Calculations on front opening fridges

[catsketcher]

I am thinking fridges again - mine is 23 years old and plugs along okay (Waeco Danfoss BD 50 - lightly used with lamella evaporator, top opening, hard to find stuff in and tricky to access - never been serviced and doesn't cool terribly well). I don't like a few things about it and rather than try to fix it I was thinking of replacement.

I would really like an upright fridge on our cat - we have the bench space. I read that front opening fridges are inefficient because you lose air every time you open the door. I am a Chemistry teacher sometimes (not often) so I had a go at some numbers.

For a 130 litre fridge packed pretty tight, I would guess that there is still about 50% of volume taken up by air. So 65 litres.

The specific heat capacity of air is 1.2 kJ/m3/K. So if we lose all the air from the fridge every time we open it (and we wouldn't at all) and replace it with normal air we need to cool down the new air.

1.2 x 65 litres/1000litres (m3) x (25-4 = 21) = 1.68 kJ (ambient air temp at 25 - I know it aint Kelvin but it doesn't matter)

The specific heat capacity of lots of watery stuff - fruit, veges, drinks, water, glassware is similar to water = 4.2kJ/L/K. So for the fridge 1/3 full of stuff the heat capacity of the other stuff is

4.2 x (1/3 x 130) x 21 = 3792 kJ

The heat capacity of all of the other stuff in the fridge dwarfs the energy required to cool down new room temp air by 3792/1.68 = 2258 times.

To get the air back down to temp requires, say, double the energy in electricity (because of inefficiencies in motors and pumps)

If we open the fridge once every half hour then the motor has 30 minutes to draw down the new air to temp

Power required = energy/time = 1680x 2 J/30x60 = 1680 x 2 /1800 = 1.8 watts. Let's say 2 watt - at 12 volts this is 0.16 Amps extra current needed. As we say in Australia - this is bugger all.

So why should we not use front opening fridges? They would be much easier to find stuff in, have less time open because you don't have to dig stuff out and are only very slightly less efficient.

Is there some error in my assumptions and maths? Please go your hardest before I commit some fundamental error and purchase the wrong setup.

cheers

Phil

[Bill O]

A little too early in the morning to fully contemplate your calculations, but will agree that the front load is easier to use.
Your 30 min. run time to chill down the air again maybe be a little too long depending on the size of your compressor and evap. plate. Yes, our 6 cu. ft. front loader does turn on after you open it due to warm air entering. It only runs for ~ 5-10 minutes (on the lowest compressor rpm) depending on fullness of the fridge and outside ambient temp.
We built our main box w/3" insulation and used a commercial door w/a magnetic seal. In general the amount of insulation around the main box is good and the magnetic seal works well, but the door would be better if it had more insulation. That said, we only consume ~30-40 ah/day (normal usage adding warm items) during the summer months while maintaining 35F (1.7C) to keep water/beverages nice and cold.

[Chotu]

there is one thing you left out. And I think it’s a really big factor.

You know how all of those items in the refrigerator have a high specific heat capacity?

Well, on crappy "pit" refrigerators, you need to pull all that stuff out and put in on the counter every time you need something.

All of that stuff with the high specific heat capacity is sitting out there in the hot weather on the counter while you are trying to find something in the bottom of the pit.

I think that’s a factor that's missing.

In addition there are several practical reasons to have a good refrigerator compared to those pits that used to come in boats.

first, you won’t have to worry about the raw water cooling pump going and clogging. I know the DanFoss and air combo is quite popular. But there are a lot of water cooled pit refrigerators out there still.

Second, it’s better for your back to not have to dig into the pit.

Third, it increases the enjoyment of using your boat because you don’t have to get annoyed trying to find some thing in the bottom of a pit.

And lastly, it will function many more years than any marine refrigerator wood. They are just more reliable. Home refrigerators last 20 years without even thinking about it. Without a single shred of maintenance.

A very passionate response I know. But I freaking hate those pit refrigerators more than anything.

My entire boat was designed with these types of ideas in mind. In fact I don’t have any through hulls. because I want low maintenance. And reliability. After 30+ years on the water, I am done fixing things that are needlessly complicated.

PS: I don’t have my numbers on me right now, but my front opening land refrigerator, thanks to modern technology, uses less power than most of those pit refrigerators anyway. when I get back to the boat I can add that to this thread. Don’t forget to take into account the new compressor technologies

[Chotu]

Got the numbers online:

My upright LG refrigerator uses 62A per day or 2.6A per hour.

Maintenance free and will likely last 20+ years.

AND looks better and cost $400

[rslifkin]

Originally Posted by Chotu

there is one thing you left out. And I think it’s a really big factor.

You know how all of those items in the refrigerator have a high specific heat capacity?

Well, on crappy "pit" refrigerators, you need to pull all that stuff out and put in on the counter every time you need something.

All of that stuff with the high specific heat capacity is sitting out there in the hot weather on the counter while you are trying to find something in the bottom of the pit.

