Consideration of flashgas in pipeline

[oplafki]

Hi, everyone.

There is a concern about flash gas in dry refrigeration systems.
I ask for your opinion on the situation below.

The refrigeration system is in its simplest form (compressor-condenser-expansion valve-evaporator), and the refrigerant is R507.
Let's assume there is no overcooling.

"case 1:"
The pipe between the condenser and the expansion valve was long, resulting in a total pressure loss of 2 Bar.
The flash gas generated by the pressure loss passes through the orifice of the expansion valve.
As such, the flow rate of liquid passing through the orifice will be reduced and the cooling capacity will be slightly reduced.

But what if this pipe was vertical?
"case 2:"
A total pressure loss of 2 bar occurred due to vertical piping.
The flash gas generated at this time collects in the upper part of the pipe, forms pressure, and pushes the refrigerant liquid level down.
Therefore, the expansion valve only passes the flash gas and no cooling effect can be achieved.

The total pressure loss in both cases is the same,
Horizontal piping reduces capacity slightly, while vertical piping loses refrigerating capacity to zero.

I think "case2" is wrong. But I don't know which part is wrong.
What is the actual flow of refrigerant in the pipe?
Is there a formula to estimate the amount of refrigerant loss in a vertical pipe?

I look forward to your wise reply.
Thanks

[Rob White]

.

Why is pressure drop over the liquid line equated to Flash Gas?

Rob

.

[seanf]

Hi, everyone.

There is a concern about flash gas in dry refrigeration systems.
I ask for your opinion on the situation below.

The refrigeration system is in its simplest form (compressor-condenser-expansion valve-evaporator), and the refrigerant is R507.
Let's assume there is no overcooling.

"case 1:"
The pipe between the condenser and the expansion valve was long, resulting in a total pressure loss of 2 Bar.
The flash gas generated by the pressure loss passes through the orifice of the expansion valve.
As such, the flow rate of liquid passing through the orifice will be reduced and the cooling capacity will be slightly reduced.

But what if this pipe was vertical?
"case 2:"
A total pressure loss of 2 bar occurred due to vertical piping.
The flash gas generated at this time collects in the upper part of the pipe, forms pressure, and pushes the refrigerant liquid level down.
As long as the vertical pipe diameter isnt oversized and the system isnt running at a low capacity, would the vapour not help move the liquid, in a mixed flow?

Therefore, the expansion valve only passes the flash gas and no cooling effect can be achieved.
Would you not still get some cooling, just that its from the superheating of the vapour and not from a phase change?

The total pressure loss in both cases is the same,
Horizontal piping reduces capacity slightly, while vertical piping loses refrigerating capacity to zero.

I think "case2" is wrong. But I don't know which part is wrong.
What is the actual flow of refrigerant in the pipe?
Would it not be a mixed flow of vapour and liquid, unless the system is very low on refrigerant?

Is there a formula to estimate the amount of refrigerant loss in a vertical pipe?
If you wanted to know how much vapour would be created and you know the mass flow rate. Could you look at your pressure-enthalpy charts Quality value?

I look forward to your wise reply.
Thanks

Would you not design, charge and adjust the system to suit the site conditions?

[MrFreez717]

If you have zero "overcooling" (we call it subcooling in the US, but I think that is what you mean), then any pressure drop all all will result in flash gas.

Passing through piping, valves, filter/driers etc. will inevitably cause flash gas in a saturated liquid due to a velocity increase and friction. We often refer to it as "line losses."

If the liquid must move vertically upward, you also will have pressure drop caused by gravity. For example, if your discharge pressure is 17 bar and your refrigerant is R-507 the pressure drop will be about 0.1 bar per meter of vertical rise. This is independent of line losses and adds to the total pressure drop.

If you have 2 bar of pressure drop between the condenser and the TXV, you will have mostly flash gas at the TXV inlet and system performance will be very poor. The solution would be to add a subcooler or a liquid line booster pump.

Best regards,
MrFreez717

[oplafki]

hi MrFreez717, Thank your reply.

First, you are right, I wanted to say subcooling,
and I understood the important of that.

But what I really want to know is the effect of the difference pipe direction.
If total pressure drop in both cases is the same and the only difference is in the direction of the piping from condenser to TXV,
(maybe the pressure drop in only few meters of vertical pipe is equal to the pressure loss in a few hundred meters of horizontal pipe)
can i consider the two system is identical?
Is there other things to pay special attention to?


Best regards,
Oplafki

[oplafki]

.

Why is pressure drop over the liquid line equated to Flash Gas?

Rob

.

hi Rob,
Because the saturation point of the refrigerant liquid changes.

[oplafki]

If you have zero "overcooling" (we call it subcooling in the US, but I think that is what you mean), then any pressure drop all all will result in flash gas.

Passing through piping, valves, filter/driers etc. will inevitably cause flash gas in a saturated liquid due to a velocity increase and friction. We often refer to it as "line losses."

