About the authorCritically Charged Systems
Since the advent of the mandated Process Safety Management program (PSM) the industrial refrigeration sector has been faced with attempts to maintain lower refrigerant charges (re: inventories of refrigerant used in the system). To date, most of the efforts expended focus on keeping the ammonia refrigerant charge under 10,000 pounds. To a degree, this has largely centered around minimizing the quantity or use of gravity-flooded air-cooling evaporators or flooded shell and tube heat chillers. While these methods will drastically reduce the total refrigerant charge, other methods can be applicable. These typically involve re-thinking the basic premise for the use of a refrigerant and the application of specific sub-systems that are used in ammonia refrigeration systems.
In older systems it is quite common to see flooded evaporators. They are simple to design and install and only require periodic maintenance, such as oil draining. In the attempt to limit the ammonia inventory it is necessary to find other heat exchanger types that fulfill the same duty. For fluid cooling applications a brazed plate or plate and frame heat exchanger will work quite well. These heat exchangers can be selected for almost any common cooling function with greatly reduced refrigerant charges.
Other applications where gravity-flooded air-cooling evaporators are used may dictate the use of another liquid feed methodology. These may be:
- Heat exchange systems such as direct expansion (DX) are already considered as a low refrigerant charge device. The use of excess liquid is not required and in fact limited by the very nature of the control valve; a thermostatic expansion valve.
- Liquid overfeed systems typically consist of a mechanical pump that circulates an excess volume of refrigerant to achieve the rated performance of the evaporators. Numerous articles have been published as to what is required for the minimum accepted overfeed rate. Higher overfeed rates tend to dictate the use of larger piping, which in turn holds more liquid refrigerant.
In practical terms, we have to ask ourselves: how can you provide the desired cooling effect with the lowest volume of refrigerant in the system? Large high pressure receivers are used to provide pump down capacity, but these inherently have some volume of liquid within them to provide submergence of the dip tube. The liquid from this vessel is traditionally used as the supply for all of the evaporators or other vessels within the refrigeration system. This brings up a secondary safety issue: what is inherently safer? High pressure liquid ammonia or low pressure liquid ammonia? In the event of a release the high pressure liquid will generate a substantially larger cloud of ammonia when it flashes from a high pressure to atmospheric pressure.
Condenser piping practices commonly in use balance pressure gradients in the condenser coil with specific piping practices. If these are improperly installed or designed this can allow liquid refrigerant to back-up into the condensers by affecting the ability to drain by gravity. The extent of the deficiencies in this piping can increase the total refrigerant charge also. Not only can this increase the volume of refrigerant required, it can also increase the condensing pressure and the size of the high pressure receiver.
The use of modified liquid overfeed systems and other methods to provide lower refrigerant charges are technically feasible. We have been involved with several that have been installed and in successful operation for several years now. Our approach is based on the general concepts suggested above and applied to the specific requirements of the performance parameters dictated by the facility. These systems also greatly lend themselves to operation at reduced discharge pressures. In effect, the utilization of these methods offers multiple benefits of significantly lower refrigerant charges, increased energy reduction, while greatly reducing issues associated with cold weather operation.
Tags: draining evaporative condensers, energy reduction, liquid ammonia receivers, liquid overfeed, low refrigerant charge, refrigerant piping, release reporting
Valves vary from the extremely basic to the extraordinarily complex, and they are one of the oldest mechanical designs. Thanks for sharing great information, Hope to hear more updates from you …
Thanks for visiting. Valves and their intended use can present some challenges when operating conditions change or if prior adjustments were made to solve what might be thought of as a capacity deficiency.