Danfoss BD50F - Freezer Thermodynamics

[Colin Stone]

I have had a strange issue with my approx 90ltr freezer which I don't understand.

It is refrigerated by a 24v BD50F with the keel condenser. And in 2017 I added an air condenser 1E53125 so that the deep freeze can operate when out of the water. It all worked well in the last time on a slip in 2017, with the refrigerant going firstly through the air condenser and then the keel cooler in the open air under the hull.

I then added the drier BMK-A50030 in mid 2021 after icing issues in the capillary coil. The freezer has been fine since then with low energy consumption.
The air condenser fan was disconnected when in the water, by simply pulling the crimp off the +ve on the controller.

I've just tidied up the installation and incorporated a 3 way 2 pole switch -
Freezer - Off - Freezer with fan. The fan is still running on the 12v +ve terminal.

When I tested the switch etc, the freezer was operating with the fan running as well as the condensed refrigerant then passing through the keel condenser.
So I thought it should be fine. But the freezer was not getting down to the temperature with the compressor consequently just running.

When I disconnected the fan, normality quickly returned and it all appears to be OK with the same noises etc.

So, I'm wondering if the refrigerant can be over condensed causing the issue, or if something else is happening??

[Bowdrie]

Originally Posted by Colin Stone

I added an air condenser 1E53125

Something isn't adding up.
That Danfoss # comes up as an electronic expansion valve, and the pic you posted seems to show such a device.
Generally speaking, those little units use only a capillary tube at the inlet of the evaporator, no expansion valve/TXV in the system.
Typical dual air/water units have the compressor feed directly to the air-cooled condenser, thence to the water-cooled condenser, and then to the capillary tube at the evaporator.

[Colin Stone]

In UK 1E53125 is the air condenser in the right of the pic.
The refrigerant route is compressor - air condenser - keel condenser - dryer - evaporator - back to compressor.
Anyway, solved. By running the fan for the air condenser, the gas turns to liquid too quickly/early, the overall system pressure drops and efficiency then falls and system doesn't cool as it should.
In normal floating use, the gas goes through the air condenser without the fan running - I guess it is just a bit more piping for the gas - and then changes state in the keel cooler.

[Bowdrie]

Originally Posted by Colin Stone

Anyway, solved. By running the fan for the air condenser, the gas turns to liquid too quickly/early, the overall system pressure drops and efficiency then falls and system doesn't cool as it should.

What you observed, (and I should have mentioned it, my bad,) is something that can easily happen on water-only condensing units.
I built a powerful engine drive system for a sportfishing boat that was going to Mexico, (85>90F water).
The next year the boat went to Alaska, (35F water,) and the system had hardly any cooling.
When they phoned me, I told them to partially open the bypass valve I'd installed, that reduced the water flow thru the condenser and the head pressure went up and cooling commenced.
When the refrigerant becomes sub-cooled way too much it won't expand into a gas properly and changing from a high-pressure liquid to a gas that is continually expanding as it goes thru the evaporator is how it removes heat.

[SailingHarmonie]

The system thermodynamics of a standard capillary system are basically pretty simple. (If you have a larger valve regulated system much of this does not apply...)

Cold gas at low pressure is turned into hot gas at high pressure by the compressor.

The condenser turns the high pressure, hot gas into high pressure cooler liquid, the exact temperature here is not critical.

Liquid refrigerant enters the capillary as a high pressure liquid, the pressure drops through the capillary, and a superheated liquid enters the evaporator in the box where it boils away. The boiling liquid is the only part of the system that is really cooling anything.

The amount of refrigerant in a capillary system is critical. The system is full of liquid, roughly from the output of the condenser to the outlet of the capillary, and then there should be JUST enough refrigerant charge so the last drop of liquid boils away as the gas/liquid mix travels out of the evaporator. If there is too little refrigerant, the liquid boils away before the end of the evaporator, and the cooling capacity is decreased.

Essentially, when you turn on the fan for the air cooled condenser, the amount of liquid available for cooling the evaporator is reduced by the volume of tubing between the air cooled evaporator and the inlet of the keel cooler. Before using the air cooled condenser this space was filled with low density gas, now it is filled with high density liquid. That volume of liquid is no longer available to boil away in the evaporator.

In some installations this change in available liquid for the evaporator is small relative to the internal volume of the evaporator, in other cases (like yours, maybe?) the increase in liquid holdup on the high pressure side significantly cuts the available heat removal capacity.

This effect would obviously be doubled down if the system was a little low on charge to start with.