About liuid line pressure drop.

As we know, the discharge line pressure drop will affect the condensing pressure, so we try all kinds of methods to reduce the pressure drop in discharge line. Even if we add 15 kPa to the discharge line, the condensing pressure will increase quite much.

My question is:
What about the liquid line pressure drop? Is it as sensitive as the discharge pressure drop? We know there is TXV in liquid line, if the pressure drop in liquid line increase , will it balance so that the TXV pressure will reduce, so the pressure drop will not affect the condensing pressure so much.

Now I am selecting a PHE as the ECO, it's in liquid line,I don't know 33kPa pressure is acceptable, but I know if it is in discharge line, it's not acceptable for me.

Hope for your answer!

Wilson

Set up a process simulator & trial in the effects of your changes. This will help you to decide if the line size is adequate for your design.

You need to have sufficient subcooling to offset the pressure loss of piping and elevation changes (when the liquid flows up, not down). If you do not provide subcooling the liquid will begin to flash off (part liquid/part vapor) and this reduces the system cooling capacity.

Thank you, Iceman. The ECO's function is to provide subcooling, it's enough. And the piping is not long, just within the unit.

I am worrying about the ECO "twin" effect: improve performance by providing subcooling, still reduce the performance by the pressure drop across it. As we know, any heat exchanger will have pressure drop, it's unavoidable.

Is the least the Pressure drop(PD) and the better? Reducing the PD may oversize the heat exchanger.

If you are using an economizer on a screw compressor the subcooling has to include the pressure loss of the economizer and valves also. It depends on what you are trying to do; provide more subcooling to gain performance (after the pressure losses are considered) or subcool only enough for the pressure losses, or how much the economizer costs.

Hi,Iceman!
I have different opinion.

Do we need to include the pressure loss of the ECO? The refrigerant need to be throttled before it goes into the evaporator,there's pressure loss in the TXV. So since there is pressure loss in the ECO, we can pen the TXV and make the pressure loss in TXV smaller. It may balance. So no influence.

Am I right?

Thanks!

Still looking forward to your answers, please don't hesitate to give me your opinion.Any answer will be welcome!

I'm not sure if I am following what you are asking here, I think you are saying that because a TXV is fitted at the end of the liquid line and as such has a PD to make it function (expand the refrigerant) you want to discount the economiser PD and open the TXV further and decrease its PD, you assume you will balance the pressure drops out so to speak. If this is what you assume then I must say my opinion is that you are incorrect.

For a given SST the capacity of the TXV is determined by the pressure drop across it and the amount of subcooling the entering liquid has. As mentioned earlier your economiser should be delivering enough subcooling to at least overcome the total liquid line pressures (including valves, fittings, piping and also the PD through the economiser itself) you do not want to deliver liquid that is already flashing due to excessive PD in the piping.

Part of the beauty of subcooling that liquid with the economiser circuit on a screw compressor is it will reduce the mass flow rate of refrigerant you need to circulate to achieve the required capacity. Which equates to smaller equipment with the only trade off being your condenser needs to be adequately sized to reject the total system heat which includes the heat absorbed through the economiser circuit as well as the usual heat of compression and evaporator loads, in effect you get a smaller compressor and retain the same condenser as you would with a normal non economised system.

Thanks, Valve!

We can discuss on PD in the liquid line(including the economizer).

In my opinion,a TXV is just a kind of pressure loss equipment(but can adjust the PD across itself), so in principle it's not quite much different from other accessory in the liquid line. Just like a system with a distributor, the pressure drop is quite big across the distributor(about 0.5 bar), but we can accept the PD and I don't think it will affect the performance of the system.

Also, in flooded chiller, it's quite common to use two perforated plates to expand the refrigerant and do not use any expansion valve, which means expansion valve's main function is to provide the pressure drop. And two perforated plates also equals to two expansion valves.

My point is: the pressure drop across the ECO equates a distributor or an perforated plate. So the PD across the ECO will not affect the performance of the chiller.

The pressure drop on the liquid side of a PHE economizer/subcooler is minute. The increased subcooling of the liqiud actually increases the capasity of the TXV much more than the slightly decreased pressure does the opposite. Therefore on a system with, say R404A, and a large subcooling from an economizer you need to recalculate the TXV based on the new cold liqiud and the same available liqiud line pressure.

Quote:

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Originally Posted by wilsoncheung

As we know, the discharge line pressure drop will affect the condensing pressure, so we try all kinds of methods to reduce the pressure drop in discharge line. Even if we add 15 kPa to the discharge line, the condensing pressure will increase quite much.

My question is:
What about the liquid line pressure drop? Is it as sensitive as the discharge pressure drop? We know there is TXV in liquid line, if the pressure drop in liquid line increase , will it balance so that the TXV pressure will reduce, so the pressure drop will not affect the condensing pressure so much.

Now I am selecting a PHE as the ECO, it's in liquid line,I don't know 33kPa pressure is acceptable, but I know if it is in discharge line, it's not acceptable for me.

Hope for your answer!

Wilson

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Do you have 33kPa pressure drop on the liqiud side or the evaporating side? What make of PHE and how have you calculated this value?

Quote:

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Originally Posted by SteinarN

Do you have 33kPa pressure drop on the liqiud side or the evaporating side? What make of PHE and how have you calculated this value?

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The 33 kPa is on the liquid side. This data is from the PHE manufacturer.

I agree with you on your above opinion. Do you think 33kPa on the liquid side is acceptable? Or should I contact the manufacturer for a smaller PD?

I dont think 33 kPa is to much pressure drop. If the PHE economizer is doing it's job then the subcooling should be large at the PHE outlet regardless of this pressure drop. The reason I ask about the pressure drop is that it seems very large compared to my experience when I calculate a PHE subcooler. I always get a minute pressure drop on the liquid side when I select a PHE with a reasonable pressure drop on the evaporator side.

Because I don't have PHE software, I always ask the manufacturers to help me with the selection. They said it's common to have 50kPa, so 33 kPa is fine. Reducing the PD may increase the size and cost, so what range of PD do you have in your selection? Thank you!

My fault. I was taking some numbers out of my head. I was mixing the liquid side pressure drop on a suction gas/liquid line heat exchanger with that of an economizer. On a suction gas/liquid line PHE heat exchanger the pressure drop on the liqiud side will always be minute. On an economizer it will often be more substanly like the pressure loss you have been given.

Thank you very much,SteinarN!
You've been great help to me!