sizing pipes

[ronaldjaysonm]

gud day to all...
im ronald and im new here and also new in the industry of industrial refrigeration...i just want to ask if what is the most effective way of sizing the ammonia pump on a pump recirculated system...thanks....

[US Iceman]

Welcome to the RE forums.

Sizing refrigerant pumps is relatively easy, however there are certain issues you have to be careful of.

First you need to know the flow rate required in GPM or liters/min. The flow rate is found by multiplying the refrigeration cooling capacity x the recirculation rate. For ammonia the rate is usually 3 or 4. The refrigeration capacity is the cooling required at the evaporators.

Next you need to know how much head (pressure) you need. The head is the amount of pressure to overcome pressure losses in the piping, valves, and elevation change (the pump is lower than the evaporators, therefore the pump has to create the pressure to push the liquid up).

The last point is to select the pump for low NPSH. NPSH is net positive suction head. This is the pressure of the liquid flowing into the pump suction. You have to allow more NPSH than what the pump may experience during any operating condition.

I have made this a very simple descritption on purpose to start with. There are a lot of details not discussed here, but this serves as a general starting point.

[boy]

US ICEMAN,

very nice fundamental info.many thanks

[US Iceman]

There are several threads on the site that have a lot of detail on this and other issues related liquid overfeed systems. You can search for liquid overfeed or liquid recirculation or refrigerant pumps.

I believe one of the better threads was started by lana. So you might search for the members name also.

[gwapa]

Hi US ICEMAN
I have been courius all the time how figure out the pressure loss from the evaporator to the LPR.
As you know in the evaporator we will have constant pressure however this is theoretically.We will have a two phase which is not easy to figurate the pressure loss
Do you have any literature about this or could you coment this topic.
Thanks
Gwapa

[US Iceman]

Hi gwapa,

What I have used is very conservative for wet suction line sizing. Here's one way...

If you take the total evaporator load connected to the pump vessel as 500 Tons for example @ 10°F. If the recirculation rate is 3:1, then what I would normally do is this: 500 Tons X 3 = 1500 Tons.

Then size the suction header for 1500 Tons of Refrigeration using vapor only (as if the pipe contains 100% vapor) for a 10°F dry suction line at whatever pressure loss value I need to achieve to limit compressor capacity losses.

A normal rule-of-thumb is to size the pipe as: 500 Tons X 1 = 500 Tons for example again. This would be meant to show the recirc. rate is 1:1 (similar to DX). Then select the pipe size for the desired pressure loss at the operating conditions required. Once you have this pipe size, then use one pipe diameter larger.

I typically use the first method as it is very conservative and allows for low velocity through the pipe. That helps to reduce the chances of liquid hammer in the piping.

It is not very scientific, but it works.

[SURESH YADAV]

Hi.... very nice explanation can you tell me wt are recommended velocity in suction line and wet return line... please also tell me velocity of refrigerant depends on type of refrigerant or not...........

[US Iceman]

...can you tell me wt are recommended velocity in suction line and wet return line...

For an ammonia system, particularly suction lines you determine the velocity after you size the pipe for the allowable pressure loss.

In halocarbon (and other) people typically size pipes by velocity, which you have to do to for oil return to the compressor. At whatever velocity they use, the pressure loss will be equal to some value. Then you have to allow for the pressure loss to ensure the compressor capacity is not affected.

The higher the velocity, the greater the pressure loss will be. This contributes to excessive capacity loss for the compressor, an increase in power use, and can allow water hammer to occur in gas lines where liquid may be present.