A DIY Water Cooled Condenser made for a RV, the water cooled condenser is putted in serie with the standard air-cooled condenser.
Taken from http://www.zipworld.com.au/~frankp/condenser.pdf
DIY Water-Cooled Condenser for Danfoss BD35 Fridge Compressor
Apologies in advance for not taking step-by-step photos of my project
. Should have done that but it was done over a year ago in hectic preparation for a 3 month trek.
I wanted to significantly improve the performance of the fridge in hot weather
. It's all about efficiently removing heat from the refrigerant, and having read about a water cooled fridge condenser I thought I'd have a go at it as a DIY. I am told by the experts that water can remove about 100 times more heat from refrigerant than air at the same temperature. With that in mind I made a water cooled condenser that I had fitted in series in the refrigerant circuit – that is, the refrigerant come out of the compressor, through the water-cooled condenser, and then through the normal air-cooled condenser.
The water cooling increases the efficiency hugely. On the last trip prior to the project
we were in the Kimberley with temps around 330-360, the duty cycle was 60%, fridge cabinet temps were too high, up around 100, even with the camper’s side wall (where the fridge is located) shaded.
After the project was completed the next trip was in hot northern Qld. Temps were up to 400, 40 warmer than on the previous trip. The compressor had a duty cycle of about 40%. As long as I kept the wall of the camper where the fridge is located shaded (I have a tarp for that) the fridge temp stayed pretty well in the blue zone - 00 - 40, maybe a tad above on occasions Additionally I was able to wind
down the compressor speed which reduces the current
draw when it is running. It was set to 3000rpm I think, drawing 4.5 amps from memory. Now, again from memory, it is about 2000 rpm, drawing 3.5 amps. That saving is partially offset by the water circulation pump which draws about 0.5 amp, but cuts in only when the compressor runs. It is switch selectable, so in cool weather
you switch it out and save that 0.5 amp. The water is circulated from the large water tank.
We also use storage
containers in the fridge to hold all the stuff. Two per shelf, these fit neatly side-by-side, are the full length of the shelves and the full height between the shelves and act as "buckets of cold" to hold much of the cold in when you open the door. You can also put more stuff in the fridge, which helps to increase efficiency, believe it or not. More thermal mass, I guess. Also, we don't use the tray under the ice-box, which I believe is there to control the "flow of cold" to stop stuff (vegies?) at the bottom of the cabinet freezing. Instead we use a slightly warmer setting on the thermostat (more power saving) and use a battery
powered anti-stratification fan to keep the cabinet air
moving. Two D cells last at least a month. Total saving on amp-hours per day for the fridge in hot weather is around 50%. The person who did the refigerant work for me is the same guy who makes the eutectic Autofridges, but any competent refrigeration mechanic
should be able to do the work. I got my technical info from Home
I was advised that all you need for a Danfoss BD35 compressor is 1 metre of water jacket and 1 litre per minute of water flow. Ozefridge will also supply the small pump and food
grade plastic tubing for about $50.00. Pipe and stuff comes from any plumbing
supplier. Adaptors (nipples) for the half inch pipe to accommodate the plastic water line came from Pirtek.
Below is a photo
of my prototype condenser. I decided to make a second one with the small pipe exiting the big one about 100mm further on so that both ends of the small pipe exit in the straight parts
of the condenser. From memory, that makes the entry and exit holes for the small pipe about 1100mm apart. Give yourself a bit of extra pipe to work with. DO NOT use a hacksaw to cut the small pipe, and preferably not the big one either - copper dust in the small pipe will kill your fridge and/or compressor, and in the large
pipe will not taste good in your coffee (you will be circulating drinking water). Get a proper plumber's small pipe cutter
- the type with a sharp wheel
. I think I used 1.5m of 12mm annealed copper pipe and 1.5m of annealed 5mm. Maybe get 1.8 to be sure to be sure Make two holes in the large pipe 1100mm apart. Make them as oblique as possible. I drilled a 5mm hole straight in then used the shank of the drill to bend the wall of the 12mm tube to make the hole oblique to follow as much as possible the path of the 5mm pipe out of the larger one. In the sidewall of the 12mm pipe aim for as much copper-to-copper contact as possible with the 5mm pipe as it exits - a spoon-shaped depression under the 5mm and a slight dome over it is ideal. This gives maximum contact area for soft solder. (Silver or hard solder would be better if you can do it.) Straighten the pipes as much as possible, but don't work them too much - the more you flex copper the harder it gets and then you might have to re-anneal it. Thread the small pipe through the large one through the holes you have made. You might have to drive it a bit with a hammer. Cut off about 20mm of each end of the 5mm pipe to remove burred ends (from hammering) and also to remove any contamination that might have been picked up. Tape up the ends of the 5mm pipe to stop contamination. Now wind
the whole lot around a 5 inch former. I found 5 inch was about the smallest inside diameter I could do without expensive special tools and without kinking the 12mm pipe, and even then you need to be careful. Arrange it so the two straight bits with the entry and exit points come out roughly parallel. Using solderable wire (I used tie wire from Bunnings) bind the small and large pipes tightly together near the entry and exit points for mechanical strength. If the coils don't lie snugly together, bind them too so that they do. Now generously solder the entry and exit points of the small pipe with the large, and also solder the wire binding.
