Refrigeration

woman blowing pedals rajdeep kataki 220911 640x309There has long been a debate about air cooling versus water cooling for marine refrigeration systems. Water cooling has traditionally been generally accepted to be between 25% and 35% more efficient in warm/hot ambient conditions, but some of that efficiency gain has to be offset by the energy required to run a water pump.

The Frigoboat Keel Cooled system is water cooled and does a fantastic job without the need for a fan or a pump. That will be mentioned later, but the following compares air cooled systems to traditional water cooled systems using sea water with a pump.

Many boaters these days seem to be opting for air cooled refrigeration systems even though they’re heading south into hot climates, and despite the notion that air cooled is inherently less efficient than water cooled. Why is that?

flasherInner Secrets of the Danfoss/Secop BD35/50 Motor Controller (aka Module)

Well I can't actually give away any real secrets, because I don't think anyone really knows what goes on under that mysterious black plastic shell except for the manufacturer, but I do know a fair bit about what can and does go wrong with them.

But first let's take a step back and recap on what the little black box actually does.

The current generation of small, mobile refrigeration compressors from Secop (formally Danfoss) utilize a very simple motor with just three equal windings and no brushes or slip-rings. These three windings terminate in three pins that protrude from the compressor shell and connect to a push-on plug connected to the controller.

It's the controller's job to take the 12v or 24v DC input and spit it out in sequence to each of the windings in turn, causing sufficient torque to turn the rotor and so push the piston in and out compressing the refrigerant gas. So the motor actually gets a modified DC signal, a form of AC in fact, and by varying the rate of delivery of the signal to each winding and effectively altering the frequency, we can make the motor turn at different speeds.

That's it. All pretty simple in theory. As a means of altering compressor speed, the controller uses the switched input from the thermostat, and by adding resistance to this circuit it alters the speed at which the compressor will run.

So what could possibly go wrong?

gauges-handheld-dreamstime m 76282015-640x427     It's happened again, and I'm getting really tired of it. Yet another call from a boat owner who called in what he thought was a trained and experienced marine refrigeration technician to fix a poorly performing system, and now it's working worse than it was before! Why? Because the technician had added refrigerant, or "put in a shot of Freon" to what previously was a perfectly charged system.

     Our office here spends way too much time on the phone and in e-mails helping customers correct mistakes made by mostly well intentioned but misinformed and inexperienced technicians. Too often it seems that adding refrigerant has been a Hail Mary move, made after diagnosis was unsuccessful, and done in an effort to show the customer that at least something had been done. And that's where all the problems start ....

man-gauges-dreamstime m 33496565Recently, a customer contacted us for some troubleshooting advice. He said that his refrigeration system was not working and the compressor not running. Apparently he had a technician look at the system who reported that he checked the refrigerant level and said it was OK.

Now that may sound like encouraging words from someone the owner perceived would know a heck of a lot more about refrigeration than himself, but it demands some investigation.

Just like the laws of physics there are laws of thermodynamics, but these are almost incomprehensible to the layman, and appear to most of us as gibberish and gobbledygook. Nothing so simple as "what goes up must come down" and "for every action ..." yada yada.

You'd think that with only four laws of thermodynamics things would be easy, but even the numbering of them seems deliberately designed to confuse. The last of the four laws is the Third Law. There no half-laws, the First Law is the second, and the Second Law the third. The wayward law is designated the Zeroth Law and comes before the first law. (This feels a bit like trying to explain the rules of cricket).

Hidden in those laws somewhere is the revelation that (in very simple terms): if a substance exists in an enclosed environment in both liquid and gaseous forms, the temperature and pressure are directly related. Let's call this the Law of Fridgernomics for ease of association.

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