An approach to controlling frost on low-temperature coils is to spray the coil with an antifreeze, such as ethylene glycol or propylene glycol. A characteristic of this equipment is that it generally provides a higher ratio of latent-to-sensible heat removal from the air than is true of a nonsprayed coil. The assembly which can maintain a continuous operation is shown schematically in Fig. 6.57.
The glycol solution is sprayed on the surfaces of the evaporator to keep the coil free of frost. The air passing in contact with both the sprays and the evaporator surfaces is cooled and dehumidified. The glycol solution absorbs water, so a fraction of it is circulated to the regenerator to drive off the moisture. An alternate is to operate the regenerator intermittently. The regeneration of the glycol is accomplished by spraying the solution over a heating coil through which scavenging air, usually from the outdoors, passes. A property of glycol/water solutions is that the water-vapor pressure at a given liquid temperature is less than that of water alone. The consequence of this property can be demonstrated on the psychrometric chart, Fig. 6.58, where the water-vapor pressure is chosen as the vertical scale.
A curve for the vapor pressure versus the liquid temperature of a 50%-bymass solution of ethylene-glycol is superimposed on this psychrometric chart. The straight-line law holds for the air in contact with the glycol solution in the same manner as for an unsprayed coil. For the unsprayed coil, the path of air is A, while the air passing in contact with the glycol solution follows path B. Because the vapor pressure of the glycol solution at a given temperature is lower than that of the water or frost on the unsprayed coil, path B is steeper than path A, which results in a greater amount of dehumidification. The freezing temperature of this solution is -36°C (-32°F), so no frost will form on the coil as long as the coil surfaces are above this temperature.
The advantages of the sprayed coil over the unsprayed coil in subfreezing temperatures are:
– no need to shut down for defrost
– energy expenditure of hot-gas or water defrost is eliminated
– high latent heat removal when excessive humidity is a problem
– glycol provides a germicidal benefit, eliminating many bacteria, molds, and other microorganisms.
The disadvantages are:
– higher first cost
– parasitic energy effects of the warm solution returning from the regenerator
– the additional operating cost of the pump and extra fan.