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gwapa
06-08-2007, 02:16 PM
Hello Gentlemen

My question is : How to figure out the presure drop in a evaporative condenser and a set of condensers installed in parallel?

As you know a presure drop of just 1Psi (0,068 bar) higher between condensers is very import to to disign the seal leg at the piping outlet . Also this liquid seal is very import to purge Non Condensables Gases.

I will appresite your opinions
Best regards

S.K.VARDE
06-08-2007, 04:11 PM
Total pressure drop is a sum of static head and the velocity head.

you can refer any text book on design of heat exchanger.

US Iceman
06-08-2007, 07:04 PM
The biggest problem with finding the answer you want is knowing the total length of each condenser coil circuit, and, the contributions/affects of the header boxes used.

The manufacturers are usually hesitant to tell anyone the actual length and number of circuits in their condensers.

In lieu of having this information I have always used the estimated pressure drop supplied by the manufacturers.

I typically use a total of 2 psi for the design condensing temperatures, and more than this if the system will operate at full load with much lower condensing temperatures.

The seal leg depth for 2 psi is about 8 feet on ammonia at 95F (35C). The manufacturers also say to double the seal leg depth if the condensing temperature is lower.

The highest pressure loss from any condenser will determine the maximum seal leg depth for all of the condensers.

You should also account for symmetrical piping versus asymmetrical piping as this affects the total inlet pressure, which changes the outlet pressure for the individual condenser coils.

The equalizing line size is also another very important item to size properly.

Hope this helps you.;)

gwapa
06-08-2007, 07:27 PM
Thanks US ICEMAN

Yes it is very dificult to have the lenth of the tubes and header lost of a condencer.

I use 5 ft as a minimun in new instalations however I am working in a project where we have to replace 3 existing very old condenser from GRAM . As you know this old units used to have tube 1/8" thinkness of wall. Biside that there are four condenser from BAC of different capacity. So when we want to design very sharp there are no information.

An error of 3 psi make you to install the condescer over a tower.
Regards

US Iceman
06-08-2007, 08:39 PM
The situation you describe is almost a worst case scenario. The only thing that can make this worse is if the different condensers are on different elevations! Then the seal leg depths have to accomodate the elevation differences too.

One way around this is to use liquid drainers on each condenser coil. This is identical to the concept of using steam traps on condenser coils.

It is the same principle, but we are just using a different fluid.

gwapa
07-08-2007, 01:02 AM
Are you talking about the model HT100 from hanssen Technologies?

It look a good idea?

How does work the Non Condensables Purger?
You should purger in the liquid trap and in the outlet of condenser. doesnt it?

US Iceman
07-08-2007, 01:21 AM
That would be one of them I have used, although you have to size it carefully because the drainer capacity is dependent on the pressure drop across the sliding block.

On the systems I have designed we take the liquid from the drainer straight into the inlet of the pump recirculator vessel. We are running critical charge systems without high pressure receivers.

Since this is a high side float, a liquid seal is formed in the chamber where the float ball is. Then we use the top connection for purging, and the bottom connection for an oil drain.

Works great!

This removes the seal leg depth problem and sizing the equalizing lines.

We are getting ready to start a new system in several weeks.

gwapa
07-08-2007, 02:48 PM
"you have to size it carefully because the drainer capacity is dependent on the pressure drop across the sliding block."

How big is the pressure lost acrros the drainer?

Have you obverved the operations of this trap for a long time? Any mantinence? automatic oil drain? does it requiere an inlet filter?

US Iceman
07-08-2007, 05:47 PM
How big is the pressure lost across the drainer?


It is just like any other control valve. The valve capacity increases as the pressure difference across the orifice increases. Like most refrigeration valves, I believe their "rated" capacity is based on 2 psi.

If your system has a controlled pressure receiver, then the drainers would have a pressure difference from: condensing pressure to the controlled pressure in the receiver.

The higher differentials allow you to use the smaller drainer for greater capacity.

We have one installation that is almost 3-4 years old right now and have not experienced any problem with Hansen drainer yet.

To my knowledge, we have not had any problems with oil or debris in the system. We have not used strainers upstream of the drainer, but it is probably a good idea.

Ponca Dave
16-02-2008, 10:02 AM
The biggest problem with finding the answer you want is knowing the total length of each condenser coil circuit, and, the contributions/affects of the header boxes used.

The manufacturers are usually hesitant to tell anyone the actual length and number of circuits in their condensers.

In lieu of having this information I have always used the estimated pressure drop supplied by the manufacturers.

I typically use a total of 2 psi for the design condensing temperatures, and more than this if the system will operate at full load with much lower condensing temperatures.

The seal leg depth for 2 psi is about 8 feet on ammonia at 95F (35C). The manufacturers also say to double the seal leg depth if the condensing temperature is lower.

The highest pressure loss from any condenser will determine the maximum seal leg depth for all of the condensers.

You should also account for symmetrical piping versus asymmetrical piping as this affects the total inlet pressure, which changes the outlet pressure for the individual condenser coils.

The equalizing line size is also another very important item to size properly.

Hope this helps you.;)
I totally agree with the IceMan. 8' min trap. And equalizing line do have pressure drop so size them right. They can't be too big.