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31-01-2010, 01:44 PM #1
Why are discharge lines smaller than suction lines?
Just read a post by MarkII and i got to wondering again
Why are they sized differently when they have the same volume of refrigerant going down them?
Cheers Jon
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31-01-2010, 02:26 PM #2
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31-01-2010, 02:40 PM #3
Re: Why are discharge lines smaller than suction lines?
Can ya not remember Boyles law i.e. Pressure and volume being inversley proportional. Its not the same volume in the discharge line compared to the suction as it is compressed hence it has a smaller volume but the same mass.
it being the refrigerant
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31-01-2010, 02:53 PM #4
Re: Why are discharge lines smaller than suction lines?
I agree with VRV 3 in relation to the higher velocities iff the discharge pipe was too large the refrigerant would change state before reaching the condenser.
The suction line will contain vapour which will require a larger CSA to ensure that there is adequate flow rate back to the compressors at lower pressures.
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31-01-2010, 03:33 PM #5
Re: Why are discharge lines smaller than suction lines?
There are loads of very knowlegable people on this forum - my guess is that Taz or gary would be some of the best to further explain
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31-01-2010, 03:36 PM #6
Re: Why are discharge lines smaller than suction lines?
In any system, what goes out in the discharge, comes back in the suction.
Vapour under high pressure require less space and when the pressure drops the vapour expand and occupy larger space.
Just as Quality wrote.
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31-01-2010, 03:45 PM #7
Re: Why are discharge lines smaller than suction lines?
Good explanation that one looking from the opposite side. as you said what goes in only comes out.
another thought:- imagine if refrigerant entered compressor through a 1/2" pipe was compressed then was discharged through a 1/2" pipe it would only expand into the same pressure and volume as it entered the compressor, making the comprssor a vapour pump
the smaller discharge volume/pipe simply maintains the smaller volume created by the compressor
hope this helpsLast edited by Quality; 31-01-2010 at 03:49 PM. Reason: addition
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31-01-2010, 04:54 PM #8
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31-01-2010, 06:13 PM #9
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Re: Why are discharge lines smaller than suction lines?
I would say because of different recommended speeds for refrigerant in these pipes.
Suction pipe recommended speed is about 10m/s.
Discharge pipe recommended sped is about 12m/s. Therefore, discharge pipe could be smaller.
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31-01-2010, 07:09 PM #10
Re: Why are discharge lines smaller than suction lines?
Mass flow is the same, volume is reduced.
At higher pressure (more dense/ heavy) you have less pressure drop through your pipe.
Also how the oil is carried is different.
Think of a sponge full of water (low pressure) now squeeze the sponge (high pressure) some water leaves the sponge. In our case the sponge is refrigerant and the water is oil. We want the free oil to travel with the refrigerant, this is done with velocity (speed), hence higher velocity for discharge.
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31-01-2010, 07:46 PM #11
Re: Why are discharge lines smaller than suction lines?
I don't think the density would change, i think this would be linked to the discharge pressure and in turn this would be a function of the condensing temperature.
I think it would just travel slower along the pipe.
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31-01-2010, 08:10 PM #12
Re: Why are discharge lines smaller than suction lines?
The mass flow is the same in the suction and discharge, in the suction you have low density, in the discharge you have higher density. What density does relate directly to your working pressures and temperatures.
If you have the same sized pipe at high density it will have a slower velocity (slow speed), what will happen is that your oil will drop out of the refrigerant. Think of an oil seperator (more to oil seps than just this), a small pipe goes into a big pipe, the velocity slows to a snails pace a large % oil falls to the bottom, the refrigerant then goes back into a small pipe and the velocity increases again, carry what ever oil is left to the next part of the system
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31-01-2010, 08:46 PM #13
Re: Why are discharge lines smaller than suction lines?
Just to confuse things further it's also possible to have the discharge line larger than the suction. I.E. Condenserless chiller with long pipe run/high head/various bends.
See below copy paste info
Recommended gas line velocities:
- Suction line: 4.5 to 20 m/s
- Discharge line: 10 to 18 m/s
Recommended liquid line velocities:
- from condenser to receiver: < 0.5 m/s
- from receiver to evaporator: < 1.5 m/s
comfort air-conditioning or other applications where the operating time is limited
to 4.000 hours per year or less and where low initial cost of the system is more
important than a low operating cost.
