Post Number: 340
|Posted on Thursday, August 11, 2011 - 08:50 am: ||
Boris makes several good points I would like to expand on. First, do sign up for MST 218 if you are in the area. Boris will be teaching. I taught the first year, but my life has gone in a different direction.
A cold plate system "stores" cold by freezing a solution of water and something that lowers it's freezing point. Automotive antifreeze (ethylene or propylene glycol) depresses the freezing point of water. A 50% ethylene glycol mixture freezes at -30 F. When the mechanical refrigeration is turned off you still have an icebox. The ice just melts 60 degrees F colder than ordinary water ice. I don't know what the concentration of antifreeze is in cold plate systems. It's probably less that my example because it's more work (energy) to freeze something at -30 than it is at zero. A 30% EG solution freezes at 5 above.
Boris zeros in on one serious problem - very poor insulation. Most iceboxes in coastal cruising boats are fabricated as a fiberglass inner shell with perhaps a half-inch to an inch of plastic foam sprayed on the outside. The R factor is somewhere in the neighborhood of 2 or 3. 3-1/2 inches of spun fiberglass insulation is rated at R 11 by comparison. In cross section the wall of a typical icebox will have some airspace between the foam and the hull or other internal cabinetwork.
The rate at which heat conducts is proportional to the temperature difference. Cooled by water ice alone, the internal temperature of a box may range from 35 F where the ice sits to 50 F up toward the top. With an ambient temperature of 65 F, the outside of the box will usually remain above the dew point. With a cold plate, the same box would have an internal temperature below 10 F. Now the outside of the box is going to be below the dew point and it will "sweat" as Boris points out. More heat is migrating into the box faster which makes the outside of the box colder.
I suspect that the amount of insulation factory applied to iceboxes was sufficient to prevent them from sweating with water ice, not necessarily to keep them cool for a long time. Sweating would produce customer complaints. That the ice melts fairly fast would have been no news.
I have pushed in the direction of improving the insulation on the icebox in my boat. How I went about that will be the subject of the next installment.
Post Number: 2
|Posted on Tuesday, August 09, 2011 - 08:20 pm: ||
I sail on an older (1973) Dufour 40 ketch that has both a small 12v electric cube refrigerator and a (retrofitted) flat plate freezer. The refrigerator was standard equipment on these boats and exhausts to the aft (owners) cabin at the foot of the berth, which is usually my cabin as I am the "senior officer aboard" when we cruise to Catalina. Two observations: this is a weekend system, even though the boat has been upgraded to 800 AH house bank the refrigerator is only good for about three days after cooled overnight on shore power prior to departure. Secondly, it is noisy at night. Not loud, but the compressor cycles on and off and dumps heat into the aft cabin. I usually sleep with earplugs.
The flat plate was a retrofit into a standard 1970's era built in ice box. As such, it is not insulated for the deep cycle cold of a flat plate system and sweats externally when in operation. It does work, and since most outbound trips to Catalina are "iron genny" reaches the Perkins has had no problems in the past cooling the box. But because the system needs regeneration due to leaking fittings, and because there is no way to pre cool the ice box anyway for short trips, we just drop a block of ice in the box if we need it. Unfortunately the flat plate has reduced the box volume.
We are going to be covering electrical requirements in MST 218, including peak load balancing, battery requirements and charging, if you are interested I suggest you sign up.
Post Number: 339
|Posted on Monday, August 08, 2011 - 08:38 am: ||
Mechanical refrigeration consumes a lot of power, more than half of the electricity on a typical 35 ft boat. It is practical to install a large enough battery bank to get through one or two nights away from the dock, but it is very hard to hang on a hook for a week. Generators are a poor solution. Batteries take longer to charge than most people are willing to put up with the noise. The largest solar panel array people find practical on boats in this size range, perhaps 10 square feet, will recharge batteries effectively. However, their energy barely carries refrigeration 24 hours under the best conditions.
Classic cruising solutions to the refrigeration problem include cold plates. The compressor apparatus is sized up to pump all the heat required for one day in perhaps two hours of run time. The evaporator freezes a solution of ethylene glycol and water within the cold plate. As the solution melts it cools the icebox for the balance of the day. Cold plate installations are costly and require the power of a diesel engine. Compared to small refrigeration systems that run from batteries, they accept energy much faster than lead-acid batteries.