The Maisotsenko Cycle

The following technical description of the Maisotsenko Cycle is meant for engineers and scientists who have a thorough understanding of thermodynamics.  A basic description and an engineering conceptual description are also available for other readers.

Steps to Understanding the Maisotsenko Cycle:

1. Review evaporative cooling.2. Review the indirect evaporative process.3. Learn the Maisotsenko Cycle.

Nomenclature

h is the enthalpy of the air G is the mass flow rate of dry air

Cp is the specific heat of the air

t is the temperature

hs is the enthalpy of the water vapor or latent heat from the evaporation of water

W is the humidity ratio or the weight of the water vapor in the air divided by the weight of the dry air

E is the effectiveness

Qo is the cooling capacity

Direct Evaporative Cooling

Evaporative coolers have been used to lower the temperature of air by using the latent heat of evaporation, changing water to vapor.  See Figures 1 and 1a.

In this process, the energy in the air does not change. Warm dry air is changed to cool moist air.

Heat in the air is used to evaporate water; no heat is added or removed making it an adiabatic process (Fan heat gain or pump energy is ignored in this evaluation.) This also assumes the water entering the system is to be evaporated at the wet bulb temperature of the entering air, and that there is no excess water.  Therefore the water has a negligible effect on the adiabatic process. The enthalpy of the system does not change; see equation (1), (2), and (3) below:

(1)    h = (Cp * t) + (hs * W), (2)    hin = hout.

The lower temperature and higher vapor content of the air can then be expressed by:

(3)    Gair * Cp * (tin - tout) = Gair * hs * ( Wout - Win) = Gwater * hs, 

        (sensible heat loss) = (latent heat gain) = (latent heat gain)

These direct evaporative systems vary from 70 percent to 95 percent effective (E) in temperature reduction to the incoming air's wet bulb temperature.  See Figure 1a, where:

(4)    E = (t1 - t1' wet bulb) / (t1 - twet bulb).

Figure 1: Cross section sketch shows direct  evaporative cooling.

 

 

 

Figure 1a: Psychrometric chart of direct evaporative cooling.

 

 

 

 

Indirect Evaporative Air Cooling

For many years indirect evaporative air coolers have been used with little success. Because of the poor heat transfer rates, commercialized units have not been able to produce a cooling capacity that justifies the excessive material and manufacturing costs.

Thermodynamically an indirect evaporative air cooler passes primary or product air over the dry side of a plate and secondary or working air over the opposite wet side of a plate. The wet side absorbs heat from the dry side by evaporating water and therefore cooling the dry side with the latent heat of vaporizing water into the air. The ideal and real conditions for indirect evaporative cooling are represented in Figures 2 and 2a. The air with temperature t1 on the dry side of the plate travels in counter flow to the air on the wet side. Ideally the product air temperature on the dry side of the plate could reach the wet bulb temperature t1wetbbulb = t2 of the incoming air (in reality only t2').

 

Figure 2: Cross section sketch shows indirect evaporative cooling.

 

 

 

 

Figure 2a: Psychrometric chart of indirect evaporative cooling.

 

 

Theoretically, the working air on the wet side of the plate would increase in temperature from its incoming air wet bulb temperature to the incoming product air-dry bulb temperature and be saturated. Of course this would require a balancing of the product and working airflow rates with an infinite amount of surface area and pure counter flow. Equation (5) shows the balance of energy, (excluding fan or pump and water temperature entering gain or losses) for any indirect evaporative cooling system, ideal or not where Qo represents the cooling capacity:

(5)    Qo = Gproduct (hproduct in - hproduct out) = Gexhaust (hexhaust out - hexhaust in)

(5a)   Qo = Gproduct * Cp (tproduct in - tproduct out) = Gexhaust * [ L (Wexhaust out - Wexhaust in) + Cp (texhaust in - texhaust out)

In practice it is not possible to have pure counter flow as the air must enter and leave from the same sides. This geometry of plate exchangers force indirect evaporative coolers to be in cross flow. The effectiveness E of these types of coolers is reported to approach 54 percent of the incoming air wet bulb temperature.  See Figure 2a, where:

(6)    E = (t1 - t2') / (t1 - t2) = (t1 - t2') / (t1 - t1wet bulb)

Figure 3 shows a typical cross flow indirect evaporative cooler. Examining the cooler across one plate shows that the highest heat flux will be where the dry channel and wet channel inlets cross. This is due to the wet channel working air having its lowest wet bulb temperature and the dry channel entrance having its highest temperature at this point within the heat exchanger.

