Refrigeration Cycle 4

[EAJ82]

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?

 

[quark]

Condensing temperature is what is forced upon by nature. Evaorating temperature is what you require. If the maximum wet bulb temperature in your area is say 25C then, using better cooling towers, you can reach condensing temperature of 28C. If it is air cooled then add 2 to 3C to the maximum DBT and etc.

 

[EAJ82]

how do u find the condensing temperature then?
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?

 

[MintJulep]

Selecting the components is an itterative process. The refrigeration CYCLE is a cycle. It has no begining, and no end. All of the components must be in harmony. Very Zen-like.

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.

 

[EAJ82]

ok great post...
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?

 

[MintJulep]

First law of thermodynamics requires that there be a temperature graident to have heat transfer. So sure, it's logical that the condensing temperature must be higher than the outside temperature.

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.

 

[AbbyNormal]

It is an interative process like Mint Julep says.

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.

 

[EAJ82]

hahahaha
sounds like a fun job to me
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

 

[EAJ82]

Hey guys, stil have one thing:
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

 

[AbbyNormal]

you go through the iterative process until you have an evaporator, condenser and compressor bablanced out. My experience to selecting the TX valve is to look at the SST and CT the design balances out with and starting with the compressor discharge pressure, go through the pressure drop of the condnesing coil, dryers, piping up to the TX valve.

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.

 

[AbbyNormal]

I have an old bulletin from sporlan on the distributers, this looks like the new one

http://www.sporlan.com/20-10.pdf

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

dumb question but you are including the full heat of rejection in your condenser?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[EAJ82]

not a dumb question actually..but im using a coil design program, i should be adding the heat of the compressor right?

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.....
but what i'm learning now will definitely help me through my masters, if i finish this project ...so thank you very much

 

[AbbyNormal]

yes condenser coil rejects more heat than the evaporator absorbs

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

divided evap load by 0.75 to begin iterating, then look at the compressor curves for the power input at the various temperature combinations to fine tune.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[EAJ82]

ok i get power input of the compressor, this is the heat added then !! makes sense i guess...i wish i had compressor curves i have several values under certain conditions the rest i created an excel sheet that iterates the values between the given values.
thnx man, uve been of great help, really appreciate it!

 

[quark]

For initial design, don't be after compressor curves and all that. Qc = Qe+W, is the simple equation that governs the condenser capacity. For design, consider FL capacity of the motor as work input to the system.

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.

 

[EAJ82]

oh i got the book few months ago from the library, unfortunately the book is taken now .
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?

 

[AbbyNormal]

If it is only being done on paper I guess you do not need real compressor curves.

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.

 

[quark]

EAJ,

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.