Industrial or commercial refrigeration applications, where the equipment runs
most of the time, should be designed with low refrigerant velocities in order to
obtain the most efficient compressor performance and the lowest equipment
operating cost.
Liquid lines from condenser to receiver should be designed for refrigerant
velocities of 0.5 m/s or less to ensure positive gravity flow without incurring back
up of liquid flow.
Liquid lines from receiver to evaporator should be designed to maintain the
refrigerant velocities below 1.5 m/s, to avoid liquid hammering when solenoids or
other electrically operated valves are used.
There are two methods to determine the refrigerant line sizing: by using the pipe
graphs or by using the selection tables. Both methods are based on the Darcy-
Weisbach formula.
Pressure loss in hot-gas lines increases the required compressor power per unit
of refrigeration and decreases the compressor capacity.
Pressure drop is kept to a minimum by generously sizing the lines for low friction
losses, but still maintaining refrigerant line velocities to entrain and carry oil along
at all conditions.
Pressure drop is normally designed not to exceed the equivalent of a 1 Kelvin
change in saturation temperature. Recommended sizing tables are based on a
0.02 K / m change in saturation temperature.Last edited by VRVIII; 31-01-2010 at 09:35 PM.
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31-01-2010, 09:08 PM #14
Re: Why are discharge lines smaller than suction lines?
P1V1=P2V2
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31-01-2010, 09:27 PM #15
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31-01-2010, 09:57 PM #16
Re: Why are discharge lines smaller than suction lines?
VRV, I presume you mean a chiller with a remote condenser, never seen a chiller without a condenser???
your determining factor is velocity if oil return is required (not so true with ammonia) So for very long long pipe runs to the condenser, you either have to size your compressor to match your required pressure drops (thus change in duty) or increase the size of the condenser, to allow for the smaller TD required to condense at the lower pressure, also allowing to resize your expansion device for the smaller pressure differential. i would also suggest if you had such a long discharge line, you would also have a long liquid line This would also cause big problems if not handled correctly
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31-01-2010, 10:20 PM #17
Re: Why are discharge lines smaller than suction lines?
Hi Mad fridgie,
Yes I was referring to a remote condenser but most manufactures class this as a condenserless model as they are selling you a chiller without a condenser.
The information is relating to a 134A chiller and of course liquid line is also taken in to account.
The pressure drop should be limited to prevent the formation of flashing gas in
the liquid line or insufficient liquid pressure at the evaporator. Liquid lines are
normally designed so that the pressure drop in the liquid line is not greater than
that corresponding to about a 0.5 to 1 Kelvin change in saturated temperature.
Liquid sub-cooling is the only method of overcoming the liquid line pressure loss
to guarantee liquid at the expansion device in the evaporator. If the sub-cooling is
insufficient, flashing will occur within the liquid line and degrade the efficiency of
the system. The amount of sub-cooling required can be calculated.
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31-01-2010, 11:01 PM #18
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01-02-2010, 02:46 AM #19
Re: Why are discharge lines smaller than suction lines?
As VRVIII correctly stated, each line in the system is sized based on:
1. Maximum acceptable pressure-drop (either in kPa, or equivalent K);
2. Minimum oil-carrying velocity.
Two property references (R-134a):
1. @ low Tc: d,d/d,s = 36.1955/18.7399 = 1.931
2. @ high Tc: d,d/d,s = 79.4757/18.7399 = 4.241
Mass flowrate: m' = d*Ac*v
So : Ac = m'/(d*v)
Let :
Ac,s = suction line area [m2]
Ac,d = suction line area [m2]
(Ac,d/Ac,s) = [m',d/(d,d*v,d)]/[m',s/(d,s*v,s)]
= (d,s*v,s)/(d,d*v,d) with same m'
= (d,s/d,d)*(v,s/v,d)
Now, select the (d,s/d,d)=1/(d,d/d,s) ratio required, & tolerable (v,d/v,s) [ASHRAE] & you arrive at the ratio of flow areas.
Ac = (pi/4)*Di^2
Where : Di = pipe ID
Finally,
(Di,d/Di,s) = sqrt[(d,s/d,d)*(v,s/v,d)]
You end up with the ratio of pipe inner diameters.
