View Full Version : Would you please help me to understand better ?

A.Mortezania

02-04-2017, 06:24 PM

Dear friend,

Would you please help me to understand this sentences below better and why it happen ?

In any system, whether it is new or old, measurably

higher pressure drops in the discharge line need to be

taken into consideration in sizing the condenser and

compressor. For example, if there was to be an 8 psi

(56kPa) pressure drop in discharge line of an NH3

system, it would amount to about a 2.5°F (1.4°C)

reduction in saturation temperature at the condenser.

This means that either the condenser should be

increased in size approximately 15 percent.

Why more pressure drop on discharge pipe force us to have bigger condenser ?

Sincerely yours.

NH3LVR

02-04-2017, 07:11 PM

Read on a bit in the Evapco book. It explains it well.

This means that either the condenser should beincreased in size approximately 15 percent or it must be accepted that the compressor discharge pressurewill be 8 psi (55kPa) higher than design.

A.Mortezania

02-04-2017, 08:21 PM

Hi,

That sentence was exactly form Evapco book, I want to know why ?

I want to know relationship between pressure drop on discharge pipe and more heat rejection on condenser. Why when we have more pressure drop we should use bigger condenser ?

Sincerely yours.

FaultCode

02-04-2017, 10:52 PM

You use a larger condenser to reduce the pressure drop, laws of pyhsics.

cduque

03-04-2017, 04:29 PM

You don't use a bigger condenser to have more heat rejection, you need a bigger condenser to exchange the same amount of heat with a lower temperature difference between de condensing temperature and the external temperature.

A.Mortezania

03-04-2017, 07:32 PM

You don't use a bigger condenser to have more heat rejection, you need a bigger condenser to exchange the same amount of heat with a lower temperature difference between de condensing temperature and the external temperature.

Dear cdugue ,

OK. Let's check this.

Suppose that :

Compressor discharge : 200 PSI

Ambient temp : 45 C

Discharge pressure drop : 0 PSI

Condenser pressure drop : 0 PSI

Condenser pressure : 200 PSI

It means , Condensing temp ~ 39 C and when ambient be 45C it means DELTA T = 45 – 39 means : 6 C

Now we have :

Compressor discharge : 200 PSI

Ambient temp : 45 C

Discharge pressure drop : 10 PSI

Condenser pressure drop : 0 PSI

Condenser pressure : 190 PSI

It means , Condensing temp ~ 37 C and when ambient be 45C it means DELTA T = 45 – 37 means : 8 C

Sorry if I bother you , But twice I do not understand why we use bigger condenser !

Sincerely yours.

A.Mortezania

03-04-2017, 07:35 PM

You use a larger condenser to reduce the pressure drop, laws of pyhsics.

Dear Faultcode ,

Thanks for your reply.

But I think bigger condenser has more pressure drop.

Sincerely yours.

The Viking

03-04-2017, 07:44 PM

Sorry A,

But there is a flaw in your reasoning...

It is the energy from the compressor we need to distribute so assuming you got the same duty and conditions on the two systems in your example above, in the second example you wouldn't have a compressor discharge of 200psi, it would actually be 210psi and the condenser pressure would stay at 200psi, this would mean that the compressor will have to work harder for no added gain.

If you instead "oversize" your condenser to keep the condenser pressure at 190psi, the compressor would still work at 200psi discharge as per design.

:cool:

A.Mortezania

03-04-2017, 08:28 PM

Sorry A,

But there is a flaw in your reasoning...

It is the energy from the compressor we need to distribute so assuming you got the same duty and conditions on the two systems in your example above, in the second example you wouldn't have a compressor discharge of 200psi, it would actually be 210psi and the condenser pressure would stay at 200psi, this would mean that the compressor will have to work harder for no added gain.

If you instead "oversize" your condenser to keep the condenser pressure at 190psi, the compressor would still work at 200psi discharge as per design.

:cool:

Dear The Viking,

Yes, You are right. I think I found my mistake. When we have pressure drop on discharge pipe we should have bigger condenser to reach to smaller DELAT T.

Is it true ?

Sincerely yours.

Segei

04-04-2017, 03:33 AM

How is condensing temperature lower than ambient? :D Do you have air condenser?

A.Mortezania

05-04-2017, 07:38 PM

Read on a bit in the Evapco book. It explains it well.

This means that either the condenser should beincreased in size approximately 15 percent or it must be accepted that the compressor discharge pressurewill be 8 psi (55kPa) higher than design.

Dear NH3LVR,

Would you please let me know where can i find Evapco book ?

Sincerely yours.

NH3LVR

05-04-2017, 10:27 PM

http://www.evapco.com/sites/evapco.com/files/131a_-_piping_brochure.pdf

A.Mortezania

06-04-2017, 08:33 PM

http://www.evapco.com/sites/evapco.com/files/131a_-_piping_brochure.pdf

Dear NH3LVR,

Thanks a lot.

Sincerely yours.

A.Mortezania

07-04-2017, 11:02 AM

Dear Friends,

I read EVAPCO book that NH3LVR did a favor and sent it.

Twice I didn't find my question, If someone can easily answer me I thanks him/her.

Question : Why when we have high pressure drop on discharge pipe, We should increase condenser heat rejection area or we should use bigger condenser on capacity ?

Sincerely yours.

Segei

08-04-2017, 04:57 PM

Usually, refrigeration system design start from choosing compressors based on refrigeration load. Than we choose condensing pressure(temperature). Actually, it is not condensing pressure but discharge pressure. Assume that discharge pressure is 180 psig. If we have pressure drop in discharge line 5 psig, condensing pressure will be 175 psig. Condensers should be designed to provide this pressure at design ambient conditions. However, if discharge line is undersized and pressure drop is 15 psig and we keep condensers chosen for 5 psig pressure drop, discharge pressure will be 190 psig. To get desirable discharge pressure of 180 psig, condensing pressure should be reduced to 165 psig and this can be done by increasing size of condensers.

A.Mortezania

08-04-2017, 08:04 PM

Usually, refrigeration system design start from choosing compressors based on refrigeration load. Than we choose condensing pressure(temperature). Actually, it is not condensing pressure but discharge pressure. Assume that discharge pressure is 180 psig. If we have pressure drop in discharge line 5 psig, condensing pressure will be 175 psig. Condensers should be designed to provide this pressure at design ambient conditions. However, if discharge line is undersized and pressure drop is 15 psig and we keep condensers chosen for 5 psig pressure drop, discharge pressure will be 190 psig. To get desirable discharge pressure of 180 psig, condensing pressure should be reduced to 165 psig and this can be done by increasing size of condensers.

Thanks Segei,

I got it very well. Thanks for your easy explanation.

Sincerely yours.

RANGER1

09-04-2017, 03:14 AM

I think it would be fair to say this is not normal on a new system.

If system was added to, short cuts or poor engineering it probably would occur.

Maybe no one knows as well!

quimsouille

22-07-2017, 04:37 AM

You don't use a bigger condenser to have more heat rejection, you need a bigger condenser to exchange the same amount of heat with a lower temperature difference between de condensing temperature and the external temperature.

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