Optimizing Defrost Systems
Defrosting an ammonia refrigeration system is important to the efficient operation of the system. But, just as important is the method used and how often it is performed. This column will be the first of a series that examines optimum defrost systems.
Several methods of defrosting, including hot gas, water, air and electric, can be used independently and in various combinations. Hot gas is the predominantly used method, so I will discuss that first.
Hot gas systems that are installed in a liquid overfeed (liquid recirculation) system are designed so that the coils are circuited for maximum cooling efficiency. In this situation, the evaporator, also referred to as the air-handling unit (AHU), can perform double duty. It also will work adequately as a condenser, assuming that the necessary piping and flow modifications are made. When the evaporator acts as a condenser, the refrigerant is giving up heat. However, if the fans are turned off, the ability of the coil to transfer heat is limited. As a result, the heat from the hot gas is absorbed in the metal and the temperature rises high enough to melt the frost and ice that are on the coil. The water from the melted frost drains off by a hot-gas-heated drain pan. This method of defrost is effective; however, it can be troublesome and inefficient if it is not engineered properly.
While the ability of a coil to transfer heat with the fans off is limited, it does not become zero. Some of the heat being given up by the hot gas is transferred to the space around the coil. The means by which the transfer takes place is a combination of radiation and convection, both occurring because the coil is warmer than the surrounding space. Some engineers estimate that 50 percent or more of the heat given up to defrost a coil is lost to the surrounding space.
Because the rate of heat transfer is a function of the temperature difference between the coil surface and the room air, the lower the refrigerant temperature and pressure of the hot gas during defrost, the lower the magnitude of the heat losses to the space will be.
Another incentive to maintain the lowest possible defrost temperature and pressure is the tendency for coils to steam when the temperature and pressure are too high. This is a major problem in low temperature freezers. When the air around a coil is heated during defrost and the air temperature rises, the relative humidity falls. This causes the evaporation rate of surface water on the coil to increase. The water that re-evaporates adds to the room refrigeration load. When steaming occurs, there are icicle and frost formations on the AHU housing, the fan blades, which will be at a lower temperature than the air, and on the structure, particularly on the structural steel. This ice formation necessitates increased maintenance. If ice attaches to the fan blades, the fan will be out of balance and may cause serious damage. Ice accumulating on fan rings also can restrict the fan operation.
At lower defrost temperature and pressures, the defrosting process takes a slightly longer time; however, at these lower temperatures, the overall defrosting efficiency is much better due to the reduction of refrigeration requirements.
Another loss component during defrost is the hot gas that blows through the coil and relief regulator and vents back to the compressor through the wet return line. This loss can be eliminated by employing a float drainer and draining the refrigerant condensed in the coil during defrost to the intercooler or the high temperature recirculator. This is a more efficient way of handling the defrost.
The defrost cycle should not be initiated any more often than necessary to keep the coils free of frost and ice. In the normal cold storage warehouse, less defrosting is required during the winter months than in the summer months, so the defrost schedules should be adjusted for summer and winter situations.
There is a better way. It is known as demand defrost. Demand defrost is actuated by a pressure-sensing device that measures the pressure drop across the coil. As the frost builds up on the coil fin surface, the pressure drop of the air across the coil increases. The pressure-sensing device initiates the defrost only when the coil needs defrosting, not when an operator feels that it should be defrosted. This demand defrost system, along with a float drainer added to the defrost piping system to drain the defrost refrigerant liquid, comprises the most efficient defrost system available when using hot gas.
Estimates indicate that the probable refrigeration load added to the total system load during defrost can be as much a three times the normal coil load during the operating cycle. This behooves you to properly engineer hot gas defrost systems. I'll explore the proper piping of the defrost system in future articles. PCE