The highest enthalpy of the working air leaving the exchanger (maximum work for the quantity of working air) will be where the dry channel product air enters.  At that point it is the hottest, and the wet channel working air leaves, as it will have reached its highest temperature and humidity. This means the cooling across the entire inlet of the product air dry channels is the most efficient portion of the heat exchanger.

The farther across the dry channel the product air travels the less heat transfer (work) is accomplished by the working air in the wet channels simply due to the cross flow characteristics. This means that about 10 percent of the working air and 10 percent of the surface area performs about 70 percent of the cooling.  

Figure 3: Diagram of indirect evaporative cooling.

Looking at product air flow change in energy or enthalpy, equation (7), where no water is added to the product:

(7)    Gproduct * (hproduct in - hproduct out) = Gproduct * Cp (tproduct in - tproduct out).

If an indirect evaporative cooler is only 54 percent effective, then the change in energy of the working air or the change in enthalpy is:

(8)    Gexhaust (hexhaust out - hexhaust in) = Gproduct * Cp 0.54 (tproduct in - twet bulb).

 

Maisotsenko Cycle

The Maisotsenko Cycle uses the same wet side and dry side of a plate as described in the above indirect evaporative cooler but with a much different airflow creating a new thermodynamic cycle. This cycle allows the product air to be cooled below the wet bulb and toward the dew point temperature of the incoming working air.

The Maisotsenko Cycle utilizes the psychrometric energy (or the potential energy) available from the latent heat of water evaporating into the air. The Maisotsenko Cycle was realized in a uniquely designed plate wetting and channel system, which achieved optimum cooling temperatures and saturated working air with the highest enthalpy possible for the exhausted working air temperatures obtained.

Counter Flow Adiabatic Heat and Mass Exchanger

To explain how the original Maisotsenko Cycle works thermodynamically, we have started with a simple adiabatic model shown in Figures 4 and 4a. This shows the cross section of plates with wet sides together and dry sides together.

In this example the incoming air I passes over the dry side of the plates and then turns as the air II passes over the wet side of the plates and then exhausted out as air III. As the air passes over the dry side of the plate, it is cooled by the water evaporating on the wet side or the latent heat of vaporization absorbs the heat form the plate. The air stream in the dry channels is cooled by the same air stream in the wet channels reducing its wet bulb temperature. The enthalpy at the point where the air turns from the dry channel to the wet channel II is at the dew point temperature of the incoming air hdew point. This pre-cooled air that turns to enter the wet channels is at the dew point temperature of the entering air stream. The energy balance would then be (see Figure 5a):

(9)    G (h1 - hdew point) = G (h1wet bulb - hdew point ) or h1 = h1wet bulb.

At any point across the plate h1 = h2 as there is no heat being added to or removed from the system.

These theoretical results have been achieved on a regular basis in several prototype models, confirming that the temperature of the air stream, after passing along the dry side of the plate, approaches the dew point temperature. In this process the cooling capacity is equal to zero because the product air is the working air.  There is only one stream of air or hin = hout.

 

Figure 4: Cross section sketch shows adiabatic heat and mass exchanger.

 

 

 

 

Figure 4a: Psychrometric chart of adiabatic heat and mass exchanger.

 

 

 

Analysis of this counter flow adiabatic heat exchanger shown in Figure 4 and 4a shows that the air stream leaving the wetted channels or plates possesses additional cooling capacity. This is characterized by the psychrometric difference in temperature between the air entering and leaving the system. The air leaving the system is saturated on line (φ = 1) with its associated potential moisture being the maximum of point 3 of Figure 5a, where t3 = t1 and φ = 1.

Heat to increase the exhaust temperature and moisture must be obtained from the dry side of the plates, as the wet side is in heat transfer only with the dry side, or h3 - hdew point > h1 - hdew point. Consequently, the flow rate of the exhaust air stream becomes less than flow rate of the entering air stream. If the reduction in the exhaust air stream flow rate would occur before entering the wetted channel where the air is coolest, then the air diverted could be used as a useful product. In this case the described heat exchanger in Figure 4 is transformed to Figures 5 and 5a.

