Problem with condenser pipe length

[Bashir01219]

We are using a AHU from Cooline, Model: TW6P3BD, CFM: 3000, Ext. Pressure: 1000 Pa, Total Pressure: 1539 Pa, Cooling load: 27.23 Kw with Split condensing unit and the specification of the condenser: Medium: R22, Evaporating temp.: 8.33 C, Ambient temp.: 40C, Condensing Temp.: 55.64 C, Overheating: 5 C, Capacity: 29.42 Kw, SHR: 0.7, SL: 1 1/8, LL: 3/4.
My question is what is the maximum copper pipe length that can be used safely for connecting AHU & condenser? Formula to calculate?

[research]

Very difficult question. The Operation and maintenance manual that came with your Condensing unit should have information regarding this.

But really, there is no maximum, there are just issues that need to be dealt with. Supermarkets can separate refrigerated cases from the machine rooms by more than 300 feet and work just fine.
Many ductless splits manufacturers can separate evaporator from condensers by more then 500ft. It just requires that you understand the issues and deal with the problems.

It costs money to deal with the issues and costs more money as the separation distance increases. HOWEVER, it costs even MORE money to ignore the issues pay for the service problems..
Spend your money wisely.

Rule of thumb. the max distance for a split AC system can be separated without adding accessories is about 45 ft.

This is not an answer your question, it is just a compilation of some the issues involved.

issues;
suction line... oil return, temperature penalty, superheat, oil trapping.

As the distance increases, the diameter of the pipe must increase to avoid excessive temperature penalty.

Suction Line. Is there sufficient velocity to return oil with the reduced velocity. (ASHRAE 2010 Refrigeration, Dupont piping expert) among other excellent sources can give you pressure drop and minimum velocities for oil return. Also is additional oil require to WHET the internal surfaces of the tubing etc.

Example a pressure drop equal to 2degrees F temperature penalty will reduce your capacity 3.6% and increase your power 4.8% (ashrae handbook 2006 pg2.2). So.. your capacity drops when there is more pressure drop.. but if you do not have oil return, the compressor will DIE. (typical compressor will pump all the oil out in less than an hour, if there is no oil return, it will die in less than a day).

Liquid Line. As the diameter and length increase. the volume of liquid refrigerant in the system increases. Question? Where will this liquid go when the system is shut off? IF it goes into the evaporator, (no liquid line solenoid) than the question is can the compressor handle this quantity of liquid without slugging when it starts. does it need a suction accumulator? If there is a liquid line solenoid, can the condenser hold this liquid.. does it need a receiver? etc.. etc..

If the condensing unit is lower then the evaporator, gravity will also be an issue. If there is more liquid in the system than the condenser can hold, the system can nuisance trip on the head pressure control (manual reset) or cause other mechanical issues. Internal reliefs opening etc.

If the evaporator is far from the condensing unit... example. 100ft, and the liquid line velocity is 200 ft per minute, then when the system starts, the liquid can take 30 seconds to get to the evaporator... you may require a low pressure bypass timer. just to get the system started.
These are just some of the issues involved.

ALWAYS. Read and follow the instructions and recommendations in the O&M manual.
You may choose to ignore the manufacturers recommendations (after all, it is your unit and you can do what you wish), but be informed about the issues and deal with them in an intelligent manner.

Good luck

[Peter_1]

Research, a very good post, very informative for many readers,... ;-)

[Bashir01219]

Thanks for your details answer, it will help me lot.
As per O&M manual recommended length 25 ft, and if we want to extend then need to use larger size pipe. Our actual requirement is more then 40 feet, so we need to clarify is it will work or not.
Present dia Suction: 1 3/8 discharge: 5/8 what will be our changed dia?

[Bashir01219]

........................... Oh!

[research]

8.5 tons. 40ft
suction. 1 3/8 is adequate. with no unloading.
Liquid 5/8 is also good.
I would recommend a thermostatic expansion valve, because of the extra charge. (you didn't say the type of metering device) and for 40 ft, I would also recommend a liquid line solenoid valve located in the air handler, near the TX valve. Wire it to close when the compressor is shut off.

[Bashir01219]

Thanks ........
System contain: Expansion valve mounted TDEXB 8, quantity: 01, compressor: 01, Fan: 02.
What is the purpose of liquid line solenoid valve?

[research]

This was addressed in the previous post. As your length increases, the refrigerant charge increases. The liquid line solenoid valve is designed to keep this excess liquid out of the low side of the system in the off cycle. So when the compressor starts up it wont slug.