Jim Scott wrote:
ISTM that there are/were fridges didn't perform well in cold conditions.
They have a single thermostat in the fridge rather then the freezer section. If the ambient condtions are "too cold", then the fridge doesn't have to run much, if at all, to get the thermostat to operate. Great! Low electric bills! But it then doesn't run long enough to get the freezer section down to low enough to keep things frozen.
Having the thermostat in the freezer section can also cause problems:
This is a control system which can have big lags. Take the situation with the conventional situation of the thermostat being in the icebox. The heat pump runs and lowers the temperature to the point that the thermostat operates. These thermostats have hysteresis - so the temperature at the thermostat will have to rise a few degrees before the thermostat will operate again. Heat energy is being transferred into the body of the fridge from the environment, mostly by conduction through the case- the higher the temperature outside, the faster the inside will warm up. Inside the fridge, heat has to transfer to the icebox, and hence the thermostat, mostly by convection. Under certain circumstances, eg when the ambient temperatures are high and the fridge has poor insulation, the fridge section can rise to a relatively high temperature much more quickly than the heat can be transferred to the freezer section and raise that temperature to the point of operation of the thermostat. Thus the stuff in the body of the fridge can "go off", even though the stuff in the icebox is fine.
Turning the thermostat to a colder setting *will* help. Set low enough, the temperature in the fridge section doesn't rise to dangerous levels before the thermostat operates and the heat pumps starts again.
Putting the fridge in a cupboard can raise the ambient temperature. Which increases the rate of heat flow through the walls of the fridge into the main body. Which means the fridge section gets hotter faster. Which can mean it can get to unsafe temperatures before enough heat can be transferred bu convection inside the fridge to raise the freezer section enough to operate the thermostat.
The outer heat sink is a different matter. It too mostly loses heat energy through convection. In a restricted airflow situation, turning the thermostat to a lower setting will increase the temperature of the heatsink (simplifying things..) thus increasing the temperature differential between it and the ambient air. More heat energy will transfer. Which raises the ambient temperature - which increases the heat flow back into the fridge body...which makes the heatpump work more, which raises the heatsink temperature...
Thus the question of the internal thermostat has *everything* to do with being in a cupboard - if the fridge has relatively poor insulation.
The question of the cupboard has little to do with the temperature *difference* between the outer heat sink and the temperature of the surroundings. It has a great deal to do with the actual temperature of the outer heat sink and hence the actual temperature of the surroundings, when hot air cannot escape to the infinite heatsink, known as my kitchen in Winter....