Look at this it may be helpful. Good luck.

https://www.quora.com/What-is-the-function-of-compressor-in-a-refrigerator

Hello zubair

I know the function of a compressor in a domestic refrigerator but my question is about discharge pressure variation with ambient temperature. Whether it is constant or varies with temperature.

@ Vipin Ok. If you want , use this forum "http://www.refrigeration-engineer.com/forums" and ask your question, it can help you . Good luck.

Increasing ambient temperature will increase condensing temperature and, thus, will increase high pressure in the cycle (condensing pressure). The expansion valve, which is located between the condenser and evaporator,the pressure in the evaporator depends only on the desire refrigerator temperature. So, for constant refrigerator temperature, increasing ambient temperature will lead to increase pressure lift and consistently compressor capacity. 

Dear Vipin Nair,

First of all, domestic refrigerators use capillary expansion devices to create a flow restriction, not expansion valves. The capillary does not adjust the restriction according to the demand as pressure or thermostat valves.

Second, most of the domestic refrigerators have a ordinary On/Off compressor (NOTE: there are many available in the market with variable speed), which means when is powered on the compressor run in one speed only.

Considering those two conditions, yes, the compressor ratio varies according to the ambient temperature. This logically implies in variation of discharge pressure with ambient temperature.

There isn't much that the compressor can do when the ambient goes from cold to warm, besides keep pumping gas. See, the compressor will build a gas flow and a positive pressure difference. But the flow and pressure restriction is controlled by the circuit, mainly by the expansion device (in this case, capillary tube).

So, in the overall, when the refrigerator is running on summer, the pressure ratio is bigger (bigger condensing pressure, small change in evaporating pressure) and the compressor have to run longer time (because the thermal load in the cabinet is bigger, and the temperature difference between evaporator and cold compartment is smaller). In winter, the pressure ratio is smaller (smaller condensing and evaporating pressure) and the compressor run short time (small thermal load in the cabinet and big temperature difference between the evap. and cold compart.)

Keep in mind here that the cooling compartment temperature has the same set point in both cold or warm ambient.

Any doubt feel free to contact me.

Regards

Thank you very much Andre. This is exactly the kind of answer which i was looking for.

So the bottom line is that the Compression ratio varies with the ambient temperature.And,you have also mentioned that the compressor speed is constant which is correct. Now i will try to find out how a compressor is achieving higher pressure ratio at higher ambient temperature.

I agree with your point that the compressor on its own cannot overcome the higher ambient temperature situation.But it varies the pressure ratio with the ambient temperature(I am interested in this particular fact).

Thank you once again.

@Rajendra Kumar

This question is not about the basic VCRS cycle. Please read my question properly before answering. It is a fact that discharge pressure must increase with increase in ambient temperature. But,how is this task achieved by a compressor in real applications(That is what i actually wanted to know).I am interested in proper technical response instead of bookish theoretical answers.

Please read the answer posted by Andre Morriesen.

Thank You.

@Rajendra 

So you believe that it is not possible to have variable pressure ratio in the compressor.Now,Andre and you have contradicting views. 

Nevertheless, i still believe that Andre is correct but let me go through the links which you have sent.Thank you for that and this is the kind of reply which i was expecting here.

Thank you

@Rajendra

The two documents you provided were of little help.

The document by NIST analyses variation in COP with ambient temperature whereas Purdue uni paper deals with variation in refrigerant charge with ambient temperature.

None of these papers specified whether the pressure ratio will remain constant or it will vary.These were basic thermodynamic studies of VCRS system.Did you even read the paper before sharing it?

Andre made the correct point and i think i will build up on that.

Thank you

Vipin Nair,

Appreciate your compliments.

May I ask, why are you heading to compressor pressure ratio vs ambient temperature variation? What is your interest in looking into that?

Cheers

hello again Andre

This doubt cropped up when i was discussing about VCRS cycle with my colleague. He believes that the compression ratio/discharge pressure of a compressor is fixed at a predetermined higher  value( that is for pressure corresponding to a saturation temperature of 55 degree or so).Hence,no matter what the ambient temperature is,the compressor can perform its task because the discharge pressure is always higher than required.

But the problem with this setting is that the compressor is always working at a higher pressure than required which is consuming higher work input/electricity. So,there must be a variable discharge pressure setting in a compressor to avoid this problem and to provide required discharge pressure for condensation of the refrigerant .We all know this fact but i couldn't justify it because i didn't know how a compressor is actually achieving this task.

Thanks for your help. 

The hermetic compressor can deal with the ambient temperature in only one case:

You have to put a speed variator (as a brain) dealing with two entrance (Ambien T° and discharge Pressure), you can then resolve the problem of excess electrical consumption. However, actual compressors are predetermined to work on very higher demand. 

There should be always 10-15 degree Celsius difference between ambient temp and condensing temp {after de superheating in the condenser} to make heat transfer possible .So when ambient temp will increase condensing temp will also increase to make 10- 15 degree Celsius temp difference. It order to increase condensing temp,

compressor discharge pressure will also increase .

the ambient temperature has a significant effect on the electrical power consumption of the compressor where the pressure ratio is increased due to increasing the condenser temperature hence increasing the condenser pressure. So it is very important to reducing or to mange the condenser temperature,