I think that’s a factor that's missing.

In addition there are several practical reasons to have a good refrigerator compared to those pits that used to come in boats.

That's a big factor. In the basic sense of "open door, wait 15 seconds, close door" the front opening fridge will take in more heat via air exchange than a top opening fridge. But in the real world, you move stuff around less in the front opening fridge and you don't end up needing to keep it open for as long, so the actual difference isn't so big (or as easy to calculate).

In my opinion, making sure the fridge is as well insulated as possible will gain you more than you'd ever gain from top vs front opening. That should just be a choice based on preference and what fits best.

[Brioche]

Another factor is condensation. When opened a front-opening cooler dumps cold air and takes in warm air holding, depending on the location, a lot of humidity, which then precipitates, either as water, or ice on the evaporator pipes. If you have a freezer compartment, it ices up. A frost-free refrigerator overcomes this, but at the cost of desiccating food. For long term freezer storage you are best off with top-loading. If you don't have frost free function, a front opening unit tends to ice up more often, so top loading reduces that. If frost free, then storage will be dryer, and harder on high moisture things like vegetables. How you plan to use it will determine what is best for you.

[Pete7]

Originally Posted by catsketcher

- never been serviced and doesn't cool terribly well). I don't like a few things about it and rather than try to fix it I was thinking of replacement. Phil

Well first thing, does it need servicing or even cleaning the cooling fins? How thick is the insulation on it particularly the bottom?

However, clearly you are thinking about changing, therefore could you fit two. One for long term stuff little used and one for the milk making a dozen cups of tea each day. Perhaps even change the existing one to a freezer ?

Pete

[cr180]

We have a two door front opening 12v vitrifrigo, freezer on top with a single air cooled secop (danfoss) compressor. In warmer ambient air temperatures (high 80s, 90s F) compressor runs nearly all of the time and Refrig internal temp hard to keep below 40F. Requires frequent defrosting.

When replace, will seriously consider unit with two compressors, separate compressors for freezer and fridge, and potentially water cooled compressors. Isotherm, believe, offers such units although likely special order.

[rslifkin]

Originally Posted by cr180

We have a two door front opening 12v vitrifrigo, freezer on top with a single air cooled secop (danfoss) compressor. In warmer ambient air temperatures (high 80s, 90s F) compressor runs nearly all of the time and Refrig internal temp hard to keep below 40F. Requires frequent defrosting.

When replace, will seriously consider unit with two compressors, separate compressors for freezer and fridge, and potentially water cooled compressors. Isotherm, believe, offers such units although likely special order.

Isotherm and Novakool both offer dual compressor units like you're describing.

In the meantime, you can likely get a little better performance out of your current fridge. A better fan on the condenser as well as better airflow through that area and across the compressor will help cooling performance a little bit. If you have some space around the fridge, added insulation around the housing will also help.

[cr180]

Originally Posted by rslifkin

Isotherm and Novakool both offer dual compressor units like you're describing.

In the meantime, you can likely get a little better performance out of your current fridge. A better fan on the condenser as well as better airflow through that area and across the compressor will help cooling performance a little bit. If you have some space around the fridge, added insulation around the housing will also help.

Thanks. Agree. Didn’t mention, but as you recommend, do use external fan and it does help.

[Uncle G]

Shouldn't the left-hand side of the equation for 'the watery stuff' be divided by 1000?
litres to cu m

[Karanga]

The calculations (even just ball-park) are very nice to see.

But really, there is no contest. If you to really want to design and build a unit into a particular space, to run on 12v, then do so. And then put up with all the compromises that have to be accepted.

Otherwise, commercial domestic 240 or 120v AC fridges or freezers are so much easier, better, more efficient, more ergonomic, readily available, cheaper (by a factor of 10), quieter, drier, longer-lasting, replaceable and cheaper to service than the usual crappy marine units available.

[Simi 60]

The money saved on buying toy sized 12v refrigeration will pay for better batteries and inverter to run 240v

Part of the reason I stopped the build on our 55ft cat was not being able to afford the amount of 12v refrigeration needed nor carry weight of battery.
These days with lifepo4 and efficient 240v, not an issue.

We have a 4 door fridge on our current vessel purchased for about the same price as a small camping engel
Everything cold 5c in fridge and -18 in freezer
Don't notice cold air spilling
And the compressor does not cut back in when door opened and closed so clearly no real temp drop

[BobFord]

I too decided to install a domestic frig. Benefits seem to be- cheap , easy to replace if needed , efficient door opening v energy loss , easy to view contents. The (2) door -top freezer ,bottom day frig , seems to work well. Good star rating available. Cheers!

[rslifkin]

At this point, the only downsides I see to using an efficient domestic fridge are the logistics of fitting it into the galley cabinetry (particularly if it radiates heat via the casing), securing it in place and providing a way to latch the doors closed for rough seas. All of which are somewhat less of an issue on larger boats and particularly cats.