Excessive pipe run??

[Collie]

Hi there, Im having a problem with a group of four split ac units running on r22. They are serving a server room so the heat load is constant. Every so often we get a call to say the temperature of the room has crept up and the customer starts to panic.

Checking the units, im seeing good air off temps of about 8 celcius with the controllers set down to lowest setpoint.
The problem im having is that im getting good cooling of the coils but the units still seem to just barely handle the room.
There is frosting on the suction line back to the accumulator in te outdoor unit and observing very low back pressures of about 2.5barg. This would indicate to me that there is liquid returning to compressor but im sure the air filters are clean and the air flow is good, evaporator is in good condition too??

The 4 units are exhibiting the same symptoms which makes me think the problem is external to them, maybe the load has changed.

Q: If for example, the load was increased beyond design spec of the units, what kind of symptoms would i expect to see?
Could i put the frosting of the suction line down to low gas pressures freezing the moisture in the air, rather than liquid, with high cooling capacity returning and causing the icing?

The idea that the pipe run is too great has been touted but i can see how units that have been installed in a place for 10 years and worked ok, could suddenly have too long a pipe run!?

Could units be installed in excess of their recommended pipe run and run perfectly ok, but just have a shorter life span or will they give trouble from day one?

Sorry for all the questions, its just a thorn in my side at the minute and would love to pin the frosting up to an increased load?? if there is any way this can happen id love to know your thoughts? thanks guys

[Brian_UK]

but im sure the air filters are clean and the air flow is good, evaporator is in good condition too??

So unless you check them you can't be 100% certain.

It may be that the evaporators are in need of a good clean.

As you say, the frosting on the suction pipework is probably due to freezing of the ambient air as against liquid feedback.

You haven't stated what make or model your units are.

I would query having the thermostats set at minimum, depending on the unit but with the low night-time ambient of late it is possible that the operating pressure have fallen due to over condensing and the unit is unable to bring the pressures back up again.

There is also the probability that the load has changed since inception, perhaps try and do a recalc of the server room.

Might be worth while pulling the refrigerant out of one unit and comparing it with the original charge weight if that is known.

[Thermatech]

Low suction pressure & frosting indoor coil is to be expected in 100% sensible heat load application because the units are designed by the manufactures for comfort cooling with some latent heat load.
14 to 15 deg C web bulb is the lowest indoor room on coil condition for most standard splits & below this almost all fixed speed split systems will be into coil frost prevention mode & some indoor coil frosting.
If the outdoor units head pressure fan speed control cannot keep discharge pressure up during cold weather then this makes indoor coil frost even worse.
Suggest carry out head pressure controls mods to make the outdoor unit run at higher discharge pressure as this will help reduce indoor ciol frost prevention mode.
If you can introduce some moisture into the room with fresh air or humidifier then that would also help.

If you search this formum you will find a number of threads about this issue.

[bundysnake]

as brianuk said i would reclaim the gas charge and ensure that the unit has the correct amount of charge as per the manufacturers specs.

Can you give us a head pressure?

I get a number of calls like this and i find that people have oversized the a/c units and to make it worse the client set the unit on cool @ 15deg. EVentually the unit cuts out on fault as per thermtech's comment.

[Collie]

Thanks guys, the charge has been weighed out and put back in and its correct to manufacturers spec. The latent load comment has interested me. How would introducing some latent load remove the icing on the suction? If its caused by a low return temp/pressure freezing the ambient air, are we not saying that the evaporator has used all the liquid and is cooling at full capacity?
Sorry for the basic questions guys. Unfortunately i cant give you too many details on the problems as it has been someone else dealing with it for the last while, i just seem more interested in fixing it then they do
I will get the hard facts on monday in the ofice and return to you guys then. Thanks very much

[Collie]

[quote=Thermatech;181267]Low suction pressure & frosting indoor coil is to be expected in 100% sensible heat load application because the units are designed by the manufactures for comfort cooling with some latent heat load.

Sorry, can you explain why this is to be expected? thanks mate

[Gary]

Checking the units, im seeing good air off temps of about 8 celcius with the controllers set down to lowest setpoint.

What are the air on temps?

[Gary]

There is frosting on the suction line back to the accumulator in te outdoor unit and observing very low back pressures of about 2.5barg. This would indicate to me that there is liquid returning to compressor...

This does NOT indicate liquid returning to the compressor. Only a superheat measurement can tell you if there is liquid returning to the compressor.

