Application of desiccant wheel to control humidity in air-conditioning systems

https://doi.org/10.1016/j.applthermaleng.2004.04.008Get rights and content

Abstract

Control of relative humidity is an essential aspect of maintaining indoor air quality in an air-conditioned space. In this paper, a desiccant wheel integrated air-conditioner is studied for low humidity air-conditioning. The desiccant wheel dehumidifies and heats the supply air. The regeneration of desiccant is accomplished by the return air, which gets cooled and humidified. The study shows that the proposed system can deliver supply air at much lower dew point temperature compared to the conventional system with a marginal penalty on COP. Its performance is better than the typical reheat system to provide the same low humidity levels.

Introduction

Air-conditioning consumes large amount of electrical energy, especially in hot and humid climatic areas. The cooling load of a building is the sum of the sensible and latent heat loads. While the former is due to the difference between indoor and outdoor temperatures, the latter is caused by the difference between indoor and outdoor humidity contents. Both these types of loads may also be generated within the building. Normally dehumidification of air is achieved by bringing the temperature below the dew point in the cooling coil to condense water vapor and then reheating it to the required temperature.

For human comfort, the relative humidity must be within a specified range [1]. Applications like library, museum, computer room, tapes/books/photo archives, etc. require not only low temperature but also low humidity. It is estimated that when room dry bulb temperature is lowered by 5.5 K, the life of the paper will double [2]. A low relative humidity of 35–45% and temperature of 20–24 °C have been recommended for library and museum considering both human comfort and shelf life of the objects [2].

Humidity control, though with certain limitations, can be achieved by air bypass control, variable speed fans and capacity control of compressor [3]. Reheat and desiccant based systems including hybrid systems can be used to lower the humidity. Energy efficiency ratio of desiccant enhanced air conditioner is more than other systems [4] at low sensible heat ratios. Even though various studies have been reported [5], [6] on the development of novel desiccant cooling systems and also on conventional desiccant wheel dehumidifier [7], [8] for humidity reduction in the conditioned space, it is observed that no system is identified which produces low humidity conditions in the conditioned space by utilizing accessories like desiccant wheel in conjunction with conventional AC system. Desiccant wheel is to be continuously regenerated by hot air for absorbing moisture in the ensuing cycle and hence energy cost for running the desiccant wheel would be large. Therefore it is prudent to utilize, either solar heat [9] or high temperature combustion gas heat/industrial waste heat [10] to accomplish the regeneration of desiccant material. The desiccant assisted air-conditioning (DAAC) system with regeneration of desiccant material by return air is theoretically analyzed [4] comparing the performance with reheat AC system, bypass control, variable speed fans, etc. However, there are no experimental studies reported on this topic. Moreover, it would be interesting to study the performance of DAAC system in terms of moisture removal capacity, cooling capacity and COP for various operating/design parameters like airflow rate, compressor speed, etc.

In the present study, the function of a desiccant wheel, which is integrated with a conventional air conditioner, is to recover heat from warm return air to reheat the cold supply air, simultaneously transferring moisture from the latter to the former. Thus, the supply air gets reheated simultaneously decreasing the moisture content thereby saving the reheat power. Also the regeneration section of desiccant wheel acts as a pre-cooler to the return air before it passes over the cooling coil, resulting in enhanced heat transfer performance of the cooling coil. With processing section of desiccant wheel simultaneously heating and dehumidifying the supply air stream from the cooling coil, the relative humidity in the conditioned space can be reduced well below 50%.

In this paper the performance of a desiccant wheel integrated air-conditioning system and its comparison with conventional air-conditioning system are presented.

Section snippets

Method of study

When moist air (return air) is cooled at constant pressure to a temperature below its dew point, some of the water vapor condenses. This technique is used for cooling and dehumidification of supply air in a conventional air conditioner [11]. Fig. 1 shows the schematic of conventional air-conditioning apparatus and its process. Moist return air enters at State 1 and flows across the cooling coil through which the refrigerant (HCFC22) circulates. Some of the water vapor initially present in the

Experimental details

The schematic of the experimental setup for desiccant wheel integrated air conditioner is shown in Fig. 4. The return air coming from room flows through the regeneration side of the rotating desiccant wheel (shown in Fig. 5). The pre-cooled and humidified air leaving the regeneration section of desiccant wheel then passes through the cooling coil for further dehumidification and cooling. The air leaving the cooling coil passes through the process side of the desiccant wheel for further

Results and discussion

The desiccant wheel integrated air conditioner and conventional systems are driven by same air-conditioner of 5.25 kW nominal cooling capacity. These two systems are tested at the following conditions:

  • Outdoor air: DBT=32 ± 1 °C and RH=65 ± 3%.

  • Indoor/return air: DBT=26 ± 0.3 °C and RH=65 ± 1%.

  • The airflow rate is varied from 250 to 750 m3/h.


The performances of the conventional and desiccant wheel integrated systems are compared. Further, the estimated performance of the reheat system to maintain the same

Conclusions

A desiccant wheel integrated vapor compression air-conditioning system is tested at various supply airflow rates to evaluate its performance and compare with those of conventional and reheat systems. The desiccant wheel enhances the dehumidification of supply air thereby increasing scope for low humidity air-conditioning. The dew-point temperature of the supply air is lower by about 2 °C and the COP is marginally lower by about 5% compared to the conventional system. However, the system COP of

Acknowledgements

The authors thank the Ministry of Human Resource Development (MHRD), Government of India, for sponsoring this project. The authors also thank Bry Air (India) Pvt. Ltd. for supplying the desiccant wheel.

References (11)

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