above shows the mess in my compressor cupboard. Much of that is camper manufacturer’s doing, but I'm afraid I have followed suit. In splicing some other mods into their wiring
I found that there is not enough spare length to re-route the original wiring
and neatly terminate it in a junction block. It's electrically sound and safe, but a bloody mess.
The photo shows the condensor mounted on a bracket made from 1.5mm aluminium sheet. The bracket roughly an inverted, squared off U shape with the ends bent under to form tabs with holes to match the compressor mounting bolts. At this stage you will have to employ the services of a refrigeration mechanic
who does small fridges because you have to recover the refigerant, cut into the gas circuit, install the condenser, evacuate the system and re-charge with refrigerant. Ideally the gas flow in the coil should be gravity assisted. So the coil you have just made should lie horizontally, with the outlet from the compressor connected to the uppermost end of the 5mm pipe, then the lower end of the 5mm pipe connected to the top of the original air-cooled condenser.
If it could be mounted on top of the compressor that would be ideal. However, space constraints in my camper made that impractical, so it had to be mounted
alongside, in a vertical configuration. I made the aluminium bracket to accommodate that.
The two condensers are in series, the water cooled one first in the gas circuit, followed by air cooled one. The compressor outlet should be plumbed to the top 5mm pipe. The bottom 5mm pipe (refrigerant outlet) should be plumbed to the top of the air cooled condenser. The water circulation should be contrary to the gas flow, so water should enter at the bottom 12mm pipe and exit at the top. I circulate water from the larger of my two water tanks.
I used a small gravity-primed centrifugal pump from Detail /Dimensions
- the P3 micro pump kit. Ozefridge will supply a kit with food
grade plastic tubing for around $50.00 plus P&P. (Food grade tubing because you may/will be circulating drinking water
.) Being gravity-primed, the pump must be mounted at about the same level as the bottom of the tank it feeds from. I built a stone-proof enclosure and hung it off the side of my #2 tank at the bottom, using a drain valve already installed by the camper manufacturer. See the photo below. (I installed another drain plug
on the other side to replace my now lost
drain.) Encase the output tube and the very light duty twin core
(figure eight ) electrical
flex in split conduit, route
it under the van and bring it up through the
compressor cupboard floor. Likewise the return line. I used the tank breather hose for a water return, and to prevent return water from being discharged to the road instead of into the tank, put a 12mm garden irrigation stop valve on the end of the breather. Just open the stop valve when you fill the tank and close it again afterwards. The tank can breathe quite adequately through the filler during use, albeit with a bit of audible gurgling. You MUST use a small gravity-primed pump. The one supplied by Ozefridge is almost silent (important for night time operation) and draws less than 0.5 amp. Self priming pumps are generally noisy, would keep you awake at night and all draw too much current
so that you will lose the benefit of your work if you use them. The electronics
of the Danfoss cannot drive the pump directly. You have to use a relay - I think I used a Jaycar SY4007 12 volt model in a plug-in base, the whole lot is mounted in the grey box in the forground of the messy picture. Only 65mA hold in current, that's the main thing, 10 amp contacts, way more than you need. Pinouts from the Danfoss are in your fridge manual. I wired the relay through a spare switch on the console so that I can switch the water cooling off in cool weather. AS mentioned in my previous post, the performace gain allowed me to reduce the compressor speed, which reduces current draw during the run cycle. The camper manufacturer has installed a small PCB on the Danfoss control unit, containing a small fixed resistor that sets the compressor speed to 3000RPM. I cut out the resistor, installed a small 10kohm preset potentiometer and set a value according to the table in the fridge book to reduce the compressor RPM to about 2000. What I should have done and didn't, was bring that pot out to the grey box to allow fiddling with compressor RPM without removing the SS grill
on the sidewall. I am advised that although you get more cooling power at higher RPM it comes as a result of a law of diminishing returns - the compressor is actually more efficient at a lower speed - ie more heat removed from the fridge per watt of power consumed. Wierd. Refrigeration is a black art.
Some tech info for the Danfoss compressor is at the end of this document Well worth doing, IMO. Cheers