Modeling a Condenser

[Targ]

Hi guys

im a 3rd year university student (mech engineering) writing a report on a refrigeration system.

i need to model a condenser.
i have the real life version working, but i need to get the numbers to match.

so i started to look at it as a heat exchanger problem.

real:
--------------R134a------------Air-----
mass flow------?????----------0.06136kg/s
cp-------------?????----------1005kJ/KG k
T in -----------35c------------20c
Tout----------17.1c------------?????

its a 20 tube pass (20mm tubes) with the where the tubes are 120mm long.
so i get the area to 7.53e-4 m2
U = 425
and i removing 442W of heat.

but the log mean temp difference comes out @ 2640*C

(base on log mean temp = Q/U.A)

which obviously isnt right.

so i decided to try and work out the mass flow and cp for the refrigerant and work it 'from the other way'

cp = 0.857kJ/kgk
http://www.airliquide.com/en/busines....asp?GasID=141
but thats @ 25*C and 1 bar, not 35C and 12 bar

i calculated the mass flow to be 4.1637e-3 kg/s
based on darcy's equation for a 1.95m cap tube @ 0.66mm dia causing a pressure drop of 11 bar, that got me the velocity, then i multiplied by the density (again at the wrong pressure/temp)

so my question(s):
is there any way to accuratly calcuate the mass flow of refrigerant?

does any one know the cp and density of R134a @ the correct conditions, or any way to work it out?

am i competely wrong on my entire approach

cheers
targ

[NoNickName]

I don't see any information on the fin pack (fin pitch and material). I don't understand the tube pitch as well. Is the fin pack 20 tubes high with 20mm wide tubes?
How many rows?
Is the fin pack as long as the pipes (120mm)?
Also: how is it the refrigerant is colder than air when leaving the condenser

[Targ]

ok sry

20 rows of 20mm tube. the tube pack is 240mm high by 120mm wide (the tubes are horizontal)

the fins are steel, as are the tubes.

they are vertical, 2mm thick there is 76 across the 120mm width. and they are 75mm deep.

does that make any sense/help?

cheers
targ

[NoNickName]

No, there's something wrong.
How many tubes in vertical? If the coil il 240mm high, and the tube pitch is 20mm, it means that there are max 12 tubes in one row, but if the tube is 20mm dia. that leaves no space for air.
you probably mean 9.35mm (3/8") dia pipe on a 20mm or 25 mm pitch. That would result in a 6 tubes in vertical and let's say 3 to 4 rows across.
Then: fins can't be 2mm thick. They are maybe 0.1mm thick and 2mm pitch.
Finally: 76 fins x 2mm pitch makes a total 152mm wide coil, and not 120mm, let alone the frame.

[Targ]

right ive decided to draw a pic, as obviously i can explain well enough.

http://img507.imageshack.us/img507/9460/73104op.jpg

thats NOT my condenser, thats what a google image search came up with, ive anotated to show what i was trying to say.

any more questions?

cheers
targ

[NoNickName]

Ok, that's a single row, 6 tubes coil, 3/8" tube dia, 25mm tube pitch, 0.1mm fin thickness and more or less 3mm fin pitch.
Does your coil like like that, but with 20 tubes and 76 fins? If yes, it will be 25mm pitch x 20 tubes=500mm high, and 76 fins x 3mm fins pitch = 228mm wide.
One row is approx 25 mm deep.
Your coil is then 500x228x25 mm

[Targ]

Ok, that's a single row, 6 tubes coil, 3/8" tube dia, 25mm tube pitch, 0.1mm fin thickness and more or less 3mm fin pitch.
Does your coil like like that, but with 20 tubes and 76 fins? If yes, it will be 25mm pitch x 20 tubes=500mm high, and 76 fins x 3mm fins pitch = 228mm wide.
One row is approx 25 mm deep.
Your coil is then 500x228x25 mm

yes!
thats it.

targ

[NoNickName]

That resolves into

Internal coil volume 0.3232 liters
Total exchange surface 1.55 sqm
Air side exchange coefficient 4022 W/sqm K
Global exchange coefficient 22.74 W/sqm K
LMTD 11.97°C
Air entering leaving 20/24.9°C
Refrigerant entering 35°C
Refrigerant flow rate 7.78 Kg/h
Cooling capacity 420W
Air flow 260 m3/h

[US Iceman]

so i decided to try and work out the mass flow and cp for the refrigerant and work it 'from the other way'

Hi Targ,
You cannot just use the specific heat of the fluid since this is a phase change process also. For any given mass flow you have both sensible and phase changes occurring in the condenser (also the evaporator).

The vapor exiting the compressor is superheated to a high temperature.

The sensible component of the total heat exchange is as you described it: mass flow X Cp X dT (temperature difference between the discharge temperature and the dew point temperature of the refrigerant at condensing pressure).

The phase change component of the total heat exchange process is: mass flow X dh (enthalpy difference between x=1 and x=0 on a Mollier diagram (log P-H diagram for the refrigerant)

Total Q = sensible + Phase change

The total heat exchange should balance with the energy transfer to the air circulated as: Mass flow (air) X Cp X dT (temperature difference between entering and leaving air from the condenser)

Does that help you?

[Targ]

That resolves into

Internal coil volume 0.3232 liters
Total exchange surface 1.55 sqm
Air side exchange coefficient 4022 W/sqm K
Global exchange coefficient 22.74 W/sqm K
LMTD 11.97°C
Air entering leaving 20/24.9°C
Refrigerant entering 35°C
Refrigerant flow rate 7.78 Kg/h
Cooling capacity 420W
Air flow 260 m3/h

thank you very much, but can you explain where/how you worked this out??

thats nearly excalty what im seeing in real life, so if i can write that up i will be forever indeted to you

cheers
targ

[NoNickName]

So, I use a coil manufacturer selection program.
Because I don't bother, and my place practice is more valuable than theory.

