You can get basic information about your vapor compression refrigeration cycle from the atatched file.

Regards

Please, visit 

http://twt.mpei.ac.ru/MCS/Worksheets/PTU/Vv-27-eng.xmcd

http://www.maplesoft.com/applications/Category.aspx?cid=2886

https://www.ptcusercommunity.com/thread/128069?sr=stream&ru=367546

Regards

Hi

In vapor compression cycle you mentioned you are facing a challenge. Lets start with some calculations. 

maximum power of compressor is 600 W. considering a usual COP of 3 for the cycle, 1800 w of cooling can be achieved. On the other hand you have 50 Liter of water from 28 to 7 C. So 4410000 J of energy should be removed from the water or 2450 second of operating time. This is a rough estimation to measure the order of working time which is an important parameter. 

lets start with the design: at first take a very close look at the minimum temperature. it is 7 C degree an near freezing point. 

since Temperature difference in heat exchangers are usually higher than 5 degree and usually up to 10 degrees, your cooling or working fluid needs to be so much near the freezing temperature of water. if it happens ice sheet formation prevents heat transfer and increases the time dramatically. 

you have to start with high and low temperatures of the cycle. design a heat exchanger for evaporator at first. this is the most important part. High performance evaporator with low temperature difference between two fluid flows.

the second important part of the refrigeration cycle which controls the power required mostly, is the condenser. if you can design a high performance condenser, then the cycle highest pressure can be lower resulting in lower work.

after that go on with throttler. chose the throttle system which could operate between two pressures you calculated. 

I did forget to mention you have to chose your working fluid at first, then solve the problem and again change it to check if it is not the best one. The first choice is a working fluid which have normal boiling at normal pressures (not too low not too high) at cycle highest and lowest temperatures ( 7 C and ambient temperature). 

About the time scale (I appreciate if you recheck all calculations) . It Shows that you should be careful about your system startup  time. you have to reduce the start up time and your system should reach its maximum capacity at the lowest time step. 

I hope you find this tips helpful. I am going to mention that what you design can not be exactly purchase on the market and wont works as analysis show. So consider this in your calculations and you might consider some compensatory ideas for such cases. 

I truly appreciate dear Mr. Islam, Mr. Voloshchuk and Mr. Barzegaravval  who answered my question concisely and precisely! Thanks you all and wish you all the best!  

Dear Volodymyr Voloshchuk,

Thank you for your information about refrigeration cycle calculations. I have a question about the pressure calculation at the compressor outlet. Though the state at the compressor outlet is superheated, the pressure there has been obtained with saturated vapor line. I know that the temperature at the compressor outlet is the T_condenser, out + DT_cond, hot but I don't know the temperature at the saturated vapor line. It seems strange to use the PSVTwf (T_condenser, out + DT_cond,) to calculate the pressure at the compressor outlet because the vapor at the compressor out is not saturated vapor. 

Therefore, I don't know why you use PSVTwf(T_condenser, out + DT_cond) to calculate P2?

I am looking forward to your answer.

best regards,

Yun Mo

Dear Yun Mo,

Thank you for the question

In the calculation I need to calculate the pressure in the condenser.

I consider the heat transfer in the condenser isobaric (pressure at the condenser inlet (compressor outlet) is equal to the pressure at the condenser outlet).

I propose to use  PSVTwf(T_condenser, out + DT_cond) to calculate the pressure of the saturated vapor in the condenser which is equal to compressor outlet (see attached file)

T_condenser, out + DT_cond - is the temperature of the saturated vapor in the condenser but not at the compressor outlet

best regards,

Volodymyr Voloshchuk

Dear Yun Mo,

In order to be more clear the phrase

"Temperature difference at the hot end of the condenser"

in the data block should be changed to

"Minimum temperature difference in the condenser"

or

"Temperature difference in the pinch point of the condenser"

Thank you for the comments!

Dear Dr. Volodymyr Voloshchuk ,

Thank you very much for your quick response to my questions. I understand what you want to do if you use the minimum temperature difference in the condenser to design a refrigeration system. I, however, still wonder if this method is a common way in practical refrigeration system design.  I was told from a tutorial that a  set of given pressures in the condenser and evaporator with the corresponding sub-cooling and super-heating or a set of condensing temperature, evaporation temperature, temperature before the expansion valve, and temperature at the  compressor suction point, is usually taken as design parameters. The process of refrigeration can be easily described in P-h diagram. 

This is my first time to study refrigeration cycle so that I've not got  much experience. Thank you very much for your kind explanation. 

best regards, 

Yun Mo

Yun Mo,

Yes, you can take a set of given pressures in the condenser and evaporator with the corresponding sub-cooling and super-heating or a set of condensing temperature, evaporation temperature, temperature before the expansion valve, and temperature at the  compressor suction point.

But, at desing stage you need to take into account such parameters as temperatures of heat sourse and heat consumer. These parameters are very important and you can "touch" them. These values are independent.

Using temperature differences you can evaluate and optimize the other parameters of your heat pump.

Best regards,

Volodya

Dear Dr. Volodymyr Voloshchuk,

I really appreciate your detailed explanation about the refrigeration design parameters to me. I see what you mean now. The design method described in the general textbook is easy to work  but is not practical because it does not directly consider  the load and environment which is not dependent our designs. We should make our design to  accommodate the user needs and the environment that is available. 

Thank you very much for your kind and nice comprehensible explanation. 

Best regards and have a nice day.

Yun

By the way,

if you are so interested in design of heat pump we can cooperate.

I am involved in this process. Now I develop exergy-based methods (advanced exergy, exergoeconomic and exergoenvironmental analysis) of evaluation of heat pump providing space heating. It is a huge, interesting and promising process.

Have a nice day,

Volodya

I think you talk about a small water cooler. You can start with spiral type copper heat exchange. The refrigerant should evaporate inside the tube while water will be assumed to be stagnant in the storage. Some useful correlations related this problem may found in ASHRAE fundamentals. You have to design the tube size, capillary tube,  and the condenser which I suggest to be plat-fin-and- tube condenser.

Hi, Amin

You must have finished your work for the design.

Here I recommend you our cloud eTools for refrigeration unit design. As they are cloud-based, you can work through using your smart phone. The eTools are available at the website www.refriglab.com.

Firstly, the cooler cools water from 28 to 7 deg.C with compressor input power

This is a heat transfer rate maximization problem in the evaporator , in which you have to find an evaporator with the maximum LMTD which means the highest efficiency. Theoritically you can cool down the water as fast as you want but you have design restriction such as refrigrerant inlet temp to the evaporator. In the best scenario the refrigerant inlet temp to evaporator cannot be higher than 2c for your design case and it leads to local water solidification which acts as an insulator. To solve this problem you can recirculate water in tank or add antifrost to tank if you are not using it for sanitary purposes. Higher evaporator efficiency inflicts alot more costs.