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Thread: suction line velocity
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25-07-2008, 07:59 AM #1
suction line velocity
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
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25-07-2008, 04:17 PM #2
Re: suction line velocity
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.
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25-07-2008, 04:36 PM #3
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25-07-2008, 04:51 PM #4
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25-07-2008, 04:57 PM #5
Re: suction line velocity
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.
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25-07-2008, 05:05 PM #6
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25-07-2008, 08:20 PM #7
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25-07-2008, 08:45 PM #8
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25-07-2008, 09:58 PM #9
Re: suction line velocity
Originally Posted by Kevin Yeo
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.If all else fails, ask for help.
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25-07-2008, 10:12 PM #10
Re: suction line velocity
You see NoNickName yet another ... in another direction!
Hi Big Brother, I erased the catalog pictures should you need them I'll re-post. Little thing called intellectual property infringement here.
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22-08-2008, 11:34 AM #11
Re: suction line velocity
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
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23-08-2008, 04:03 AM #12
Re: suction line velocity
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....and she said "give it to me you big fridgie"
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23-08-2008, 04:54 AM #13
Re: suction line velocity
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.
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23-08-2008, 08:05 AM #14
Re: suction line velocity
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.
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24-08-2008, 02:25 AM #15
Re: suction line velocity
Yupz, thanks a lot for your advise....
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24-08-2008, 01:25 PM #16
Re: suction line velocity
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.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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24-08-2008, 01:26 PM #17
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