Question about liquid line sizing

[kengineering]

If an existing system has a 1/4 inch connection from the liquid reciever going to an expansion valve that has a 1/4 inch liquid line connection is there any benefit to using a larger liquid line like 3/8 inch? I can't imagine why it would be but my boss insists that it is a good idea. I say if the valve is 3/8 inch then maybe ,,, Ken

[US Iceman]

What matters is the pressure loss in the piping. NEVER size piping based on the connection/valve sizes.

A larger line size will have a lower pressure loss for a specific mass flow rate, than a smaller pipe will have. If there is enough pressure loss in the piping the liquid will start to flash off. This means you will have less liquid to feed the evaporator and in all likelihood less cooling capacity (because there is not enough liquid).

I agree with your boss, with one condition. The 3/8" pipe is properly sized for the intended application and NOT guessed at. That's why they call it engineering and not guess work!

[desA]

^ I agree with US Iceman's post.

The line sizes are based on acceptable flow velocity & pressure drop. Often, valves & other equipment have specific reasons for sizing their inlet/outlet connections. Generally it has to do with making sure that the fluid enters the equipment evenly.

In other industries, control valves are almost always smaller than the line they control.

[D.D.KORANNE]

Sub-cooling on the existing size might help reduce flashing and yet achieve the same result without increasing liquid line size ; if possible

[kengineering]

I agree with your boss, with one condition. The 3/8" pipe is properly sized for the intended application and NOT guessed at. That's why they call it engineering and not guess work!

OK since this is a remote job I could see how pressure drop might play a part. However we do not know anything about the installation. I believe 3/8 inch pipe was chosen because it is the next size in stock. Not that any look-up on a chart was done. BUt I can see now how the larger tube will hold more liquid against the TEV for a wide open big load situation.
Thanks, Ken

[US Iceman]

BUt I can see now how the larger tube will hold more liquid against the TEV for a wide open big load situation.

That has nothing to do with it Ken.

If the liquid line of any size is too long or too small the pressure loss can affect the refrigerant causing it to flash off. If you do not know how long the liquid line will be, this could be a problem installation someone has to work on under the warranty.

A better way of attacking this is say: the liquid line sizes provided by us (the manufacturer) are intended to be no longer than XX feet. If the evaporator is remotely located any distance farther away from the condensing unit, then the liquid line should be XX inches in diameter (and be prepared to tell them what size the liquid line should be). This also applies to the suction line diameter.

[Magoo]

I agree with US Iceman,
The longer the liquid line the greater the losses, and flash gas if undersized. TX vavs are spec'd on specific pressure drop across vav, flash gas will cause TXV to hunt. Same losses apply to suction line sizing. The next problem that everyone forgets with long pipe runs, is liquid receiver volume, and pump down ability .
magoo

[lowcool]

i also agree with iceman,oversized liquid lines also create problems but i cant remember the specifics,so if you could please reply to this iceman being the general nice guy i will be forever greatful

[kengineering]

If you do not know how long the liquid line will be, this could be a problem installation someone has to work on under the warranty.

Iceman,
I am not personally responsible for the warranty issues just a builder/mechanic thats looking for knowlege. But I see your point. Most remote stuff I build is to specs for someone who has done the engineering. My company is not all that concerned with us builders knowing anything about flashgas or any others technical things. But very concerned that it is built as fast as possible and shipped. That's not to say that quality suffers. We use top of the line materials and parts. Their position is that they are not a school and we are just workers/ assemblers.
I really wished I worked with someone like yourself , or the many other great minds on this forum, but alas my supervisor has about the same level of technical knowlege as myself and it doesn't lend itself well to learning. These days this forum is my school and it's members are my mentors. Thanks to all for your detailed info...Ken

[US Iceman]

...The longer the liquid line the greater the losses, and flash gas if undersized. TX vavs are spec'd on specific pressure drop across vav, flash gas will cause TXV to hunt. Same losses apply to suction line sizing. The next problem that everyone forgets with long pipe runs, is liquid receiver volume, and pump down ability .

All valid points magoo. If it's not one thing it will be something else!

[US Iceman]

Ken,

One of the points I always try to make with people is this: Any refrigeration system can be designed for zero faults, if someone is willing to enable this process.

Not to long ago, the manufacturers made unitized equipment you could plug-in and walk away from. And...they would last for 40 years or more.

In today's market everyone is trying to save money and push the product out the door. If the product last long enough now to exceed the warranty they have succeeded.

The owners are the ones who inherit the design effort expended by the manufacturers.

One of the items I started when I worked for a manufacturer was: tell the customer or installing contractor what they had to do to properly install the equipment. If they do not follow your instructions then they own the problems.

If they do follow your directions (and the directions are appropriate for the task) then the system should start-up OK and last for a very long time.

I also realize this type of emphasis/thinking starts at the top and is difficult to push uphill from the bottom. Almost always it falls into the category of: It's not my idea and there is always someone else to blame.

The end result is the equipment owner looses...

rant off......

[US Iceman]

Generally, it's safe to say the liquid can be sized for a larger pipe diameter. Not too much will go wrong if you do.

If the liquid line flows in a vertical direction upwards you need to evaluate how much subcooling you have. Subcooling overcomes pressure losses and vertical liquid lines are critical for this.

A second point on larger liquid lines is: the larger the pipe is the more heat it can absorb from the surrounding environment. Remember what happens when liquid refrigerant absorbs heat? It starts to flash off!

Subcooling and pipe insulation (on liquid lines) are your friend!

[lowcool]

thought it was a flash off situation,had one supermarket with oversized lines and did have problems,had to go back to the nitty gritty to find out why,thanks for the refresher iceman

[Magoo]

Hi US Iceman, we concurr with reagrds to fault free maintenance only systems.
Here we have to contend with bean counters that totally influence corporate decission making.
I can design a total system, fully balanced and reliable, added maintenance program etc, etc,. The bean counter flicks through all contract options and states the cheapest, total disregard for design and efficiency. One up costs is the prime factor.
Fortunately I have a couple of logical clients that bar the bean counter from technical design meetings. They come back to me with on-going plant upgrades, with out others envolved. A solid client trust.Everyone is happy

[US Iceman]

They come back to me with on-going plant upgrades, with out others envolved. A solid client trust.Everyone is happy

I love it! A win-win situation for everyone involved (except the bean counters).

[kengineering]

Subcooling and pipe insulation (on liquid lines) are your friend

I have seen the liquid and suction lines tied together as a heat exchanger. Is this a good thing if subcooling is too low? Ken

[kengineering]

Does this help? If so how much line should be used? I imagine there can be too much of a good thing.Ken

[FreezerGeezer]

Ken, it's very common practice in supermarket refrigeration to braze the liquid & suction together within the cabinet for cooling the liquid line (& I was taught also ensuring any entrained liquid droplets in the suction have an extra chance to boil off). It does help, but how much I'll admit I don't know.