Please check my typing - done on the fly, but the logic is basic.Last edited by desA; 01-02-2010 at 03:04 AM. Reason: Correction of typo's... hope it's correct now
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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01-02-2010, 03:05 AM #20
Re: Why are discharge lines smaller than suction lines?
I put my hand up in disgust!
I use a software and a gut feeeling (which you need when dealing with software)
can not add 2 + 2 together without using a calculator
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01-02-2010, 03:09 AM #21
Re: Why are discharge lines smaller than suction lines?
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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01-02-2010, 10:48 AM #22
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01-02-2010, 07:08 PM #23
Re: Why are discharge lines smaller than suction lines?
On a purely simple level.
Assuming that the liquid is water and say one litre of water occupies one metre of 25mm diameter pipe.
If you now heat the water enough to turn it into steam consider the size of pipe that needs to accommodate that volume of steam.Brian - Newton Abbot, Devon, UK
Retired March 2015
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01-02-2010, 11:16 PM #24
Re: Why are discharge lines smaller than suction lines?
Why are they sized differently when they have the same volume of refrigerant going down them? To keep you from piping it wrong way!
I did here of a lad who did just that if you can believe
I was told by the tech who found it the guy used all the right bushings wow
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10-05-2010, 08:16 AM #25
Re: Why are discharge lines smaller than suction lines?
In addition vapour in suction pipe has high density in discharge lower. For good oil-carrying velosity in discharge pipe must be higher.
Pressure drop is normally designed not to exceed the equivalent of a 1 Kelvin
change in saturation temperature.In some places will have to think ...
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10-05-2010, 09:22 AM #26
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10-05-2010, 11:57 AM #27
Re: Why are discharge lines smaller than suction lines?
Hi!
About liquid pipe from receiver have some questions... In many literature sources liquid velocity must be 0.3-1.2 m/s.
Cuold anybody tell physical side of minimum and maximum velocities in liquid pipe?
I have view that liquid is sudcooling only after receiver in pipe, usually pipes situated in room with temperature of air smaller than temperature of pipe and liquid is subcooling. And velocity of 1.2 m/s of liqid don't give to appear fleshing in gorizontal pipe.
In receiver vapor and liquid be in equilibrium. What subcolling is can be if vapor and liquid be in equilibrium?
May be I don't understend something, could somebody explain?In some places will have to think ...
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10-05-2010, 12:16 PM #28
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Re: Why are discharge lines smaller than suction lines?
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10-05-2010, 07:02 PM #29
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11-05-2010, 04:04 AM #30
Re: Why are discharge lines smaller than suction lines?
I read a member say that the smaller discharge pipe size on a compressor is to maintain the higher gas pressure produced by the compressor. "if the lines were 1/2 " in and out the compressor would be a vapor pump".
In my mind I would think that if the discharge line were the same size as the suction line size, it would not effect the discharge pressure, but would only effect the discharge velocity. Right?
JP Newbie
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11-05-2010, 07:53 AM #31
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Re: Why are discharge lines smaller than suction lines?
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11-05-2010, 08:05 AM #32
Re: Why are discharge lines smaller than suction lines?
Hi there,
One thing I read above is not correct.
In refrigeration system compressor volume flow is constant (m³/hr). Compressor is running at the same speed and the cylinder capacity is the same. BUT BUT the mass flow is different according to the refrigerant density.
mass flow (kg/s) = Density (kg/m³) x Volume flow(m³/s)
In my opinion desA's formula and reasoning is correct.
CheersEven Einstein Asked Questions
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11-05-2010, 09:36 AM #33
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Re: Why are discharge lines smaller than suction lines?
Isn't that a fact that, when compressor works in same operating conditions, he moves same mass of refrigerant in time. If compressor is pump, than that same mass circulate all over the circuit. How pressure, temperature and volume changes thru circuit, same mass has different density.
Only when conditions on compressor are changed we have different mass in circulation.
That is how I see things.
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11-05-2010, 09:46 AM #34
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Re: Why are discharge lines smaller than suction lines?
Minimum velocity is all about oil return. We must maintain sufficient velocity in pipes in order to ensure oil return to compressor. Certain minimum velocities for certain refrigerants and oils are for horizontal pipes and certain are for vertical pipes.
I don't know physics of that. There are many practical guides in piping books and some software.
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11-05-2010, 09:57 AM #35
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11-05-2010, 11:04 AM #36
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11-05-2010, 11:28 AM #37
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