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The Cromer Cycle is a thermodynamic cycle proposed and patented by Dr. Charles J. Cromer, PhD, PE in 1988.The Cromer cycle uses a desiccant to interact with higher relative humidity air leaving a cold surface. When a system is taken through a series of different states and finally returned to its initial state, a thermodynamic cycle is said to have occurred. The desiccant absorbs moisture from the air releasing heat and drying the air which can be used in a process requiring dry air. The desiccant is then dried by an air stream at a lower relative humidity, where the desiccant gives up its moisture by evaporation, increasing the air’s relative humidity and cooling it. This cooler, moister air can then be presented to the same cold surface as above, to take it below its dew point and dry it further, or it can be expunged from the system.
The desiccant undergoes a reversible process whereby it absorbs or adsorbs moisture from air leaving a cold surface releasing heat and then in the second part of the cycle evaporates moisture, absorbing heat and returning the desiccant to its original state to complete the cycle again. The original Cromer cycle concept patent expired in May, 2006.
Cromer Cycle Psychometrics
The Cromer Cycle is primarily used in air conditioning and drying applications. The cold surface portion of the cycle is most often a result of a Reversed Carnot or Refrigeration Cycle. For the Cromer Cycle to operate, a desiccant must be exposed to two air streams, one higher humidity from a cold surface, and one lower humidity to dry it. This is most easily accomplished by moving the desiccant. Any cycling mechanism can be used, such as pumping a liquid desiccant, however an easy mechanical application is a rotating wheel, loaded with desiccant, through which the different air streams pass. This is shown in Figure 1 where a desiccant wheel has been applied to a standard air conditioning set-up.
The psychometric process of the air passing through the system with four state points is shown on the psychometric chart of Figure 2 as 1, 2, 3 and 4. The state points of the air are also depicted on Figure 1. In this application, the air returning from the space, typically around 50% RH, is presented to the desiccant wheel and dries the desiccant. The air picks up moisture and cools, process 1 to 2. The moist air is now presented to the cooling surface (cooling coil of the air conditioner) which cools it below its dew point and dries the air, process 2 to 3. This represents the work done by the cold coil. In the meantime, the dried desiccant from below is rotated to the upper air stream. The saturated air leaving the coil, typically 93 to 98% RH, is presented to the desiccant at 3 where the air is dried further, process 3 to 4, where it is presented to the space as supply air. The desiccant, now loaded with moisture, rotates to the return air, where the cycle repeats. Typical cooling and drying by the cold coil without the Cromer cycle is depicted on the psychometric chart and is also shown in Figure 2. State point 1 is the air that returns from the space to the system (return air). For a typical air conditioning system, this air at state point 1 enters the cooling coil and leaves at about state point 4' after cooling and drying. State point 4' represents the temperature and moisture content of the air that leaves the typical unit, about 45 to 50 degrees F and 95 to 98 percent RH.
Changes to the Standard AC System by the Cromer Cycle
By observation of the psychometric process, there are a number of changes to the air conditioning cycle that should be apparent. First, the end state point 4 for air from the wheel represents a significant latent ratio increase (moisture removal), to about 45% as opposed to the 25% of the typical coil shown. Secondly, the air quality delivered by the cycle is much dryer, i.e. about 55% RH (state point 4) rather than 98% with the standard coil (state point 4'). This keeps the supply air delivery ducts much dryer, providing less opportunity for mold and mildew growth. Third, this is accomplished with a higher average evaporator temperature. Compare the midpoint of the evaporator’s temperature, line 1 to 4’, to the midpoint of the Cromer cycle’s evaporator’s temperature, line 2 to 3.This is significant because given a constant condenser temperature, the higher the evaporator coil temperature, the more efficient is the Carnot refrigeration cycle and the greater capacity any particular system can deliver.
The Cromer cycle requires an energy use penalty due to the increased pressure drop through the system caused by the desiccant. However, this penalty is similar to that caused by heat pipes or run-around coils (other strategies for enhanced dehumidification). But the Cromer cycle penalty can be offset by the warmer coil improvement of the efficiency on the Carnot refrigeration side, whereas the heat pipe and run-around coil strategies, produce colder coils with an attendant reduction of efficiency of the Carnot refrigeration side.
