Pipe diameter, real life and theory

[gwm121]

Id like to understand "pipe size"
Modern split sytems we use generally have only one recommended size liquid/suction pipes, typically 3/8 5/8. and have eiter fixed capillary, or electronic exp valve. They have a maximum recommended pipe run.

When we come up against replacing am old system because the pipe runs hidden through the building, what is the effect of installing onto one or two size up liquid, or one or two size up suction line.

If we are going to instal a new system and the pipe length is significantly longer (50% and often not much height difference) than the manufacurers recommended, is it am advantage to increase the size of the liquid line.

In practice we have done this over the years and find that the systems give no trouble, however the manufacurers claim the compressor could either be starved of refridgerent or flooded, and destroyed. Id like to understand the theory and real life aspects.

[mad fridgie]

How to cover this in afew words Hmmm.

Firstly velocity, you need a certain velocity to ensure oil return.(a bigger pipe slows the velocity "gas speed")
refrigerant charge these are +/- a bit critical charge, so incorrect pipesizing ****s up the charge.
Incorrect charge can cause floodback or starving (overheating the compressor)
AC do not tend to have some of the mechanical niceities that refrig systems have?

[Thermatech]

I was looking at this with an engineer a few weeks ago.

Take a japan manufactured split system heat pump size 100 on R22 from the 1990's. It typically had 3/8L & 3/4S with max interconnect pipe lenght of 50 meters.

The current R410a size 100 will have 3/8L & 5/8S with up to 75m interconnect pipe lenght.

So for example you can use a Mr Slim power inverter system with 3/8L & 3/4S but pipe length is limited to 50m for this pipe size & you have to set dip sw at outdoor unit so the system can trim operation for larger pipe size.
We do not know exactly what difference this dip sw setting makes to the system operation as this is control stratagy trade secret. But I suspect the system will change the target SH & SC & expansion valve control stratagy for optimum refrigerant flow control for the larger pipe size.

Ofcourse you need to look carefully at other factors like the copper tube wall size to make sure that the tube is ok for the higher pressure of R410a but for most applications this type of new split system should make a good retro fit replacement with the max pipe lenght being the same as the R22 system at 50m.

The Mr Slim system can clean up the existing tube with the special 'replace filter' being used for a 2hr test run with any contamination being removed from the system by the filter.

If you look in the technical manual you will see that the cooling capacity drops with longer pipe lengths & you need to make a correction to the pipe length to allow for the number of bends in the pipe because to more bends the higher the pressure drop / reduced refrigerant flow / reduced cooling.
Typically at max pipe length the system will have about 80% cooling capacity compaired to 100% at 5 meters.

The trim charge calculation is important & with very long pipe length the system will tend to run at low suction pressure due to the pressure drop & so the compressor will run hot which will reduce its life.
But as the system looks SOG some engineer may add refrigerant & overcharge. So getting the refrigerant charge at optimum is not so easy as the pipe length gets very long.

[multisync]

How to cover this in afew words Hmmm.

Firstly velocity, you need a certain velocity to ensure oil return.(a bigger pipe slows the velocity "gas speed")
refrigerant charge these are +/- a bit critical charge, so incorrect pipesizing ****s up the charge.
Incorrect charge can cause floodback or starving (overheating the compressor)
AC do not tend to have some of the mechanical niceities that refrig systems have?


If these are charge critical why do they come precharged up to 30m? That alone tells you they are not that critical

[Thermatech]

On the older R22 split systems some were pre charged up to 50m. To accomodate the refrigerant not needed for shorter pipe runs the outdoor units had large suction accumulators which store the excess refrigerant.

On some R410a split systems there is now a liquid line reciever instead.

But in both cases the service manual will show how to calculate the refrigerant charge for exact pipe length.
So if you have less than the precharged pipe length & you carry out service work with full refigerant charge recovery then when recharging you can calculate the exact charge for the pipe lenght.

Also if you exceed the precharged pipe length at installation then you can calculate the extra trim charge.

The manufacturers have to make a ballance between cost of supplying expensive refrigerant with the outdoor unit & making commissioning of the system easy for the installer by making it precharged.

The cost of manufacture large suction accumulator or liquid receiver is an item the budget cost manufactures remove from the system to make cheaper. In this case the exact refrigerant charge is critical as there is no flexible storage of refrigerant at the outdoor unit.

[Contactor]

Because it's a function of many different variables the optimum compression ratio is merely a design target and is rarely achieved once the system is in service. In some circumstances, therefore, you may be improving conditions at the compressor by changing the pipe sizes.

[mad fridgie]

If these are charge critical why do they come precharged up to 30m? That alone tells you they are not that critical
"+/- a bit is critical charge" I think is what I said.
What should also be noticed with these systems is the reqirement for minimum pipe lengths. I think thermatech covered this quite well:D So as the maximum lengths have increase so have the minimum lengths. "Critical Charge"
The minimum length came about due to high compressor failure during a 5 period in Asian countries. Many splits, were mounted back to back, with the outdoor unit underslung from the balcony above. The tail from the indoor units could be connected straight to the outdoor unit.
What was happening is floodback, not enough to smash the compression parts, but more to do with diluting the oil, cause bearing failure.

[gwm121]

Thank you for all these replies, is it possible to say ok or not ok on for example... hidden pipe in building, old r22 system, 3/8 and 1/2 say 15mtr, new 410 system 1/4 1/2. ((obviously wall thickness and cleaning pipe has to be done, efficiency may reduce,but im interested in an opinion if the change of pipe size will destroy the system in time)) The second opinion im intereseted in, was if installing on a longer than recommended run, would increasing liquid line a size be beneficial. It seems strange that the manufacurers are so adamant the system will be destroyed quickly, as ive had to instal on wrong pipe sizes on several occaions an the systems have been running for years.....

[Peter_1]

Thank you for all these replies, is it possible to say ok or not ok on for example... hidden pipe in building, old r22 system, 3/8 and 1/2 say 15mtr, new 410 system 1/4 1/2. ((obviously wall thickness and cleaning pipe has to be done, efficiency may reduce,but im interested in an opinion if the change of pipe size will destroy the system in time))

No, only efficiency will be less, what could destroy the system, especially if the sizes is one bigger then usual.
AC lines already have high to very high speeds to ensure oil return but going from 3/8 to 1/2 will not harm.
You apparently have additionally your own practical experience who tells you that it will not harm.


The second opinion im intereseted in, was if installing on a longer than recommended run, would increasing liquid line a size be beneficial. It seems strange that the manufacurers are so adamant the system will be destroyed quickly, as ive had to instal on wrong pipe sizes on several occaions an the systems have been running for years.....

I should not increase liquid line size but suction line size taking care that you follow the basic rules for lines a much as possible (suction inclined towards compressors, U-traps when going more then 4 m upwards, qualitative isolation..)