suction line velocity

[Kevin Yeo]

Hi all,
Hope everyone is fine.May I ask for a given suction line diameter say 54mm, what affects the suction line velocity? Is it the compressor suction cfm or the mass flow through txv or exv? When the 54mm main suction line is connected to say 10 no of evaporators, how do we compute the total suction mass flow and subsequently the suction velocity.
Rgds,
Kevin

[GXMPLX]

You have no way of estimating INSTANTANEOULS mass flow through a suction because the fluid is compressible and you have all kind's of wave like effects BOTH from the compressor and from the t/e-xv.

Fortunately for all, the flow in the high side of the system is incompressible and therefore the AVERAGE INSTANTANEOUS flow trough any point in the system is CONSTANT.

Notice I mentioned flow not velocity.

So the average mass velocity will be the average mass flow divided by the area of your pipe, should you use consistent units.

The average flow is given by the compressor given working conditions of the system.

[NoNickName]

You have no way of estimating INSTANTANEOULS mass flow through a suction because the fluid is compressible and you have all kind's of wave like effects BOTH from the compressor and from the t/e-xv.

but in engineering, most of the time the design is done on mean values.



So the average mass velocity will be the average mass flow divided by the area of your pipe, should you use consistent units.


And of course it changes everytime one compressor starts or stops as the mass flow changes accordingly.

[GXMPLX]

but in engineering, most of the time the design is done on mean values.



And of course it changes everytime one compressor starts or stops as the mass flow changes accordingly.

Agreed, but I don't find much sense in mentioning compressor and TXV affecting unless he WANTS instantaneous velocity. My interpretation.

[NoNickName]

What he wants us to suggest is how to correctly size the TXV, provided the mass flow through it changes on a range of 1:10. What instead should be done is to have as many evaporators as many txv's. This way, everytime one load goes off, the associated txv and liquid solenoid valve close.
The user shall note that when the flow rate is 1/10 of the design rate, the oil return is insufficient unless double or triple risers are built.

[GXMPLX]

What he wants us to suggest is how to correctly size the TXV, provided the mass flow through it changes on a range of 1:10. What instead should be done is to have as many evaporators as many txv's. This way, everytime one load goes off, the associated txv and liquid solenoid valve close.

The user shall note that when the flow rate is 1/10 of the design rate, the oil return is insufficient unless double or triple risers are built.

Really??!!


..., what affects the suction line velocity...


Masaaannnn my English must be getting worse by the minute!!

Or your ESP is much better than mine!!

:off topic:
By the way Kevin you don't happen to know a Maxwell (and exclude any media former tycoons plse).

[NoNickName]

Really??!!


Well, no.. :o at least that was what I understood out of it.

[GXMPLX]

Well, no.. :o at least that was what I understood out of it.

Do not worry ... my interpretation may be way off ... in another direction!

[US Iceman]

Is it the compressor suction cfm or the mass flow through txv or exv?


Well, let's see. You have volume flow or mass flow.

Volume flow = mass flow X the specific volume of the gas

If the compressor has no capacity control then it continues to pump the same CFM. If multiple evaporators are used then the mass flow through each is determined by the specific evaporator load. The total mass flow of the evaporators = the compressor suction mass flow. Another way of looking at this is; the compressor volume flow = the total volume flow of all evaporators connected to this compressor.

If you turn off a liquid line solenoid valve to one of the evaporators, the mass and volume flow from that evaporator = zero. That simply means the mass flow going to the compressor will decrease by the same amount.

[GXMPLX]

You see NoNickName yet another ... in another direction!

:off topic:
Hi Big Brother, I erased the catalog pictures should you need them I'll re-post. Little thing called intellectual property infringement here.

[Kevin Yeo]

Dear all,

Thanks for your reply. I'm asking this question as I'm worried about over design of suction line that will cause problem to oil return even though all conventional pre-cautions such as U or N trap is used every 15ft elevated distance and slopes towards compressor.

Compressor loading/ unloading is governed by a pressure tranducer that feedback to compressor manager. So the loading / unloading is only controlled by SST temperature / pressure transmitted via a tranducer. On the other hand, evaporators mechanical TXV or EXV is only controlled by thermostat temperature. Assuming 10 evaporators is linked through a common suction pipes, 7 evaporators achieve temperature and closes its EXV or solenoid valves, doesn't the mass flow through the pipe reduces and oil at the moment will have difficulty flowing back to compressors (compressor located higher than evaporators).
So do we size the suction pipe for 10 showcases based on its total mass flow (cooling capacity) or perhaps 70% of its cooling capacity?

Thanks,
Rgds,
Kevin

[750 Valve]

you have to size for 100% of the load, if you are worried about a part load scenario then you may want to look into double risers to get up any vertical lifts and remember to keep a gradient on the pipes for the oil to flow towards the compressors under a lower than ideal velocity scenario


Or you could just run a few separate stubs back to the plantroom, by grouping a few cases together (say 5 and 5) you will limit the chances of going from one end of the spectrum to the other ( full load to no load).

Or just set up the compressor unloading so that the lowest SST cases DO NOT cycle on temperature but rather run on rack suction pressure with case temp setpoint lower than is achievable, this is most common in supermarket applications and allows a more energy efficient operation while ensuring there is always a load on the rack.

[fridgidman]

This is a question that should be answered by someone who majored in physics or engineering that studied thermodynamics, because that's exactly what it is......thermodynamics.

[CHilly1]

Design for 100% of load, Have oil supply sufficient to handle 25% out in the system for low flow periods, keep piping sizes as small as possiable but at their minimum sizes for rated capacity, Trap rises and travel as nessary and pitch all verticle piping toward traps. Alternate defrost times for evaporators never run more than one at a time. And then if you note a problem under low system loads use a third pipe to provide hot gas at teh evaporator outlet through a PEV set at your antipicated low load but this will impact your effeciency.

[Kevin Yeo]

Yupz, thanks a lot for your advise....

[Peter_1]

If you wanna be 100% on the safe side, install 10 risers, one for each evaporator.

It's up to you and based on what information your client gives you to determine what the smallest capacity will be that must be transported along the suction line.

If he runs in the weekend only one evaporator, then your smallest capacity will be that of this one evaporator.

You could also use the system we're using in supermarkets: open all the SV's at once 4 to 6 times a day so that you have max speed in the lines and oil returns easy.
Install then also a suction accumulator so that the returning oil doesn't harm the compressor.

[Peter_1]

...he flow in the high side of the system is incompressible and therefore the AVERAGE INSTANTANEOUS flow trough any point in the system is CONSTANT.

Notice I mentioned flow not velocity.


Mass flow or volume flow? ;)