Refrigeration

question-mark-light-string-dreamstimemedium 63743826Anyone with a refrigeration system on their boat or RV that is powered by a Danfoss (now Secop) BD35 or BD 50 compressor should be aware of the diagnostic diode function, which, with a simple flashing LED, tells you the fault reason that caused your compressor to stop.   Much of the following also applies to the BD80 compressor controller.

By connecting a simple 10mA 12v LED across the "D" terminal and a "+" terminal on the electronic controller attached to the compressor, you will have added a powerful fault-finding tool that may save you a lot of time and trouble later on.

Why the manufacturer elected not to incorporate a diode in the controller housing is anyone's guess, but Coastal Climate Control comes to the rescue here with three options:

1. A Coastal Diagnostic LED kit with an LED that you can either install permanently or keep in the spares kit.

2. A Merlin II compressor speed controller that also incorporates a diagnostic LED.

3. A Guardian digital thermostat/compressor speed controller that features a diagnostic LED

OK, so now you have some form of diagnostic LED installed, what is it going to tell you? If your fridge or freezer system unexpectedly stops working, i.e. other than the thermostat stopping the compressor, and you go take a look at the diagnostic LED, what exactly are you looking for?

ice box over frostingRemember back when you had to defrost your home refrigerator every couple of weeks? It was a fact of life. And then those frost-free refrigerators came about. No more defrosting!!

So why have we seemingly gone backwards in evolution and are having to defrost our boat's refrigerator/freezer boxes so often? Why can't our boat's fridge be frost-free too, you ask as you drape your frozen food in towels to keep it safe as you patiently wait for the ice to melt from your evaporator plates.

Natural defrosting is the only safe method, although a small amount of addition heat is OK, say from a portable inspection light in the box. Those who are impatient might consider using a hair dryer or heat gun, hot water, or, (shudder), an ice pick, but these are not recommended, as high heat and physical force are very likely to cause nasty and highly undesirable side-effects.

And who gets the finger pointed at them as the culprit for this lack of frost-free living? Why, the refrigeration manufacturer of course, although after reading the following, you may shift the blame elsewhere.

danfoss-secop bd compressorMost marine refrigeration, and all Frigoboat systems, use the Danfoss BD 35 and BD 50 compressors which are incredibly reliable machines and very robust. It takes a lot of abuse to "kill" one (they don't simply die from natural causes), so if you suspect that your compressor is faulty, it's important to know how these compressors work, how they can be damaged, and what symptoms to look for.

The Danfoss BD 35 and BD 50 compressors are identical except that the BD 50 has a little more cubic capacity and therefore a slightly higher Btu rating. The compressor and motor are hermetically sealed inside a steel canister and supported on vibration-absorbing springs. There is no shaft seal that could leak refrigerant.

COMPRESSOR – The compressor is a single-cylinder device with a crankshaft that when rotated pushes a piston in and out of a cylinder, compressing the gas. A simple valve plate allows gas to be drawn in on the down-stroke, and the gas to be compressed and then released on the up-stroke. The compressor assembly is mounted above the motor, and the vertical crankshaft is a continuation of the motor shaft.

MOTOR - The compressor crankshaft is rotated by means of a poly-phase inductively coupled motor that has no brushes to fail or wear out. There are three sets of windings arranged around the stator, and the ends of these windings terminate on a three-pin connector that provides for the electrical connection through the compressor shell. The rotor is built around the shaft that connects directly to the compressor crankshaft.

ELECTRONIC COMPRESSOR CONTROLLER
– Also known as a "module", this is a vital part of the compressor, and the motor cannot run without it. The controller is mounted on the compressor on a special bracket, and secured with a screw. It is electrically connected to the motor via a three-wire plug that is pushed on to the three-pin connector on the compressor shell. The compressor controller basically takes the 12v or 24v DC input and energizes each of the three windings in turn, causing the motor shaft to rotate. Varying the speed at which the windings are energized varies the speed of rotation of the shaft.

So what could possibly go wrong?

air-cooling-dreamstime m 42985008Which marine refrigeration system is better when cruising in warm waters: air cooled, pumped-water cooled, or Keel Cooled?

A well designed and fabricated air cooled refrigeration system, like the Frigoboat Capri 50, should be able to maintain refrigerator and freezer temperatures in the tropics if the application, installation, and operation are all within the manufacturers' guidelines.

But in tropical/Caribbean conditions, air cooling will be 25% to 35% less efficient than water cooling. As a result, the overall power consumption of an air cooled system will be considerably higher than for a pumped-water cooled system, and very much higher than for a Keel Cooled system.

Many serious cruisers these days install hybrid Air-plus-Keel Cooled systems, where the air cooling is only used when the boat is hauled for any reason. For a new installation, this could be a Frigoboat Capri 50 installed together with a Keel Cooler, with a switch installed in the fan circuit of the Capri 50. Existing Frigoboat Keel Cooled systems can be easily converted to an Air-plus-Keel Cooled system by installing an Air Add-On condenser. This is accomplished with the use of just basic tools, and without any adjustment to the refrigerant charge.

If your boat is aluminum, then due to corrosion considerations that prohibit the installation of a Keel Cooler, you would be best served by installing a pump-water cooled system like the Frigoboat W50. That, together with the Air Add-On air-only condenser for use when hauled out, will provide the same efficiency as a Keel Cooled system, although the added current draw from the water pump will negate some of the power savings normally experienced over an air cooled system.

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