Counter Flow Heat and Mass Exchanger

Moving to Figures 5 and 5a, if a portion of the total air stream I, has the product stream II, split off, then the working air stream III will receive additional heat from the dry channel as the total

air stream I is greater then the working air stream III. This forces additional evaporation and now htotal in > hproduct out or h1 > hdew point or there is cooling. The working air stream temperature will increase in the saturated condition moving up the saturation line j on the psychrometric chart shown in Figure 5a.

At any point across a plate there is cooling of the total dry air G total -- point 1 and evaporation of the working air Gworking -point 3. For an ideal cycle the temperature of the working air leaving the exchanger will equal the temperature of the air entering the dry side or t1 = t3. The ideal cycle heat balance, ignoring fan and pump energy loss and water temperature-entering gain or loss is (see Figure 5a):

(10)    Gtotal (h1in - hdew point) = Gworking (h3 - hdew point)    or(10a)   Gproduct (h1in - hdew point) = Gworking ( h3- h1in)

 

Figure 5: Cross section sketch shows counter flow heat and mass exchanger.

 

 

 

 

 

Figure 5a: Psychrometric chart of counter flow heat and mass exchanger.

 

 

 

Due to the steep curve of the saturation line, a small increase in the working air humidity on the saturation curve creates a larger temperature decrease of the product. Therefore the flow rate of

the working air stream becomes less than flow rate of the product air stream. The amount of energy the working air can then remove in an ideal cycle is h3saturated - h1, and that is also the amount of cooling. In practice the exhaust enthalpy will always be less then ideal, h4 saturated.

A variation of this cycle is to use perforations in the working air channels to reduce the pressure drop. The cooler cycle shows where the working air leaving the cooler reaches the temperature of the air entering the dry channel.  The working air is saturated, and the product is at the dew point temperature. Cooling capacity for this ideal model is:

(11)    Qo ideal = Gproduct (h1in - hdew point)

The heat balance of the ideal model:

(12)    Gtotal (h1in - hdew point) = Gworking ideal (h3 - hdew point)    or(12a)   Gproduct ideal (h1in - hdew point) = Gworking ideal (h3 - h1in)

 

Figure 6: Cross section sketch shows perforated cross flow heat and mass exchanger.

 

It is interesting to note that the performance of the cycle is thermodynamically dependent on the inlet air conditions as this determines the ideal coldest product temperature (the dew point temperature of the working air), and the ideal maximum temperature the working air can reach (the product air inlet temperature).

A Perforated Cross Flow Heat and Mass Exchanger

This system uses the previously described thermodynamic process, which is capable of sensibly cooling out side air.  However, this new process is capable of cooling any fluid (liquid or vapor) below the wet bulb and toward the dew point of the working air.  In addition, because the product channel is completely separated from the working channel, this process works at any pressure or temperature.

The working air stream is first passed over the dry side of a plate where it is pre-cooled and then passed to the wetted side of the plate. More particularly, the working air stream, (see Figure 6) Gworking passes through the perforations in the plate over the length of the plate from its dry side

to the wetted side.  There the working air stream then cools the dry side by evaporating water off the wet surface. Simultaneously the product air, (or any fluid), Gproduct is passed along a different portion of the dry side of the same plate as the dry working air and having heat exchange relationship with the wet side working air. The heat balance where the product and working air enter at the same temperature and humidity is (see Figure 5a):

(13)    Ideal Gproduct (h1 - hdew point) = Gworking (h3 - h1)

(14)    Real Gproduct (h1 - h2) = Gworking ( h4 - h1)

Effectiveness:

(15)    E = (t1 - t2) / (t1 - tdew point).

Figure 7: Diagram of actual perforated cross flow heat and mass exchanger with air flow paths.

This heat and mass exchanger delivers cooled air at some point below the wet bulb temperature of the air without adding humidity to the air. This provides two advantages over existing evaporative air cooling systems, as the air is cooler and dryer. Currently this atmospheric air cooler is made of a cellulose fiber coated with polyethylene with hot melt channel guides.

An independent testing lab has tested one of the Coolerado heat and mass exchangers. Tests obtained a wet bulb effectiveness of 110 percent to 122 percent and a dew point effectiveness of 55 percent to 85 percent.

The Maisotsenko Cycle has broad applications in many industries to increase the efficiency of cooling and saturation beyond any previously considered cycles while reducing initial and ongoing operating cost.

This cycle opens up a new way to arrange the flow in heat and mass transfer equipment obtaining better and more economical results. This technology is proprietary, patented and patent pending technology.