Technological Advances in Marine Refrigeration
In the recent history of technological advances there are some profound developments in industrial and marine refrigeration that have completely changed the way we work. There are also simple yet remarkable advances that have greatly improved the service life of our systems. We live in a time when we are surrounded by fabulous technological developments that are changing the very nature of our interaction with the machinery of refrigeration, and improving the way we chill and freeze our catch.
New Compressor Types
The most significant change in marine and industrial refrigeration has been the migration from reciprocating (piston) compressors, where refrigeration technology began, to screw compressors that now dominate our industry. The trend began some thirty years ago, and as screw compressor technology improved, more and more owners were encouraged to invest in screw compressor equipment. Today, very few reciprocating compressors in the 100-HP and up range are being considered for new projects, and many of the existing reciprocating plants are rapidly being replaced by screw compressors.
The main reason screw compressors are so attractive to owners of fishing vessels and processing plants is the long service interval – the hours between partial and complete overhauls. For reciprocating compressors the typical regimen was for regular internal inspections at 10,000-hour intervals, with substantial rebuilds expected at 15,000 to 20,000 hour intervals. With modern screw compressors, having fewer wearing parts, thrust bearing inspections are typically done at 30,000 hours with many machines reaching 100,000 hours before any major work is indicated.
Other advantages include the ability to pack much more capacity into a small package. For example, a single 300-horsepower compressor package would take only a fraction of the space of three separate 100-horsepower reciprocating compressor packages. Furthermore, in an ammonia plant, a reciprocating compressor requires two steps in compression to achieve low suction temperatures (-40F). A screw compressor can do the same job in a single stage.
Does that mean that reciprocating compressors will become obsolete? No. For small plants – especially plants that are used seasonally, a reciprocating compressor of 100 HP or less is the most economical choice, especially for halocarbon (Freon) plants that can easily achieve low temperature refrigeration with single stage machines.
Changes in Materials of Construction
One of the most practical changes seen lately is the use of stainless steel refrigerant piping and titanium in chillers and condensers.
Stainless steel pipe is more expensive per foot than carbon steel, but it does not require the expensive process involved in removing the mill scale, varnish, and grit that comes with carbon steel pipe to be used for refrigeration. Furthermore, once a contractor has invested in implementing a stainless steel program that assures compliance with standards set forth by IIAR, ASHRAE, and ASME, then the costs tend to equalize, and the contractor can then deliver a stainless steel piped system at the about the same cost that his competitor can in carbon steel. Look at how that affects the value and safety of a vessel in the seafood industry: piping that won’t corrode, and a system that doesn’t continually wear itself out over the years by circulating the contaminants that are introduced by uncleaned carbon steel pipe!
Titanium, likewise, has become more affordable in the construction of seawater pumped refrigerant condensers and RSW chillers. Yes they are still more expensive, but they have a very long lifespan compared with carbon steel condensers and chillers, even in well-zinced seawater systems. Another advantage is the resistance that titanium has to erosion from high water velocities, and the resistance to biological growth and fouling that is inherent in carbon steel condensers. Biological growth and fouling decrease the capacity of condensers, and this decrease in capacity is reflected in much higher energy costs to operate the system.
Networks: Refrigeration Plants Meet the Internet Age
One of the major advances in the seafood industry is the use of web based automation systems that allow for remote monitoring, alarming, and control.
These are systems that can connect to all of the devices in the refrigeration plant for coordinated control and monitoring. They provide a graphical interface on any desktop computer on the local network, such as the machinery room, the wheelhouse, the engineer’s stateroom and even the galley. They can provide the same graphical interface on a remote laptop that has a wireless Internet connection, so that the engineer on vacation in the Caribbean can help diagnose a refrigeration problem with his factory trawler at port in Alaska. If a laptop is too much trouble to lug around while on a carefree vacation, the engineer’s smart phone will do just as well.