If the liquid must move vertically upward, you also will have pressure drop caused by gravity. For example, if your discharge pressure is 17 bar and your refrigerant is R-507 the pressure drop will be about 0.1 bar per meter of vertical rise. This is independent of line losses and adds to the total pressure drop.

If you have 2 bar of pressure drop between the condenser and the TXV, you will have mostly flash gas at the TXV inlet and system performance will be very poor. The solution would be to add a subcooler or a liquid line booster pump.

Best regards,
MrFreez717

hi MrFreez717, Thank your reply.

First, you are right, I wanted to say subcooling,
and I understood the important of that.

But what I really want to know is the effect of the difference pipe direction.
If total pressure drop in both cases is the same and the only difference is in the direction of the piping from condenser to TXV,
(maybe the pressure drop in only few meters of vertical pipe is equal to the pressure loss in a few hundred meters of horizontal pipe)
can i consider the two system is identical?
Is there other things to pay special attention to?


Best regards,
Oplafki

[oplafki]

Would you not design, charge and adjust the system to suit the site conditions?

hi seanf,
This is not really situation in site.
I just have question about theoretical part.

Liquids and vapors are mixed in horizontal piping, but they are divided in vertical piping.
Could there be some fatal problem causing from this?

[seanf]

hi seanf,
This is not really situation in site.
I just have question about theoretical part.

Liquids and vapors are mixed in horizontal piping, but they are divided in vertical piping.
Could there be some fatal problem causing from this?

On the systems I've seen with liquid line sight glasses on the vertical pipework, or even at the top of the vertical just before the expansion valve. You see a mixed flow of liquid and vapour, mainly when the system is slightly low on refrigerant, or when the system hasnt been set-up quite right.


But what if this pipe was vertical?
"case 2:"
A total pressure loss of 2 bar occurred due to vertical piping.
The flash gas generated at this time collects in the upper part of the pipe, forms pressure, and pushes the refrigerant liquid level down.

As long as the vertical pipe diameter isnt oversized and the system isnt running at a low capacity, would the vapour not help move the liquid, in a mixed flow?


the expansion valve only passes the flash gas and no cooling effect can be achieved.

Would you not still get some cooling, just that its from the superheating of the vapour and not from a phase change?


total pressure loss in both cases is the same,
Horizontal piping reduces capacity slightly, while vertical piping loses refrigerating capacity to zero.

I think "case2" is wrong. But I don't know which part is wrong.
What is the actual flow of refrigerant in the pipe?

Would it not be a mixed flow of vapour and liquid, unless the system is very low on refrigerant?


is there a formula to estimate the amount of refrigerant loss in a vertical pipe?
If you wanted to know how much vapour would be created and you know the mass flow rate. Could you look at your pressure-enthalpy charts Quality value?

[Rob White]

hi Rob,
Because the saturation point of the refrigerant liquid changes.

But if the liquid is subcooled as it comes out of the condenser
why would there be saturated gas in the liquid?

If you have saturated liquid / gas in your liquid line your condenser is for
whatever reason not doing its job.

Rob

.

[MrFreez717]

The condenser is not designed to subcool liquid, just condense it. Some are equipped with a subcooling loop that takes condensed liquid out of the condenser and runs it through an additional coil to get a few degrees of subcooling. I usually see these on ***** systems that are DX and therefore rely on having high pressure liquid travel a significant distance to the evaporators, to assure it is still all liquid when it gets there.

That said, I usually see a few degrees of subcooling in liquid leaving condensers because they do not drain perfectly. A ***** system equipped with a flooded head pressure control can have a LOT of subcooling at certain times, as the condenser may be half full of liquid during periods of low ambient and/or part load.

Making sure there is enough subcooling to get high pressure liquid where it needs to go is an important part of the designer's job, and one that is often overlooked.

MrFreez717

[Rob White]

The condenser is not designed to subcool liquid, just condense it. Some are equipped with a subcooling loop that takes condensed liquid out of the condenser and runs it through an additional coil to get a few degrees of subcooling. I usually see these on ***** systems that are DX and therefore rely on having high pressure liquid travel a significant distance to the evaporators, to assure it is still all liquid when it gets there.

That said, I usually see a few degrees of subcooling in liquid leaving condensers because they do not drain perfectly. A ***** system equipped with a flooded head pressure control can have a LOT of subcooling at certain times, as the condenser may be half full of liquid during periods of low ambient and/or part load.

Making sure there is enough subcooling to get high pressure liquid where it needs to go is an important part of the designer's job, and one that is often overlooked.

MrFreez717

Every Condenser I have ever worked on was designed to deliver subcooled liquid out
and I have never known a condenser not to be so designed. The level of subcooling
and the quality of the liquid can be debatable but unless there is an issue with the
condenser, a fault, blockage, air/water flow or the landlords dog sleeping by the
condenser to keep warm. Any issue that causes the condenser not to produce liquid
is a fault and should be dealt with.

Rob

.

[NH3LVR]

Rob:
How about Evap Condensers for NH3 that are operating well. I always thought that they would have little to no Sub cooling.

