Planning Your RSW Project
Several “direct-replacement” refrigerants are on the market, but none is a simple “drop-in” replacement. There is labor involved in flushing and readjusting an existing system. As the cost of R22 continues to rise, these replacements become somewhat more affordable. Before switching to one of these replacement refrigerants, the vessel owner should assure himself that this particular refrigerant is readily available wherever the vessel intends to fish.
The approach we prefer is to continue using R22 in an existing system as long as the system is leak-tight. When in time the opportunity to upgrade your system arises, the owner may consider installing larger capacity chillers and condensers to permit using a higher-capacity refrigerant, such as R507, with an existing compressor and other components. Another course of action is to replace the system in its entirety, which in the long run often makes financial sense.
Retrofitting Into an Existing Vessel
Limitations to the size of the system to be installed include the amount of drive power available for the compressor, condenser pump, and circulation pump. A gen set must be able to supply the electrical spike to cycle on the compressor while the pumps and any other loads are on line.
The seawater circulation system must match the chiller capacity: the circulation system line sizes and pump capacity must provide flow adequate to match the refrigerant load on the chiller.
Planning for a New-Build Vessel
The vessel owner has the opportunity to determine how much refrigeration capacity is desired, then to size the gen set and the seawater circulation accordingly. There is also the opportunity to determine the optimum location for the refrigeration components, so the system is readily accessible for monitoring operation, for maintenance, and for service. Space may be available for a pre-fabricated skid-mounted system, or space may be better utilized by having the components installed on site.
One consideration is to install multiple RSW systems, generally two matching systems, installed along with two gensets, one sized to run both systems, one sized to run just one of the systems. Both systems may be run at once on the larger genset for maximum capacity, as when starting the refrigerating cycle with warm seawater in the tank. Then once the tank temperature approaches set point, only one system needs to run to pull down the last few degrees and to maintain tank temperature once fish begin to come aboard. The smaller gen set can be cycled on, and the larger gen set cycled off, saving on fuel and wear-and-tear.
Logging operating conditions in an engine room log is recommended. Useful benchmark information includes time taken to completely fill the hold with raw seawater while bypassing the chiller, and the time taken to tank down through the chiller. Also, at the start of the chilling cycle, ambient seawater temperature and temperature and pressure of the recirculated brine in the hold, then refrigerant pressures (discharge, suction and oil pressures) at regular intervals as the system chills. Recording brine temperature at regular intervals logs temperature pull-down rate. The desired brine temperature is setpoint. Monitoring brine pressure when the system is near setpoint can reveal freeze-up of the chiller.
When tanking down, try to ensure the water pumped into the fish hold is full-strength seawater. Take water from the middle of a sound, away from glacier and river run-off, if possible. If you must tank down in harbor, do so towards the peak of the flood. Seawater that is diluted with fresh water might freeze up in the chiller before desired setpoint is reached. Adding freshwater ice to the hold before tanking down, to “jump-start” the system, reduces the salinity of the water and raises its freezing temperature.
Seasonal System Start-up
If your unit is equipped with a compressor crankcase heater, the heater should be energized for several hours before starting the compressor. On many systems the heater is energized whenever the control power breaker is on. Check oil level in the sight glass located on the compressor base--a level should be visible. Keep in mind that refrigeration oil is highly refined and is nearly colorless under normal operating conditions.
Check the readings on the refrigerant pressure gauges. Some residual pressure should read on the gauges. If the gauges read zero, the refrigerant has leaked out. Check the liquid line sight glass. If the gauges read zero and the sight glass moisture indicator shows “wet”, it is time to call for service and repair. Running the system could compound a relatively simple repair job into a major one.
Make sure all refrigerant hand valves are open, including any valves to control lines. Assure that the brine in the hold is circulating through the chiller only (no bypass valves left open) and that the condenser pump is running raw seawater through the condenser. Switch on the control power. Switch on the compressor: if the compressor comes on, allow it to pump down the residual refrigerant left in the chiller before switching on the liquid line solenoid.
When the compressor starts, immediately check the oil pressure gauge. It should read significantly higher than the suction pressure gauge. If oil and suction pressure are the same, there is no net oil pressure, and the compressor should be shut down immediately. Don’t wait for the oil failure switch to do this, as it operates on a time delay.
Once the compressor has pumped down the system, it should shut itself off on the low pressure switch setting. This setting should never allow the compressor to pump into a vacuum; the suction gauge pressure at cut-out should always be above zero.
