturated at a dry bulb temperature of approximately no greater than the incoming wet bulb temperature at state 1 (Figure 7). This is to assure an energy balance, representing a direct swap of sensible for latent heat.
Satu
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umid
ity
Figure 7. Adiabatic evaporation with air heat regeneration
2. With a product dry channel (Figure 6) or partial extraction (Figure 4, B), there is an additional supply of sensible energy being swapped for latent energy with the working wet channel. Thus, depending on the dry mass flow of the product dry channel or extracted air, the maximum exiting dry bulb temperature at state 3 is only limited by the incoming dry bulb temperature at state 1. That is, a dry bulb temperature gradient is required between states 1 and 3. In infinitely long channels, the dry product mass flow could be increased such that state 3 exited at a dry bulb temperature of 86 °F. Thus the latent for sensible enthalpy swap is bound by both the dry bulb and dew point temperature of state 1, the larger the difference the higher the cooling capacity.
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Thermodynamic & Graphical Explanation of the M-cycle feasibility to cool down to the dew point
As an alternative to the psychrometric thought exercise discussed in the previous section, one may prove the feasibility of cooling to the dew point temperature with the M‐Cycle through a thermodynamic analysis. As the key to the M‐Cycle is the integration of the working dry and wet channels and the use of an additional product dry channel (Figure 6) is immaterial to sub‐wet bulb cooling, we will use this working dry/wet channel as shown in Figure 7 as a starting point. Let us break this diagram into its wet and dry channels, as shown in Figures 8 and 9.
Satur
ation
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Abso
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Hum
idity
Figure 8. Air cooling down to saturation with no humidification
Satu
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idity
Figure 9. Heating of saturated air
Dry Channel (Figure 8). First we assume a mass of dry air with tDB=86°F and TWB=66°F in a dry channel is cooled to the dew point temperature 54.7°F, with a heat sink of sufficient magnitude. No moisture is added to the air (constant absolute humidity). Using psychrometrics, cooling state (1) down to its dew point requires that this heat loss, Q1‐2, be 7.7 Btu/lb dry air. Wet Channel (Figure 9). Now assume that an isolated wet channel heats saturated air (RH=100%) with tDB=tWB= 54.7°F at state (3) up to the temperature 66 °F along the saturation line. The heat required to accomplish this, Q3‐4, is also 7.7 Btu/lb dry air.
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We purposely chose that outlet parameters (temperature and absolute humidity) of air in the dry channel (Figure 8) and inlet parameters of air in the wet channel (Figure 9) were the same. With the heat reject by the dry channel equal to the heat required by the wet channel, Q1‐2 = Q3‐4, and saturated air at (2) and (3) at equivalent psychrometric states, one may integrate the two into a feasible combined process. This integration is shown in the diagram below, which shows a now adiabatic process.
dh1-2=dh3-4=7.7 Btu/lb - heating capacityIf m1=m3 then Q1-2=Q3-4
Saturated air, m1=m3
t2DB = 54.7 Ft2WB = 54.7 FH=0.0091 kg/kgRH=100%
t1DB = 86 Ft1WB = 66 FH=0.0091 kg/kgRH=34.36%
2 1
t1DP = 54.7 F
Air, m1Dry channel
Q1-2=Q3-4
t3DB = 54.7 Ft3WB = 54.7 FH=0.0091 kg/kgRH=100%
t4DB = 66 Ft4WB = 66 FH=0.0137 kg/kgRH=100%
3 4 Heated saturated
airWet channel
86 F66 F
1
2t1DP=54.7°F
Psychrometric Chart(afterheating of
cooled saturated air)
Dry Bulb Temperature
Satu
ratio
n lin
e
54.7 F
t1WB=66°F
3
4dh1-2 =dh3-4 A
bsol
ute
Hum
idity
Figure 10. Afterheating of cooled saturated air
As we can see from the diagram and psychrometrics in Figure 10 the dry air at state (1) can be cooled down to its dew point temperature 54.7°F by indirect evaporative cooling. What is important to note about this discussion is that it did not require an a priori assumption that cooling the ambient air to its dew point temperature is possible within an adiabatic system.