The engineer can assign different levels of access to different personnel. For his senior operator, he may give open access to operate the system and change set points. To a new hire, he may give view-only access to the animated system diagram, so that this apprentice can have an intuitive picture on how the refrigeration system works. The production foreman may wish to see when different freezers will be ready to turn around. If this type of display cuts fifteen minutes of confusion out of the turn-around time on each freezer, the production foreman will have dramatically increased the production of his operation. Integrated controls can also communicate with other on-board systems and software so that the production manager can see what the daily production and recovery was, and what the fuel cost per pound of finished product was for that day.
In the past, much of the technology depended heavily on proprietary software and graphics. Newer web-based systems ride on Internet technology and graphics that are inherent in the devices that we own. Yes there is still a degree of proprietary dependence, but as long as the engineer has the ability to operate his system effectively in “hands-on” manner in the event that the main control system goes down, integrated controls with remote capabilities will be a tremendous asset to any large operation.
Energy Recovery Using Heat Pumps
All heating energy on board the vessel, whether powered by generators or boilers, requires fuel; and fuel is very expensive. But seafood processors spend enormous amounts of energy driving their refrigeration system for the purpose of extracting heat from seafood products, and rejecting that heat into the condenser seawater stream to be pumped overboard.
Can we recover that heat? Is there enough of a payback in energy savings to justify the cost? Could we replace the use of the boiler during processing hours; or at least replace the portion that we use for comfort heating, domestic hot water, hot water wash-down in the processing areas, and boiler preheat? We can indeed.
Consider for a moment your refrigeration compressors; put your hand on the hot compressor discharge pipe for a few very short seconds; that pipe is hot! Your intelligent conclusion is that there must be a lot of heat to recover! However the “hotness” of that discharge pipe is what we call “sensible heat” - heat you can feel; and the sensible heat content of the compressor discharge vapor is a very shallow well - not much heat energy there at all. The real source of heat suitable for recovery is what we call “latent heat” or the heat hidden in the vapor state of the refrigerant that can be recovered by condensing that refrigerant vapor into liquid. The problem is that your refrigeration system is probably condensing at 65F to 85F. That is the temperature at which that the vapor is giving up its heat, and that temperature is of very little practical use.
We need to have a way of raising the condensing temperature to 160F in order to make hot water that has a high enough temperature to be of good use. Water at that temperature could be used for space heating, boiler preheat, and mixed with cold water for factory wash-down, and domestic hot water. We can raise the condensing temperature by raising the compressor discharge pressure; but the increase in pressure of that magnitude, would require an additional compressor stage – what we refer to as a “third stage heat pump”. This system would divert the majority of the refrigerant vapor that normally flows through the discharge pipe through a high pressure heat pump compressor where it would compress up to a much higher pressure - 474 psig, for an ammonia system, and have the ability to produce 150F water for all kinds of uses.
Heat recovery with the use of the third stage heat pump is a proven technology, but at present, it can only be found in very large land-based food processing plants. Some of these plants have had such a profound experience in lowering their process and space heating costs, that they have installed evaporators outdoors in order to provide additional load for heat recovery. This is a technology with a huge market in the fishing industry, especially in remote locations where diesel generator and boiler fuels are very expensive. While it is true that on a fishing or processing vessel, there is large volume of waste heat from the engine room, consolidating that heat and making enough hot water for process area wash down without a huge storage tank may make a third stage refrigeration heat pump more practical. Currently, equipment costs are high, but recent experience has produced paybacks that are in the three to five year range. Keep your radar on - within a decade, this equipment will be available, affordable, and scaled to fit the size and type of freezing operations that dominate our industry.
Technology is the art of doing better with what we have. As technology changes and attractive new systems are introduced, it’s important to not lose sight of the product or system that will do the job in the most practical, safe, and reliable way. The answer is to design, operate, and maintain refrigeration systems that are, above all, trustworthy, reliable, safe to operate, and have the means in place to get beyond operational difficulties, so that valuable production time isn’t lost.
Rick Greenquist, a project engineer with Highland Refrigeration, has taught classes as an adjunct instructor through the Seattle Community College Marine Training Center, and through the Dutch Harbor, Alaska chapter of Refrigerating Engineers and Technicians Association. His focus in the last two decades has been in mechanical design of refrigeration systems, electronic controls and energy efficiency. He can be reached at firstname.lastname@example.org