Now, switch on the liquid line solenoid. The compressor should start when the suction pressure reaches the cut-in setting on the low pressure switch. After the compressor starts, you may see foaming of the oil in the compressor oil sight glass. This is generally a normal condition as suction pressure drops on start-up, however once suction pressure stabilizes the foaming should subside and an operating oil level should be visible.
Gauge Readings During Operation
The suction gauge gives an indication of chiller temperature. Pressures correspond to saturated temperatures, which may be thought of as the boiling point of the refrigerant. The suction pressure generally corresponds to a saturated temperature about 10 to 15 degrees colder than the temperature of the recirculated brine.
Compressor discharge pressure is controlled by the amount of raw seawater being pumped through the condenser. For most efficient operation, the discharge pressure should correspond to 85-95-degree saturated temperature (the point where the refrigerant gas condenses into a liquid). Decreasing condenser water flow increases the discharge pressure. Higher ambient seawater temperatures result in higher discharge pressures, so the flow through the condenser may need to be increased.
Discharge pressure will drop as suction pressure drops, as brine temperature is pulled down, so occasional adjustment of condenser water flow is desirable to keep the system operating at best efficiency. On many systems the compressor discharge pressure is controlled by an automatic water valve that adjusts condenser water flow.
Liquid Line Sight Glass
Once the condenser water flow has been adjusted to get the optimum compressor discharge pressure, look at the liquid line sight glass. The glass should be clear, showing a solid stream of refrigerant with no bubbles. If you are unsure if the glass is completely full or completely empty, cycling the liquid line solenoid will make it obvious. Bubbles in the liquid line sight glass show that the expansion valve and the chiller aren’t being fed a full supply of refrigerant. This may indicate a short refrigerant charge, in which case the leak should be found and fixed. Bubbles in the liquid line sight glass, particularly if this occurs close to setpoint, can have other causes, even if the system is fully charged, so the operator shouldn’t just add refrigerant.
Freezeup and Refrigerant Floodback
As temperature setpoint is approached the engineer should start checking system operation more often. The compressor is a vapor pump, and if liquid refrigerant enters the compressor through the suction side, the compressor suction valves will be stressed and eventually will break. During refrigerant floodback conditions, such as when the chiller freezes up due to reduced brine flow through them, frost will extend down the suction line all the way to the compressor, and may cover a portion of the compressor. The cause may be low brine salinity, too low temperature setpoint, chiller clogged with debris, too many fish in the hold, circulating brine bypassing chiller (valve in wrong position or not sealing), or circulating pump failure. A pressure gauge to measure circulating pump pressure on one side of the chiller or the other becomes useful as setpoint is reached. A reading that starts to drop on the downstream side, or to increase on the upstream side of the chiller, indicates the freezer is freezing up.
System Shutdown (“Pumpdown”)
For compressor protection, this shutdown procedure should always be followed. The system thermostat (controller) does this automatically at temperature setpoint. To shut down the system, switch off the liquid line solenoid valve (or manually close the king valve).The compressor will continue to run as suction pressure drops, until the compressor starter coil is de-energized by the low pressure cut-out, and the compressor stops. The compressor should never be allowed to pump into a vacuum.
The chiller should be back-flushed thoroughly each time the catch is unloaded to flush out debris and reduce bacteria levels in the chiller.
Any time the RSW system isn’t going to be used for several days, and for seasonal lay-up, a few extra steps should be taken. With the compressor running and the liquid line solenoid valve energized, close the king valve and allow the system to pump down. If the system is equipped with a receiver, close the isolation valve, which may be provided between the condenser refrigerant outlet and the receiver inlet.
The compressor suction service valve should be front-seated to isolate the compressor from the chiller. Be sure 1/4” flare service port on the service valve is tightly capped, and be sure valve packing and valve cap is sealed.
Seawater should never be allowed to stand in the chiller or condenser for an extended period. In a well-designed system these components are self-draining. Many vessel owners flush their chillers with fresh water, before draining them. To maintain the condenser, secure the raw seawater inlet and remove the end plate from the return end. Check the zinc for action, and inspect the tubes for fouling. The end plate can then be bolted loosely into place, to be tightened next time the system is to be operated.
Wally McDonald is the owner of Petersburg, Alaska-based Fleet Refrigeration.