[Targ]

what is the name of this mythical program?

many thanks for your efforts.
targ

I do love that quote.

[NoNickName]

www.stefani-online.it

I don't remember if you can directly download the software or have to ask for a license unlocking code, which is free nonetheless.

[Peter_1]

Where's the link for the software selection program please?
Is it a large file to post?

[Julian]

Mass Flow (m) = Refrigeration capacity of 1 Kw (Qe) / Refrigeration Affect (qe)

Refrigeration Affect (qe) = Hc-Ha (on a PH chart this)
Hc = enthalpy of 134a at your saturaterd suction temp
Ha = enthalpy of 134a at your saturaterd discharge temp

this divide by 1Kw will give you as mass flow rate of refrigerant per kilowatt in Kg/s

Alternativly mass flow can be calculaterd by
Mass Flow rate (m) = Piston displacement (Vp) x Density (p)
Density(p) is needs to taken your saturaterd suction temp (SST)

Hope this helps

[NoNickName]

Where's the link for the software selection program please?
Is it a large file to post?

Yes, it is big. Sorry, it can't be downloaded, probably they restructured the website.
Feel free to ask them.

[Targ]

i have email them to no response.

does anyone know where or how i can model future condensers.

as i think this software or something like it could be extremely useful for the future.

or even if some one knows the theory behind this or has a link, ill try and write my on program.

cheers
targ

[autt]

That resolves into

Internal coil volume 0.3232 liters
Total exchange surface 1.55 sqm
Air side exchange coefficient 4022 W/sqm K
Global exchange coefficient 22.74 W/sqm K
LMTD 11.97°C
Air entering leaving 20/24.9°C
Refrigerant entering 35°C
Refrigerant flow rate 7.78 Kg/h
Cooling capacity 420W
Air flow 260 m3/h

Compare to my calculation:

Internal coil volume 0.3277 liters
Total exchange surface 2.36 sqm
Ref side exchange coefficient 1558 W/sqm K
Air side exchange coefficient 27.68 W/sqm K
Average LMTD 10.06°C
Air entering leaving 20/24.77°C
Refrigerant entering leaving 35/23.95°C
Refrigerant flow rate 7.78 Kg/h
Cooling capacity 417W

Tube: copper, Fin: Aluminum; both steel cause little difference

[Targ]

Compare to my calculation:

Internal coil volume 0.3277 liters
Total exchange surface 2.36 sqm
Ref side exchange coefficient 1558 W/sqm K
Air side exchange coefficient 27.68 W/sqm K
Average LMTD 10.06°C
Air entering leaving 20/24.77°C
Refrigerant entering leaving 35/23.95°C
Refrigerant flow rate 7.78 Kg/h
Cooling capacity 417W

Tube: copper, Fin: Aluminum; both steel cause little difference

can i ask how you calculated this?

many thanks
chris

[autt]

I calculated that use my program.

I check my input, a 2mm fin pitch results to 419W cool capacity, 3mm pitch results to 411W, should I doubt myself??

In my program, a condenser is devided into 3 region along duct flow: super heating, two phase and super cooling section, which have different heat transfer equation.

First, suppose ref side outlet state, i use the state that ref dryness=0 and super cooling temperature=0; then use both side heat transfer to calculate three section tube length, get a new suppose state by comparison to real length, iterate the calculation to a satisfied precision.

Vertical and horizonal ref flow coil use different LMTD and two phase heat transfer coefficient calculation equation.

There are so much to explain my calculation so i suggest you look up articles about heat transferation needed in a condenser, such as single and two phase, finned surface heat transfer coefficient or mechanism. After trying your calculation, you will find more.

[Peter_1]

As far as I understand, I see extrme diffrences in what Autt calculated and what NoNickName calculated.

[autt]

As far as I understand, I see extrme diffrences in what Autt calculated and what NoNickName calculated.

Air side exchange coefficient 4022 W/sqm K
Global exchange coefficient 22.74 W/sqm K

Ref side exchange coefficient 1558 W/sqm K
Air side exchange coefficient 27.68 W/sqm K

Do you mean this? I can affirm that air side heat exchange coefficient should not be so big, normally it should no more than 100, it will be very small in this case by that air speed is as slow as 0.64m/s in calculation, I think NoNickName just miswrite it, it should be a ref side value.

Different suggested ref side exchange coefficient equations get varies value but cause small difference in air cooling condenser, for air side has the main heat resistance, so the total pricision mainly relies on air side calculation, fortunately it can be done well.

cheers

[Josip]

Hi,

As far as I understand, I see extrme diffrences in what Autt calculated and what NoNickName calculated.

I believe Peter_1 mean about this:

NoNickName = Cooling capacity = 420 W
Autt = Cooling capacity = 417 W

Try to imagine if they express capacity in eV (electronvolt)

Best regards,

Josip

[Peter_1]

Sorry, wrong read/interpreted.

[NoNickName]

My mistake.
Though ref and air side coeff. change in relation to geometry, pitches, etc.

[Targ]

does any one have useful links or articales i could read, as this topic is bugging me that i dont get it.

and i can find a program to calculate if for me, and they must be out there some where.

cheers
targ

[autt]

Here is a pdf link for heat transfer
http://www.sae.org/altrefrigerant/pr...esw-mathur.pdf

[US Iceman]

Hi autt,

That was an excellent resource you posted.

Thank you.

[Targ]

Here is a pdf link for heat transfer
http://www.sae.org/altrefrigerant/pr...esw-mathur.pdf

thanks that seems to outline the whole area very well, and should give plenty to work from.

many thanks
targ