How good is your split system running?

[acnerd]

Hey folks, let's start a discussion on your basic split system.

You all make me jealous with measuring SH or SC on cold rooms and chillers and being able to fine tune them with refrigeration fundamentals, but let's take a look at a basic split unit for comfort cooling:

It has a capillary after the condenser on the outdoor unit that feeds a partially expanded liquid to the indoor unit where it evaporates in the coil, and depending on pipe run, may or may not completely evaporate before returning to the outdoor unit. They chuck in an accumulator to mop up any liquid. They will come pre-charged for pipe runs up to 50m, but you may only have a 10m pipe-run.

Some of the very basic units will give you 1 access point for gauges on the suction valve on the outdoor unit.

A typical check for good running would be air-on/air-off temperatures on the evaporator with a delta-T of 12 to 14 degrees, and ensuring a suction pressure above freezing. (hopefully) In fact, some technical help desks tell you to ignore the pressures and work on temperatures only.
We all know that on critically charged systems, when performance is lacking, (after checking good airflow) we first do a "pressure test" and weigh out the charge and see if anything has been lost.

In general if the evaporator off-coil temperature is below 10 degrees in cooling I am happy, and if the heat rejection outside is in the 30 degree range (fixed speed compressor) I am happy. [Depending of course on ambient]

So the question is, do I need to get more technical with my measurements? Would trying to measure a subcooling value off the outlet of the condenser tell me much? The superheat values can vary so much.

Before answering, please remember I am talking about a BASIC BASIC split system. No EEV's, no inverter driven compressors, but I will allow you a condenser fan speed controller! ;)

[nike123]

Please clarify in what measuring system are your temperatures. I presume SI (Celsius).

[acnerd]

You presume correct!! Wouldn't life be easier if we all stuck to one unit of measure!! ;)

[nike123]

You presume correct!! Wouldn't life be easier if we all stuck to one unit of measure!! ;)
Yes, that is why SI system is introduced and recommended for use in whole world.

[marc5180]

When are you wanting to take these measurements? On a maintenace visit or a breakdown?

If it was just a maintenance visit them usually temperatures alone can tell you if the system is running ok. No need to start taking subcooling and superheat measurements unless the system is first being commissioned or there is a problem with the unit.


The reason most people say work on temperatures rather than pressures are because each refrigerant has different pressures
but the temps needed around the system are always roughly the same no matter what gas it is e.g R22 4bar 0°C but R410 0°C is 7bar

If there was a breakdown for example and you weren't getting cooling at your indoor and both your coils
were clean and airflow was correct then you would have to connect your gauges and begin to check your temps around the system.
Your subcooling is important because by measuring it you can check if you have enough or not enough liquid at your metering device once you are sure that you have the correct amount of liquid at you metering device then you start to look at your superheat.

[acnerd]

Thanks Marc, it would seem that I am doing enough. In reply to your question, it would be on maintenance and breakdown visits. I think temperatures can tell you a lot about system performance.

I've read some of your posts and know your are well qualified, so here's a harder question. 2 identical splits : One has a "liquid line" of 10m, and the other 50m. Assuming they have no drop or lift (ie indoor and outdoor unit at same level), and all other influencing factors are identical, would they have the same subcooling value? The reason I ask is because the metering device is on the outdoor unit, so one line has a 10m run of flashing off liquid, and the other 50m?

[marc5180]

Thanks Marc, it would seem that I am doing enough. In reply to your question, it would be on maintenance and breakdown visits. I think temperatures can tell you a lot about system performance.

I've read some of your posts and know your are well qualified, so here's a harder question. 2 identical splits : One has a "liquid line" of 10m, and the other 50m. Assuming they have no drop or lift (ie indoor and outdoor unit at same level), and all other influencing factors are identical, would they have the same subcooling value? The reason I ask is because the metering device is on the outdoor unit, so one line has a 10m run of flashing off liquid, and the other 50m?

Subcooling is the amount of refrigerant up to the metering device and superheat is the amount of refrigerant after the metering device. (after the evaporator for multisync)

They both should be in the same region of subcooling.
(4-7K).
You say one line is 10m and one is 30m but since this is 'after' the metering device this is no longer measured as 'subcooling' but instead 'superheat'. By measuring the superheat you can start to add refrigerant slowly to bring superheat up to around 7-10k??

If the metering device was on the indoor ( very common with VRF's) then the liquid line would be long as you say and you would need to add refrigerant to increase the subcooling to 4-7K.

[acnerd]

OK. It's what I thought, but always trying to learn something new! Thanks again.:cool:

[marc5180]

No probs...

[multisync]

Subcooling is the amount of refrigerant up to the metering device and superheat is the amount of refrigerant after the metering device.

They both should be in the same region of subcooling.
(4-7K).
You say one line is 10m and one is 30m but since this is 'after' the metering device this is no longer measured as 'subcooling' but instead 'superheat'. By measuring the superheat you can start to add refrigerant slowly to bring superheat up to around 7-10k??

If the metering device was on the indoor ( very common with VRF's) then the liquid line would be long as you say and you would need to add refrigerant to increase the subcooling to 4-7K.


Run that by me again.

Subcooling is a volume and superheat is a mixture of liquid and gas?

[marc5180]

Subcooling is a volume and superheat is a mixture of liquid and gas?

I've just re-read my post and im sure i didn't say that.

Ok your being pedantic. Superheat is measured after the evaporator.

[multisync]

I've just re-read my post and im sure i didn't say that.

Ok your being pedantic. Superheat is measured after the evaporator.

Originally Posted by marc5180 http://www.refrigeration-engineer.com/forums/images/VA_RE/buttons/viewpost.gif (http://www.refrigeration-engineer.com/forums/showthread.php?p=155651#post155651)
Subcooling is the amount of refrigerant up to the metering device

Plus it's not being 'pedantic' to point out you are completely wrong.

[acnerd]

That's ok Multisync, I knew what Marc meant. As all 3 of us know, subcooling tells us if the charge is correct in the system; and that's why he said AMOUNT!! :p

[marc5180]

Seeing as you know everything, rather than pick at my posts, why dont you help out answering questions on the forum?
Or is that beneath you.

[paul_h]

Nah, he's a one trick pony.

Joking of course :D

[nike123]

That's ok Multisync, I knew what Marc meant. As all 3 of us know, subcooling tells us if the charge is correct in the system; and that's why he said AMOUNT!! :p

Subcooling doesn't tell you if the charge is right in capillary system like you speaking here.
Superheat is what tells you if your charge is right.
How much it should be, depend on design.
Here (http://www.mediafire.com/?wzgtwmyhzty) are some text about that and some generic superheat tables.

[acnerd]

Thanks Nike, I did know about that, but wasn't thinking of it. That's a good little document. Ta!!

[multisync]

Seeing as you know everything, rather than pick at my posts, why dont you help out answering questions on the forum?
Or is that beneath you.


I did, I told you you were wrong and wrong.

Basically you didn't like it and pretended you didn't write what you obviously did and rather than saying "opps my bad" you thought the best way of deflecting attention was to attack me in a very unfriendly way.

[marc5180]

If it was anyone else who had said it, then i would have said "oops my bad"