Why are discharge lines smaller than suction lines?

[monkey spanners]

Just read a post by MarkII and i got to wondering again :eek::D

Why are they sized differently when they have the same volume of refrigerant going down them?

Cheers Jon :)

[VRVIII]

Just read a post by MarkII and i got to wondering again :eek::D

Why are they sized differently when they have the same volume of refrigerant going down them?

Cheers Jon :)
Hi Jon,

I’d say this is due to higher pressure in discharge line = higher velocity, so basically the gas is moving faster through the smaller discharge pipe :D:D:D

[Quality]

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:)

[brunstar]

Hi Jon,

I’d say this is due to higher pressure in discharge line = higher velocity, so basically the gas is moving faster through the smaller discharge pipe :D:D:D

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.

[Quality]

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:)

[chemi-cool]

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.

[Quality]

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.

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 helps

[VRVIII]

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

But the discharge pressure is aprox 4 x higher than suction pressure, surely this would still take up a smaller volume even though both pipes are 1/2"???

[nike123]

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.

[mad fridgie]

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.

[monkey spanners]

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.

[mad fridgie]

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.
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

[VRVIII]

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 :o




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
Higher gas velocities are sometimes found in relatively short suction lines on
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.

[multisync]

P1V1=P2V2 :cool:

[VRVIII]

P1V1=P2V2 :cool:

No you've got it all wrong multi,
Head loss: Δh = λ (l / dh) (v2 / g 2)
Friction loss: 1 / λ1/2 = -2 log [ 2.51 / (Re λ1/2) + (k / dh) / 3.72 ]
:p:D:D

[mad fridgie]

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

[VRVIII]

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
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.

[mad fridgie]

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.

Hi VRV, yes just different termnology around the world.
You must be getting this info out of a good book, or you are in the wrong Job, you should writing manuals 'training' ect :D

[desA]

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.

[mad fridgie]

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

[desA]

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

Stick the equations into a spreadsheet, plug in the property values & you'll have the line sizes. It's easy, really - for single-phase fluids.

2-phase fluids become a little more interesting... :)

[multisync]

No you've got it all wrong multi,
Head loss: Δh = λ (l / dh) (v2 / g 2)
Friction loss: 1 / λ1/2 = -2 log [ 2.51 / (Re λ1/2) + (k / dh) / 3.72 ]
:p:D:D

No surprise me being a split basher 'n all..:)

[Brian_UK]

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.

[tonyhavcr]

Why are they sized differently when they have the same volume of refrigerant going down them? To keep you from piping it wrong way! :o

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:rolleyes:

[Aik]

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.
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.
For every 30 m of pipe.

[nike123]

Why are they sized differently when they have the same volume of refrigerant going down them? To keep you from piping it wrong way! :o

It is not same volume. It is same mass flow!
That doesn't mean same.

[Aik]

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.


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?

[nike123]

http://hvac-talk.com/vbb/showthread.php?t=56495&page=2

[Aik]

http://hvac-talk.com/vbb/showthread.php?t=56495&page=2
Thanks, nike!
What about:

Cuold anybody tell physical side of minimum and maximum velocities in liquid pipe?

Minimum velocity is more interest.

[Zpoint]

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

[nike123]

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?



Discharge pressure is condensation pressure + pressure loss in pipes and parts from condenser to compressor.
Condensation pressure depend on saturation temperature of refrigerant in condenser, condenser TD and condenser air inlet temperature.

[lana]

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.

Cheers

[nike123]

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.

[nike123]

Thanks, nike!
What about:

Minimum velocity is more interest.

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.

[Aik]

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.
I mean liquid pipe, not vapor.

[nike123]

I mean liquid pipe, not vapor.


In liquid pipe your only concern is noise and pressure drop.

[Aik]

In liquid pipe your only concern is noise and pressure drop.
I also think so, and suppose that minimum velocity in liquid pipe depends by economical considerations.