Refrigeration Cycle
Refrigeration Cycle
(OP)
Hey All
My question is about the setting of condensing and evaporating temps in the cycle. Suppose u want to design an 18000 BTU/Hr cycle. to choose the compressor u choose it according to the BTU/Hr rating and it usually has different performances at different temps. then u go to choose the Thermal Expansion Valve, also this valve is rated according to BTU/Hr in terms of evap and cond. temperatures. Then u design the Evaporator and Condenser which are also in terms of the EVP and Cond temperatures. My question is how do u set those temperatures? what will make sure that i end up with a High temperature cycle for example? and not with a low temp cycle?
My question is about the setting of condensing and evaporating temps in the cycle. Suppose u want to design an 18000 BTU/Hr cycle. to choose the compressor u choose it according to the BTU/Hr rating and it usually has different performances at different temps. then u go to choose the Thermal Expansion Valve, also this valve is rated according to BTU/Hr in terms of evap and cond. temperatures. Then u design the Evaporator and Condenser which are also in terms of the EVP and Cond temperatures. My question is how do u set those temperatures? what will make sure that i end up with a High temperature cycle for example? and not with a low temp cycle?





RE: Refrigeration Cycle
RE: Refrigeration Cycle
and about the EVAP temperature is what i require, but suppose u r using a selection program for the coils, u have to set the EVP temp. and when u change it u change the capacity of the coil !! in that case if the compressor is working at 18000BTU so the EVP will work at the temp. that would give the 18000BTU?
RE: Refrigeration Cycle
There are no short cuts. So start by typing "you" and "are" 100 times each to get yourself in the correct frame of mind.
You should know what evaporator capacity you need, and you should know the evaporator temperatures you need. You should also know the environment that the condenser will be operating in, so you should have a reasonable idea of the condensing temperature. So start with the evaporator coil selection program. Keep those three parameters fixed, and play with the coil dimensions until you get something reasonable.
Then go to the condensor coil. You've already made a guess at the condensing temperture, so start with that. You should have a reasonable idea of the compressor power required, so put that in. Play with the rest of the parameters until you get a coil that will reject the correct amount of heat.
Then go to the compressor selection program, and using the values from the coil selections, see how close a match you can find. You probably won't find a perfect match. Select the closest one, and make adjustments to the coil selection accordingly.
Repeat until everything is in harmony.
RE: Refrigeration Cycle
just one thing remains...the condensing temperature is very close to the ambient temperatures right?
just for an example, i was trying to match capacities with the catalogued capacity of some companies...
the trend goes :
for outside temperatures of 95, 104,110 Fahrenheit the condensing temps will be 110, 120, 131 Fahrenheit !?
is that logical?
RE: Refrigeration Cycle
The values you list would be reasonable first guesses to use in the selection process I described above. Those values have some set of assumptions associated with them regarding air flow through the condenser and fin conditions. Most likely they assume a uniform face velocity and nice clean new fins.
Neither of those conditions exist in the real world.
RE: Refrigeration Cycle
I must have been through it a few hundred times back in the early 90s.
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Refrigeration Cycle
sounds like a fun job to me:D
Thank you guys...i think its clearer now !
technically between low,medium,and high temperature refrigeration only the evaporator temperature will differ according to the design i specified, but the condensing temperature will stay the same and only vary according to the outside ambient air going thru the fins !
sounds logical..
thanks
RE: Refrigeration Cycle
Lets suppose i fixed a certain condensing temperature at Y1 and designed a condenser that gives me 20 MBH, then i fixed the evaporator temperature at X1 and got an evaporator that gives me 20MBH cooling, then i look at the compressor rating at X1 and Y1 and discover that it gives me 19MBH, so i have to go back and change condensing temperature to Y2 and EVP tempr X2 both gave me 19.5 MBH at this temperatures, and the compressor also gives 19.5 MBH at X2 and Y2 here the system is balanced..What remains is the Expansion Valve !! how can i be sure that the evaporator wil have the temperature i want it to have after the flow passes through the thermal expansion valve ?
Thnx in advance
RE: Refrigeration Cycle
Then go with the pressure drop expected for a distributor, its tubing, the evpaorator coil and the suction piping back to the compressor.
It will leave you with a differential pressure. Your valve must be able to pass enough refrigerant with this remaining differential as the driving force.
The system will find its own balance with the load and the ambient. Under a design condition you should be close.
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Refrigeration Cycle
http://www.sporlan.com/20-10.pdf
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Refrigeration Cycle
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Refrigeration Cycle
without adding the heat of the compressor the program is giving me answers close to the designs of carrier and york which i'm using to compare my answers to. how do u find the accurate heat of the compressor?
i just increase the design capacity of the condenser over the capacity of the evaporator, but i don't have a systematic way of finding the total heat of rejection..
guess i look dumb now..:D...
but what i'm learning now will definitely help me through my masters, if i finish this project :)...so thank you very much
RE: Refrigeration Cycle
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Refrigeration Cycle
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Refrigeration Cycle
thnx man, uve been of great help, really appreciate it!
RE: Refrigeration Cycle
Mass flowrate times the enthalpy difference at evaporator gives you the heat absorbed by the refrigerant at the evaporator. If you already know the evaporator capacity, you can get the mass flowrate of the refrigerant.
Now, you know the suction and discharge conditions of the compressor. (suction condition is evaporator outlet+superheat and discharge condition is condenser outlet-subcooling, but gaseous phase). Get the compressor power my multiplying the mass flowrate with the enthalpy difference across the compressor and divide it with compressor efficiency. This gives you theoretical power of compressor at full load.
You can go through an excellent discussion about this topic in Roy J Dossat's Fundamentals and Refrigeration and Air Conditioning.
Abby has given you good advice, as usual.
RE: Refrigeration Cycle
but according to what u said quark, then a program developer may do a program that gives u the condensing capacity of the condenser after it removes the loss of cooling due to the compressor heat?
A program like USA coil would it take into account the compressor heat or not?
RE: Refrigeration Cycle
I only went through the process when a custom unit was sold and had to be designed, never as a project.
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Refrigeration Cycle
Just check the condenser input parameters. If there is no temperature difference between the gas and liquid phases, except the subcooling part, the compressor load is not considered. If the condenser design considers gas temperature equal to that of compressor discharge then the motor power is implicitly considered.
RE: Refrigeration Cycle
feels wierd to design a 60,000 BTU/hr evaporator and have an evaporator working at close to 80,000 Btu/hr (compressor power is adds a 1/3)...takes my condensing temperature a little higher than i expected. :)..
so now i got condensing temperature of 130F for 95F ambient air...i wonder what happens on a 115F day :D reaches 160 i guess...
RE: Refrigeration Cycle
You need some head pressure to push it through the valve but consider a bigger condenser coil
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Refrigeration Cycle
install it in Canada.
Lool...why u need a new air condition? :D
RE: Refrigeration Cycle