Refrigerated Seawater System Rebuilds for Bristol Bay Gillnetters
Here's a little boat maintenance job you might feel like taking on, but may be better left to professional help instead. By law, the maintenance and repair of refrigerated seawater systems (RSW) on fishing vessels requires a technician certified to EPA standards when handling the refrigerant.
An RSW is an often-overlooked maintenance item that normally doesn't get attention until it fails. The RSW systems are important as they bring added value to your fish, and your bottom line. In today's terms a red sockeye salmon gillnetter in Bristol Bay delivering 150,000 lbs. of refrigerated reds at $0.90/lb., in lieu of $0.75/lb., means a $22,500 season bonus on your paycheck for that 15 cent differential and higher quality fish. That's almost enough to pay for a new RSW packaged unit, rather than trying to rebuild after 8 years' service.
The 32' Rozema-built Bristol Bay gillnetter F/V Finnegan has been operating a 7.5-ton IMS RSW unit since 2007. It is a self-contained, second generation, hydraulic driven, skid mounted RSW unit with compressor, condenser, chiller and seawater pumps. It is capable of refrigerating (6) 250 cubic foot fish holds to pack 14,000 lbs. of salmon in RSW.
The RSW unit has a single hydraulic motor that runs the compressor and seawater pumps for the condenser cooling pump and fish hold circulation. Hydraulic oil required to run the system is 8 to 10 GPM at 2,800 PSI at start-up of 55-degree Fahrenheit seawater, reducing to 2,300 PSI at an ending temperature of 33-degree seawater. Hydraulic oil is typically supplied by a hydraulic pump off the main engine. A 2.8- to 3.8-cubic-inch displacement pump will require a main engine horsepower draw of 13 to 16 HP, depending on the efficiency of your engine driven hydraulic pump.
The rebuild of the RSW unit was taken on by Kurt Manchester, father of the F/V Finnegan's owners Griffin and Willie Manchester. Kurt didn't have time on his hands but did have the mechanical expertise from years of working with his Grandpa Ole Bardahl's hydroplane, car and motorcycle race program.
Upgrades included a new Refrigeration Research accumulator with liquid line heat exchanger. These heat exchanger/suction accumulators help the system by protecting the compressor from refrigerant flood-back, sub-cooling the liquid refrigerant for greater system efficiency, and stopping condensation and frost from forming on the accumulator.
A new, quieter hydraulic motor with a stronger shaft replaced the original. Self-closing hydraulic hose ends were added to facilitate the sliding out of the lazarette-mounted unit without tools. Cam locked fittings added to the pumps allow for conversion to emergency bilge pump service in less than one minute. They also provide ease of maintenance draining the system and servicing the pumps when needed.
2-Way Pressure Actuated Water Regulating Valve
In addition, a pressure-actuated water regulating valve was added to the condenser cooling water circuit. The water regulating valve is designed to maintain a constant head pressure by regulating the flow of water in the condenser. More water lowers the pressure while less water increases the pressure. Constant, consistent head pressure helps to keep the thermal expansion valve operating properly and efficiently. With varying seawater temperatures, the amount of water needed to maintain the correct head pressure will vary. The head pressure valve regulates this automatically. Most small RSW systems do not have one because the same pressure regulation can be accomplished manually with a 3-way ball valve, with less capital and installation cost.
Low-temperature freezing systems will almost always have these, because it's more important to keep high pressure at a closer range. If head pressure is too low, the thermal expansion valve will not work properly. If too high, the valve will work but the system is not efficient, and capacity will be reduced. The trick is to keep the SST (Saturated Suction Temperature) up as high as possible while keeping SDT (Saturated Discharge Temperature) as low as possible.
The SST and SDT are ratings the compressor manufacturers use to state the capacity of their compressor. Note the SST and SDT will change throughout the run cycle of the system. For example, at 55-degree seawater you would see 40 degrees for SST and 90 degrees for SDT. With 33-degree seawater you would see 20 degrees for SST and 90 degrees for SDT. As the system's water chills, the compressor capacity reduces.
There are two common types of valves: 2-way direct acting and 3-way direct acting. The two-way valves are for pumps that can operate with back pressure. The three way valves suit pumps that cannot. This particular system needs a 3-way valve because the system's pump can't handle backpressure. Rebuilding included going through most of the components listed below:
• Heated Accumulator Receiver
• New sweated Copper Tubing
• Seawater Pump
• Compressor Belt Including Spare
• Electric clutch for the Compressor
• Seawater Pump Belt Including Spare
• Hydraulic Motor
• Hydraulic Motor Coupling
• Hydraulic Motor Mount
• Condenser Cooling Bypass Valve
• Thermal Expansion Valve
• Hydraulic Hoses
• Refrigerant Lines
• Seawater Hoses
• Self-Closing Hydraulic Hose Ends
• Refrigerant Pump Down by Certified Technician at Highland Refrigeration
• Refrigerant Charging by Certified Technician at Highland Refrigeration
• Stainless Steel Nuts, Bolts and Washers
• Electrical Components including Solenoid Valve
Many items just could not be accomplished without professional help, such as the certified technician pump down and recharge, condenser and chiller cleaning, items requiring a hydraulic press and aluminum welding of wasted mounting and foundation components.
Your RSW system adds a lot of value to your catch. Keep it running properly, and make sure to have it serviced by a qualified refrigeration technician.
Doug Short of IMS Integrated Marine Systems contributed technical expertise to the preceding article. John Douglas Dixon, P.E. is a Naval Architect and Marine Engineer and General Manager of Pacific Fishermen Shipyard and Electric.