Air-Cooled Compressor vs Air/Water-Cooled Compressor

[melidramatic]

Hello Fellow Cruisers,

I am in the process of doing up a 1978 38ft Downeaster and am looking to install a new refrigeration system. I am considering a Dometic CU-100 Air Cooled Compressor, with a Danfoss BD50F compressor, which is said to be highly efficient for temperate climates. The CU-200 Air/Water Cooled Compressor however is said to be a better unit for long cruises in tropical waters. We have plans to spend six months sailing between Southeast Asia and the Pacific, so I guess we will be in tropical waters. :-)

Any advice out there on which is a better unit to consider? We don't want to cut corners and only choose the Air Cooled compressor because it's less expensive option. We want the best option for us.

Thanks in advance!

[djmarchand]

Practical Sailor (the CU of sailboats and gear) did a test of an air cooled Danfoss compressor system and a water cooled Danfoss system. Considering the power required for the seawater pump, the water cooled system only used less power in tropical conditions. So go with the seawater system.

But consider the Frigiboat keel cooled system. It uses a passive block of bronze that is mounted on your hull. Freon circulates through the block and condenses. So it saves the power and complexity of a seawater pump.

I have one on my boat and between its efficient controller that adjusts the compressor rpm and the keel cooler it used 1/3 less power than an air cooled system.

David

[Mark Johnson]

We've only had Air cooled, so can't make a comparison... But our 2 cu/ft (interrior) fridge, super insilated using vacuum panels, to = 7" of foam. has made our air cooled Sea Frost, (enclosed, 12V, AIR cooled unit) a perfect compressor choice for us. We did add insulation to the elephant trunk ducting in and out, and an extra muffin fan on the end of the exhaust hose. In a moderate climate it uses about 20 A/H per day, or less. In The heat of a Rio Dulce, or Trinidad summer, more like 30...
Mark

[Maren]

Originally Posted by melidramatic

We have plans to spend six months sailing between Southeast Asia and the Pacific, so I guess we will be in tropical waters. :-)

Any advice out there on which is a better unit to consider? We don't want to cut corners and only choose the Air Cooled compressor because it's less expensive option. We want the best option for us.

I'm pretty sure one of our members, Richard Kollman, has answered this question a couple of times both here and on his marine refrigeration forum. If I recall, he favors the air cooled systems rather heavily. Here is a google search for this site to help.

[Richard Kollmann]

Risk assessment of water cooling small Danfoss compressors.

It is not a question of whether water is a better conductor of heat than air, it is a question of what cooling medium provides the best overall performance and reliability when controlling a small Danfoss compressor powered condensing unit. The condenser’s cooling medium air or water temperature has a major affect on a refrigeration units performance especially with a fixed refrigeration volume capillary tube expansion device used with small compressors. If condenser’s medium is to cool or to warm on a cap tube system overall performance is degraded. Boat interior ambient air temperature is much easier than seawater to control when used as a condenser cooling medium. If air and seawater temperatures remain between 65 and 75 degrees F and refrigerant volume was set to provide good performance in this temp range performance of both mediums should be equal. Under these equal conditions condenser inlet gas pressure will be 105 to 120 psi depending on Btu work done.

When considering water cooling a Danfoss BD powered condensing unit one should consider the performance and reliability risk assessments involved with water cooling:

• Using a 12 volt pump to circulate seawater through a water cooled condenser is not only less efficient but creates system down times when any of the following can be restricted by sea growth, through hull, lines, strainer and even pump. Another problem is service life of pump. It is risky to leave a pumped water cooled system unattended as food can spoil. When water fails to circulate through condenser compressor’s electronics will sense an overload stopping compressor if this condition is not corrected repeated attempts to start compressor will over-temp module causing it to fail. Service life of one of these pumps is less than 2,000 hours or two and one half years on a live aboard boat. Condenser air cooled fans have a service life of 7 years.
• A pumped water cooled condenser should not be used on a variable speed compressor. The purpose of optimizing daily energy consumption with a variable speed mean running compressor longer. Adding water pump’s increased energy to the longer running variable speed compressor can offset any energy savings from compressor’s lower daily Coefficient Of Performance (COP).
• Because of the problems boaters are experiencing with pumped water condenser cooling one small system manufacturer is suggesting using water from fresh water tank as a cooling medium to dispose of refrigeration process heat. If low voltage leaks into water tank and it will where will it go from there? And how can fresh water in tank be maintained at an efficient temperature to balance refrigerant high and low pressures?
• Any time the process heat of refrigeration is disposed of by water only eliminating fan cooling of compressor there is a risk of heat damaging compressor and electronic control module. All hermetically sealed compressors rely on super cooled gas 65 to 80 degrees F returning to cool compressor. Anytime a design exceeds compressor manufacturers applications specifications the unit is at risk.

• The major risk involved with water cooled condensers or keel coolers is total wipe out if water inters refrigerant requiring complete system to be replaced.