[MrFreez717]

Liquid is not the same as subcooled liquid. Condensers are designed to condense liquid, not subcool it. If condenser surface area is being used for subcooling, then you no longer have enough surface area for the amount of actual condensing the unit was designed for.

As I stated previously, some condensers are built for subcooling, typically in Fre*n systems (not sure why I can't use that word here, but okay). Those employ a separate loop, so that the condensed liquid is drained and then run back through the condenser.

All that said, most condensers do provide a little bit of subcooling, as it is about impossible to drain liquid from the coils as soon as it is condensed. But subcooling is not a design criteria and in fact condensers are designed to avoid it.

As NH3LVR posted, an ammonia evap condenser at full load will have little to no subcooling. But a Fre*n condenser with flooded head pressure control could have 20-30F degrees of subcooling in low ambient conditions.

I believe the OPs question was about flash gas in vertical piping, so we may have strayed a little off topic.

MrFreez717

[RANGER1]

This Trane link explain a lot about liquid lines & other

https://www.tranebelgium.com/files/book-doc/20/en/20.aqerykdx.pdf

[Rob White]

Rob:
How about Evap Condensers for NH3 that are operating well. I always thought that they would have little to no Sub cooling.

Mmmmmm maybe I'm being too simplistic??

I know liquid draining out of the cond at the saturation temp is liquid so
I suppose I must agree with you on that point.

It's just that I was always taught that the lower the liquid temp the better the efficiency
but I know that is not always true in real life :)

Rob

.

[Rob White]

Liquid is not the same as subcooled liquid. Condensers are designed to condense liquid, not subcool it. If condenser surface area is being used for subcooling, then you no longer have enough surface area for the amount of actual condensing the unit was designed for.

As I stated previously, some condensers are built for subcooling, typically in Fre*n systems (not sure why I can't use that word here, but okay). Those employ a separate loop, so that the condensed liquid is drained and then run back through the condenser.

All that said, most condensers do provide a little bit of subcooling, as it is about impossible to drain liquid from the coils as soon as it is condensed. But subcooling is not a design criteria and in fact condensers are designed to avoid it.

As NH3LVR posted, an ammonia evap condenser at full load will have little to no subcooling. But a Fre*n condenser with flooded head pressure control could have 20-30F degrees of subcooling in low ambient conditions.

I believe the OPs question was about flash gas in vertical piping, so we may have strayed a little off topic.

MrFreez717

:)

You have valid points and I can't disagree with you in principle.
The vast majority of condensers I have ever come across deliver subcooled liquid out
and yes there is a section of the condenser that is designed to subcool the liquid.

So maybe I'm being too simplistic in my argument by saying that the condenser delivers subcooled liquid
because I am talking about condensers that have subcooled sections built in.....

Apart from a very bespoke design I can't see why a condenser would not deliver liquid that is colder than the saturation temp??

Rob

.

[Rob White]

As NH3LVR posted, an ammonia evap condenser at full load will have little to no subcooling. But a Fre*n condenser with flooded head pressure control could have 20-30F degrees of subcooling in low ambient conditions.

I believe the OPs question was about flash gas in vertical piping, so we may have strayed a little off topic.

MrFreez717

Your correct so I'll shut up :):D

Rob

.

[MrFreez717]

:)

You have valid points and I can't disagree with you in principle.
The vast majority of condensers I have ever come across deliver subcooled liquid out
and yes there is a section of the condenser that is designed to subcool the liquid.

So maybe I'm being too simplistic in my argument by saying that the condenser delivers subcooled liquid
because I am talking about condensers that have subcooled sections built in.....

Apart from a very bespoke design I can't see why a condenser would not deliver liquid that is colder than the saturation temp??

Rob

.

I think we have different experiences - guessing yours is mostly with Fre*n systems that have a subcooling loop built into the condenser. Ammonia systems are not usually equipped like that.

Subcooling can be very beneficial for some systems, but the condenser may not be the most efficient way to accomplish it. Normally a low temperature system will use a separate heat exchanger connected to the intermediate suction level to add significant subcooling to the liquid being supplied to the low side evaporators.

MrFreez717

[oplafki]

This Trane link explain a lot about liquid lines & other

https://www.tranebelgium.com/files/book-doc/20/en/20.aqerykdx.pdf

Thanks RANGER1, !!
I think that it's the easiest book to understand i had seen, I loved it.

[oplafki]

But if the liquid is subcooled as it comes out of the condenser
why would there be saturated gas in the liquid?

If you have saturated liquid / gas in your liquid line your condenser is for
whatever reason not doing its job.

Rob

.

is it right that you wanted to say that it is unnecessary worry if there is enough subcooling in system?

[oplafki]

I believe the OPs question was about flash gas in vertical piping, so we may have strayed a little off topic.

MrFreez717

yes... that is exactely what i want to know.

[RANGER1]

Thanks RANGER1, !!
I think that it's the easiest book to understand i had seen, I loved it.

Oplafki, glad someone read it, I agree good easy to understand