Page 42

FISH FARMING TECHNOLOGY #2

A

ALL YOU NEED TO KNOW ABOUT WATER CHILLERS IN AQUACULTURE American aquatic equipment manufacturer Aqua Logic Inc., shares their expertise in an overview of water chillers; their function, features, and best use practices for recirculating aquaculture systems (RAS).

n aquaculture water chiller is a refrigeration system that cools and maintains recirculating fresh or saltwater tank systems to the desired water temperature. Many of these chillers are used for research, egg rearing and brood stock development. Consistent, reliable control of water temperature is key to a successful recirculating aquaculture system. Maintaining a consistent water temperature will reduce mortality rates, increase growth rates, and produce a quality product. The ability to reliably manipulate temperature is critical for control of spawning cycles.

Types of water chillers

There are several types of water chillers in the aquaculture industry, the most common being air-cooled, water-cooled and ground source cooling. The supply water for water-cooled units can be freshwater or seawater, sourced from a building water supply, well, reservoir, lake, bay, or other coastal access. These chillers can range from one quarter up to 1000 tons in size. A chiller used in the aquaculture industry would typically use reciprocating, semi-hermetic or scroll compressors, which are powered by electric motors. So how does an aquaculture chiller work? The chiller is comprised of four basic components that make-up the refrigerate compression cycle in which the refrigerant passes; (1) the compressor, (2) the condenser, (3) the thermal expansion valve (TXV), and (4) the evaporator. The cycle begins with the compressor converting a vapor (gas) into a liquid (Condensation). The condenser serves as a two-fold component. Before any condensation occurs, the high-pressure refrigerant vapor must be first brought to a saturated condition (de-superheated). Enough rejected heat must be transferred from the refrigerant to lower its temperature to the saturation temperature. At this point, condensation can begin. As heat continues to be transferred from the refrigerant vapor to the air (or water, if a water condenser is used), the quantity of the refrigerant (% of the refrigerant in the vapor state) will continue to decrease, until the refrigerant has been completely condensed into a liquid. In the ideal system, this occurs at the outlet of the condenser. In the real world, some subcooling would be expected at the condenser outlet. Subcooling is when the liquid refrigerant temperature is below the normal boiling point. Subcooling provides insurance against liquid flashing as the refrigerant experiences pressure losses in the tubing and components. The refrigerant is now in the liquid state, and at a high pressure and temperature. It must undergo a further reduction in temperature before it becomes a useful heat transfer medium. This is accomplished by reducing the pressure. In order to reduce the temperature, the pressure has to be reduced, which is accomplished by restricting the flow of refrigerant to the evaporator. It is preferable that the restriction regulates itself as the system load demands change. This is the role of a thermostatic expansion valve (TXV); it is an adjustable restriction which causes a varying reduction in liquid refrigerant pressure, yet will modulate in an effort to maintain constant superheat at the evaporator outlet. Superheat is the amount of heat added to a refrigerant after it has already turned to a vapor from a liquid. It must be measured to make sure that the liquid refrigerant in the evaporator coil has fully changed from a liquid to a 100 percent vapor. The thermostatic expansion valve is a superheat control, and will not maintain a constant vapor pressure. It only provides the restriction necessary to reduce the pressure to some level, which will be determined by compressor size, thermostatic expansion valve (TXV), size, load demand, and system conditions. If a constant evaporator temperature is required, it can be achieved very simply by maintaining the pressure corresponding to the saturation temperature required. This is 40 | February 2017 - International Aquafeed

FEB 2017 - International Aquafeed magazine  
FEB 2017 - International Aquafeed magazine  
Advertisement