To operate in this cycle, the desiccant is required to absorb moisture from air coming off of the coil that is colder and about 98% RH, and desorb moisture to air that is warmer and at a lower RH. The desiccant is regenerated by the vapor pressure differential inherent in the RH differences rather than heat or temperature difference. Desiccants that have a moisture sorption isotherm of the type shown in Figure 3 (Type III), are common such as many formulations of silica gel. Type III desiccants absorb little moisture below 70% RH but many will take up more than their own weight in water from the air when presented with over 90% RH. The absorption isotherm is very steep between 90 to 100% RH. Desiccants of Type III have plenty of potential for the cycling of moisture from the air off of the coil, around 98% RH, over to the return air stream, typically around 50% RH.
Simulations of the Cromer Cycle
Independent simulations have been conducted to determine if the Cromer cycle concept is scientifically valid. The first, conducted by the inventor, used a wheel simulation model developed by Kirk Collier (DCSSMX1) which incorporates the finite difference algorithms for moisture sorption and desorption developed by Ian McClaire-Cross in Australia (MOSHMIX) into the DESSIM wheel model developed at NREL (then SERI). The simulation data provided the state points of air before and after the wheel. The air response through the coil and performance data were provided by a set of equations developed from measured data on a 3.5 ton split system heat pump unit. Two desiccant types, three wheel sizes, and three wheel thicknesses were simulated. All showed excellent moisture transfer of the desiccant and a doubling of the system moisture removal.
The second set of simulations were completed by Dr. Bruce Nimmo and Kannan Rengarajan at the Florida Solar Energy Center (FSEC) in 1993 and published in ASHRAE Transactions.The addition of heat pipes, to exchange heat from the incoming air stream to the outgoing air stream, is an alternative method to increase the moisture removal of a heat pump system. In the Nimmo study, the Cromer cycle was compared to heat pipes. Dr. Nimmo's simulations found the Cromer cycle to provide better moisture removal capability than the alternatives simulated, and at a LR of 52%, showed an improvement in EER from 10.1 to 11.1, a 10% energy savings compared to the heat pipe application. Simultaneously, the Cromer cycle increased capacity from 30.8 kBtu/hr to 34.0 kBtu/hr, a 10% increase in capacity compared to the state-of-the-art heat pipe system. Dr. Nimmo used an upgraded version of the Collier-Cross simulation developed by H. Henderson at FSEC, and the HVAC response was simulated by DOE-2. A Type III silica gel was used as the desiccant in the simulation.
A third set of simulations was conducted by Dr. E. Chant and Dr. S. Jeter at the Georgia Institute of Technology, Atlanta and also published by ASHRAE.Dr. Chant used a simulation developed at Georgia Tech. in 1991 using a parabolic concentration profile model (PCP) to model the desiccant moisture and sensible exchange. Chant's model showed that the Cromer cycle, when providing additional moisture removal, i.e. a latent ratio of 52%, vs. heat pipe, 41%, provided an energy savings (improvement in COP) of 2.58 to 2.68 (4%) and also an improvement in total cooling capacity from 9.23 Kw with the heat pipe to 10.39 Kw with the Cromer cycle (12.6% improvement). Dr. Chant assumed the desiccant wheel would have a greater pressure drop than the double coil heat pipe system and consequently the simulation added an additional fan energy penalty to the Cromer cycle.
In 2004 through 2006, the U.S. Department of Energy, through the Oak Ridge National Laboratory, sponsored a series of field tests of air conditioning equipment using the Cromer cycle manufactured by Trane, under their brand name CDQ. The tests covered three geographical areas: a 30 ton unit in Farmington Maine, a 12 ton unit in Birmingham Alabama, and a 3 ton roof top unit in Cocoa Florida. The Maine and Alabama tests were before-after tests, and the Cocoa test was a side-by-side test (alternating every week). All units were required to control both temperature and humidity in their respective spaces, and all units prior to switch-out used electric reheat as the humidity control mechanism.
The measured test results showed all Cromer cycle systems were able to provide more stable indoor conditions of both temperature and humidity. The total power use for each system was monitored and the energy use of the reheat coils on each system was also monitored. The results showed 40% energy savings for the Maine system, 63% energy savings for the Alabama system, and 71% energy savings for the Cocoa system. It is possible to provide re-heat by the condenser heat available on the compression side of the reverse Carnot refrigeration cycle, sometimes called “free” reheat or “hot gas bypass.” Because the actual energy use of the electric reheat coils was determined, the comparison was made as if the systems were provided with this reheat energy at no cost. Under this consideration, the Cromer cycle systems would have provided an operational energy savings of 23%, 33%, and 35% in Maine, Alabama, and Florida respectively.
