Unexplained efficiency

[hcdebey]

My refrigerator seems to get more efficient the longer I run it. Am I missing something?


I'm a newbie to refrigeration.
The system is installed on a boat.
The system is 22 years old.
The condenser is cooled by circulating sea water.
The system has a eutectic tank.
5 seconds after startup, watt meter indicates 1540 W.
Over the next 60 minutes, the power consumption decreases by about 15 watts every minute.
After an hour the consumption stabilizes at about 640 watts.
The thermostat is set at maximum and I do not hear the compressor cycling on an off.


Can anyone offer an explanation as to why this system seems to run with less power the longer it runs?

[Brian_UK]

Every refrigeration system when it starts consumes the most power because it has the maximum heat load.

As the heat load comes down then so does the power demand.

You say that you don't hear the compressor switching on or off, do you mean that it is not running or that it is running continuously?

[hcdebey]

Every refrigeration system when it starts consumes the most power because it has the maximum heat load.

As the heat load comes down then so does the power demand.

You say that you don't hear the compressor switching on or off, do you mean that it is not running or that it is running continuously?

Brian,

You just about doubled my knowledge of refrigeration by telling me about heat load reducing the power required. Makes sense. Thanks.

I don't hear the compressor switching or or off. It is on all of the time for 60 minutes. That is about as long we need to run it each day to keep the fridge cool.

Hank

[monkey spanners]

To work out the efficiency you need to factor the power used against the heat removed, i think you are just measuring the power used so if its taking 640watts to move 200watts of heat out of the evaporator its not working very efficiently but if its moving say 1kW that would be better.

[hcdebey]

I have a watt meter to measure power used. Is there something similar to measure how much heat is moved?

[Rob White]

.

No that is done by calculation.

You need to know what your product is and then how
much. All food stuffs have a specific heat capacity
(every thing has a specific heat capacity)

see below

http://www.engineeringtoolbox.com/specific-heat-capacity-food-d_295.html

Then it is just time and temperature difference.

It is a mathematical equation.

Apples = 3.64 (KJ/kg degC)

1kg of apples has 3.64 KJ of heat per per 1 degree temperature difference.

So 10 kg requires 36.4 KJ of heat to be removed every degree difference.
10 degC difference is 10 x 36.4 = 364 KJ of heat to be removed.


http://www.rapidtables.com/convert/electric/Joule_to_Watt.htm


Regards

Rob

.

[hookster]

Interesting set up did you construct yourself?

The eutectic tank will be a large heat load on initial start up of the plant but this is then stored energy for release during high demand periods. but allows for compressor etc to be sized smaller, Think system flywheel. The longer you run your system the closer you will come to stability and actual operating input power requirements (Unfortunately ambient conditions are dynamic and must be accounted for)
The eutectic tank should then store energy during low demand periods (night, low ambient etc) and release the load during high demand.

This makes your system efficiency increase as higher input power is required during off peak times where energy savings are made due to improved condensing etc. and lower power input required during high demand (reaching for the icy brews during midday sun) as eutectic material gives up its stored energy.