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2008-08-26 18:46:31

Compressor duty and suction temp

Hi,
could anybody tell me a calculation for the loss of duty for a reciprecating compressor if the suction line temp is 5k above designed temp? Say for theory sake unit evaporating at -8'c expected gas return to comp 2'c, however gas returning at 7'c.
Many Thanks Rowan

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RE: Compressor duty and suction temp

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The problem i have is with a co2 pack running 50 or so cabs. the pack seems to be struggling on duty, as the controls system is not what i'm used to I stand to be corrected. Cabs are making temp however run with very wide superheats. Thought that if the gas was returning warmer than the design that this would mean that the gas would be less dense than design and knock the duty of comps?? Just didnt know by how much. Co2 is very new to me so i'm still trying to grasp it and any advise would be great.
Cheers

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RE: Compressor duty and suction temp

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Hi Matthias thanks for your help. I work for a Project First Facilities Management. We work supermarkets mainly.

RE: Compressor duty and suction temp??

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Hi Rowan,

I take it we are talking R744 Transcritial application here (Dry suction)?

If you let me have the compressor details (It may take me a while to trawl through my info) I will come back to you. 


Regards


Martin Ritchie

RE: RE: Compressor duty and suction temp??

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Hi Martin, hows tricks?

Yep, should be a dry suction.

The comps are Bitzer, mod; 4HTC-20-40P. No rush for the info mate just something I'm trying to justify something in my head.

Cheers Rowan

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I am currently trying pipe sizing calculations for R744, is there a Nomograph available to check computer caculations?

R744 pipe sizing for both transcritical and subcritical

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We are in the beginning stages of designing a transcritical 4C chamber.  What are the expe cted velocities we should design to for oil movement.  With R404a we have had good results at 1500 FPM vertical and 750 FPM horizontal.  Is there line sizing criteria available I here the sizes will drop but want to size the lines properly especially since this is an R&D project.
Thanks for your help

RE: R744 pipe sizing for both transcritical and subcritical

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As far as i am aware Brad the same velocities should be used for R744 as R404a

RE: RE: R744 pipe sizing for both transcritical and subcritical

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no. lower velocities must be used, otherways pressure drop is too high
Darcy-Weisbach equation gives correct results

RE: Compressor duty and suction temp

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Sergio,

What velocities are recommended for R744 to acheive 2 psi drop?

RE: RE: Compressor duty and suction temp

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the question is too simple. you should give much more data (i.e. enthalpy difference available, suction temperature level, lenght, diameter/cpacity) and so on.
in general the right velocity for a supermarket installation (pipe length 50-80 m) never exceeds 5-6 m/s for a pressure drop lower than 1-1,5 K.
you are asking for 2 psig pressure drop. the pressure drop is normally given in drop in saturation temperature, because regarding efficiency tloss his is the right parameter to consider. 2 psig is too low, you would end up with too small velocities

RE: RE: Compressor duty and suction temp

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Rowan,
What do you mean by "the pack seems to be struggling on duty"? Is it frequent compressor on/off cycling? If it is, it could be compressor capacity design issue (too big capacity) or it could also be system control issue.

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RE: RE: Compressor duty and suction temp??

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I guess what I am looking for is guide lines for R744 piping since my last post we have decided to go with stainless tubing and swage locs in lieu of copper tubing.  We are developing a redundant chiller in the 10,000 BTUH range for our test system.  My concern for velocities is for oil return problems.  I have looked for a two stage compressor for primary cooling on one stage and gas cooling on the other stage.  Since I haven't found one that someone will sell to us we are looking at one compressor with a gas cooler not as efficient as the two stage but at this point we can only build what is available.  We are also adding a oil cooler as per discusions from Bitzer.  Will keep yu posted on our findings and results. Sanyo has a two stage but they will not sell to someone doing developement.  Don't know how we can futher the process with out developement it just cause us to think a little diffeerently.

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?

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Hi Rowan,

Just curious what display cases you are referring to?

mark

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Hi Mark
I dont look after this system anymore but the whole site was Carrier/Linde except coldstores and a couple of serve over cabs.
Rowan