Fresh Air Exchange
To maintain indoor air quality, it may be desirable to expunge return air from the conditioned space and replace it with fresh outdoor air, sometimes called “make-up air.” The optimal location to expunge return air from a Cromer cycle system is just after the desiccant (location 2 on Figure 1). At this point, the return air has been loaded with moisture from the desiccant, and expunging it removes additional moisture from the space. Furthermore, this expunge air is cooled below the return air condition by the desiccant’s evaporation of the moisture into it. Location 2 (but before the fan) is also the ideal place to bring outdoor air into the system, as the coil can then reduce its temperature and moisture before it enters the space. Also, if heat exchange is provided between the expunged air and the outdoor air, the incoming air can be cooled and brought near or to saturation before it enters the cooling coil for process 2 to 3, enhancing its dehumidification. The concepts of this fresh air application to the Cromer cycle were patented in a concept patent by Dr. Cromer, which expired November, 2019.
Cromer Cycle Dehumidifier-Dryer
When the process needed is more dehumidification or drying, the Cromer cycle can be enhanced by using the free heat available from the condensing side of the reverse Carnot refrigeration cycle. This heat, sometimes called “hot gas bypass” can be added before the desiccant wheel to enhance the drying of the wheel at location 1 of Figure 1 (but after the filter), called pre-heat. Hot gas bypass heat can also be added to the process at location 4, called reheat, which delivers warmer but even lower RH supply air. Either one or both hot gas bypass locations can be used. When a Cromer cycle air conditioning system is enhanced with hot gas bypass, it is typically referred to as “Active” Cromer cycle air-conditioning. When the cycle is used as equipment designed for dehumidification or drying, it is typically called a Cromer cycle dehumidifier or Cromer cycle dryer, respectively. The addition of hot gas bypass to the original Cromer cycle was patented by Dr. Cromer in 1998. That patent expired August, 2019.
- Cromer, C.J., “Cooling system”, United states Patent Number: 4,719,761, Jan 19,1988, United states Department of Commerce Patent and Trademark Office, Washington, D.C.
- MClaine-Cross, I. L., 1974, “A Theory of Combined Heat and Mass Transfer in Regenerators”, Ph.D. Thesis, Monash University, Australia
- Cromer, C. J., "Desiccant Moisture Exchange for Dehumidification Enhancement of Air Conditioners," Proceedings, Fifth Annual Symposium on Improving Energy Efficiency in Hot and Humid Climates, Houston, Texas, September 12-14, 1988.
- Nimmo, B. G., Collier, R. K. Jr., and Rengarajan, K., "DEAC: Desiccant Enhancement of Cooling-Based Dehumidification," ASHRAE Transactions: Symposia of the 1993 ASHRAE Winter Meeting, CH-93-4-4, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, N. E., Atlanta, GA, Jan., 1993, pp. 842-848.
- Nimmo, B. G., Rengarajan, K., Desiccant Dehumidification Enhancement of Electric Air Conditioning Units, Final Report, Oct. 1993, DOE Contract No. DE-FC03-86SF16305, A007 & A011
- Chant, E. E., and Jeter, S. M., "A Steady State Simulation of an Advanced Desiccant-Enhanced Cooling and Dehumidification System," ASHRAE Transactions 1994, V.100, Pt. 2, #3816, American Society of Heating, Refrigeration and Air Conditioning Engineers, Atlanta, GA, 1994
- Chant, E. E., Transient and Steady State Simulations of an Advanced Desiccant Enhanced Cooling Cycle, Dissertation for Doctor of Philosophy in Mechanical Engineering, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, November 1991, P. 224.
- ORNL Contract DE-AC05-00OR22725, Final Report: Trane Active Cromer Cycle (TACC) Field Performance Study, 2006, Oak Ridge National Laboratory, P.O. Box 2008,Oak Ridge, TN 37831, National Laboratory,U.S. Department of Energy, Washington, D.C.
- Cromer, C.J., “Cooling system”, United states Patent Number: 6,237,354 B1, Oct 27,1999, United states Department of Commerce Patent and Trademark Office, Washington, D.C.
- Cromer, C.J., “Heat Pump Dryer with Desiccant Enhanced Moisture Removal”, United states Patent Number: 6,094,835, Aug 1,2000, United states Department of Commerce Patent and Trademark Office, Washington, D.C.