Introduction

In hot and humid climates like Florida the ability to provide adequate dehumidification is critical to maintaining suitable indoor air quality. In some cases, the process of dehumidifying the air can use the same or even more energy than is used in cooling the air. Hy-Save, Inc., of Portland, Oregon, has developed the Hy-Dry coil to work in conjunction with their Liquid Pressure Amplification (LPA) technology to increase the dehumidification capacity of DX A/C units.

The Hy-Dry system consists of the LPA pump and the Hy-Dry coil. The LPA system employs a small centrifugal pump between the outset the condenser and the expansion valve to increase the pressure of the liquid refrigerant. This increases the saturation (evaporation) temperature of the refrigerant without increasing the actual temperature of the liquid, and this prevents boiling, or flash gas formation, between the condenser and expansion valve. The presence of flash gas at the expansion valve greatly degrades the performance of the A/C system. The LPA system is designed to ensure that a solid column of liquid is delivered to the expansion valve, so that the expansion valve can, in turn, provide an adequate supply of liquid refrigerant to the evaporator.

The Hy-Dry technology uses a subcooling/reheat coil to subcool the liquid refrigerant between the condensing coil and the expansion valve. (The amount of subcooling is defined as the difference between the measured liquid temperature and the liquid saturation temperature at a given pressure.) The Hy-Dry coil is installed on the supply air side of the evaporator coil. The refrigerant, after leaving the condenser coil, is pumped through the Hy-Dry coil and subcooled before entering the expansion valve. The Hy-Dry coil also serves to reheat the overcooled 48F supply air and bring it back up to a normal supply air temperature. The relative humidity of the air is lowered as the air is reheated.

ETL Testing Laboratories, Inc., of Cortland, New York, evaluated a13-ton DX unit (unitary system with air-cooled condenser) both before and after an LPA pump and Hy-Dry coil were added to the unit.

Summary of Test Results

The effects of the LPA pump and Hy-Dry coil on the sensible and latent capacity of the A/C unit are shown in Table I on the following page. The latent capacity of the system increased significantly while the sensible capacity dropped slightly. The LPA pump and Hy-Dry coil increased the latent capacity of the unit by 38.8%.

The latent capacity is the amount of latent heat that the unit can remove from the conditioned space per hour. Latent heat is the energy that changes the state of a substance without changing the temperature of the substance. Condensation and vaporization are examples of latent heat processes. In contrast, sensible heat is the energy that causes a change in the temperature of a substance, without changing its state. The sensible capacity of the A/C unit is the amount of sensible heat that it can remove per hour from the conditioned space. The sensible heat ratio of the unit is simply the sensible capacity divided by the combined latent and sensible capacity.

Economic Analysis

Due to the tremendous variety of installations as space conditioning systems, it is impossible to do a general economic analysis. Because savings will be site-specific, all economic analysis should be done on a case-by-case basis.

Conclusions

Test results show that while there is a minor loss in overall capacity (-1.8%) there is a significant gain in latent capacity (+38.8%). This would indicate that if a facility is having difficulty controlling humidity (maintain below about 55% R.H.)' but has plenty of overall capacity then this would be a potential application for the Hy-Dry system. Anytime a tradeoff' between sensible capacity and latent capacity is practical, the Hy-Dry system is a conceivable solution.

Humidity control is a critical component of proper HVAC design. The American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) Standard 62-1989 recommends indoor humidity be maintained between 30% arid 60% RH to minimize the growth of allergenic or pathogenic organisms. However, this Standard also increases the requirements far outdoor air far ventilation from the previous Standard (ASHRAE 62-1981). In humid climates such as Florida the increased outside air will negatively impact efforts to maintain proper relative humidity by increasing the total cooling load as well as increasing the latent load. In an FP&L commissioned study performed by the Florida Solar Energy System, it was determined that the new ventilation requirements would cause a small office building to experience RH > 60% between 500 and 800 hours per year. For an elementary school the new ventilation requirements doubled the number of hours where the RH exceeded 60%. Humidity control will become more crucial with these new standards. Humidity control will also be very important in special applications such as clean rooms (very low RH) and supermarkets where high humidity (>55%) adversely affects display refrigerator case performance.

Humidity control in Florida is as complicated as it is important. Anytime you consider a significant change in a facility's HVAC system, FP&L recommends that you review all the potential alternatives with a qualified HVAC contractor or a mechanical engineer. FP&L recommends that you fully evaluate all dehumidification technologies, including the Hy-Dry System before making changes to your system.

10/24/97
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