[Thermatech]

Standard mass produced split systems are designed for comfort cooling.
They tend to have 'Sensible Heat Ratio' of 0.75 or less depending on manufacturer & exact operating conditions.This is good for the manufacturer because they have compact indoor unit which is cheaper to make & compact indoor unit is better for domesic type installation.
In this case the indoor unit has small indoor coil & low air volume which makes low refrigerant evaporating temp & low off coil.
It needs some latent heat with air moisture to work otherwise it keeps dehumidifying the room air & when the relative humidity gets down to arround 40 to 30% & the wet bulb is down lower than 15 to 14deg C then with low refrigerant evaporating temp the coil starts to frost.If the indoor unit has some form of coil frost prevention mode then this is activated to prevent coil ice up.
For fixed speed compressor split systems this makes the compressor stop for a few mins to allow frost on the indoor coil to melt & drain away.
Typically this makes the unit run breifly & then stop. In extreem cases the compressor may only be operating 50% of the time due to this constant on off cycle.
The system is not controlling temp in the room just stoping & running on coil frost prevention mode.
In this type of application the system is operating well outside the operating envelope which it has been designed to operate within.
If you look at the manufacurers cooling performance data you will typically find no data for room air on coil below 14 deg C because the manufacturer knows that below this condition the unit will be operating in coil frost prevention mode.


But systems designed for high sensible heat load has indoor coil about twice the size with much larger air volume. This will give SHR of 0.95 or better & the refrigerant evaporating temp will be high enough to prevent any coil frost problems.

You can deploy some common sense modifications to help the system operate in this extreem operating condition.
Max out the indoor fan speed / select slightly higher set temp.
Make the system run with higher discharge pressure always will help because if you can run the system with a discharge pressure which results in an evaporating temp higher than +1 deg C then the indoor unit does not need to go into coil frost prevention & the compressor will then run for longer untill the room set temp is reached.

The very cold winter weather tends to make overcondensing & low discharge pressure for cooling operation & this make coil frost prevention even worse.
On twin fan outdoor units some enginners remove one fan & motor & just run with one fan.
Or use a pressure switch & contractor to switch fan on off can be deplyed to force the system to run at higher discharge pressure.
Make sure wind cannot blow though outdoor coil & reduce discharge pressure.

Some enginners install a small humidifier in the comms room & this helps to increase the humidity & with some latent load the system can run without coil frost prevention. In some cases free cooling with fresh air is also deployed & helps keep up the humidity level.

Other engineers just keep adding more ac split systems systems into the room. Although they are operating with poor performance if you install enough systems you will keep the room temp under control.

You also tend to see alot of additional belt & braces drip trays installed under wall mounted indoor units in comms rooms because they tend to get iced up & a drip tray contains the water when thawing out the ice.

[Collie]

Thanks, perfect and comprehensive explanation!

[Gary]

They would have been better off tying the 4 evaporating units into 2 condensing units.

In the meantime, these units need all the evap airflow they can get... and more. The coils, blower wheels, air filters, etc. need to be kept immaculately clean.

[nike123]

They would have been better off tying the 4 evaporating units into 2 condensing units.

In the meantime, these units need all the evap airflow they can get... and more. The coils, blower wheels, air filters, etc. need to be kept immaculately clean.

Or they just need to buy equipment designed for task instead of buying cheap "suppose to do task" equipment!
Would you cool your bear with AC! No, You buy fridge.
Why is that different?

[Gary]

Or they just need to buy equipment designed for task instead of buying cheap "suppose to do task" equipment!
Would you cool your bear with AC! No, You buy fridge.
Why is that different?

You are right, of course... but in the real world, when it is the company you work for that is going to be sued for putting in a system that doesn't do the job, you do an on-the-spot re-design. I have gotten several of my employers out of such messes... lol

[nike123]

You are right, of course... but in the real world, when it is the company you work for that is going to be sued for putting in a system that doesn't do the job, you do an on-the-spot re-design. I have gotten several of my employers out of such messes... lol

I am pretty sure, at least in these days, that responsible one is one who ordered job and not installer. They are usually one who want cheap (as possible) conditioning (which is expense) while their real investment is in IT equipment (which brings money). I could understand that philosophy from point of investor.

[Gary]

I am pretty sure, at least in these days, that responsible one is one who ordered job and not installer. They are usually one who want cheap (as possible) conditioning (which is expense) while their real investment is in IT equipment (which brings money). I could understand that philosophy from point of investor.

I suppose it all depends on how the contract is written and who determined that it would do the job and who guaranteed what.

Sometimes its wrong equipment... sometimes its bad design... sometimes its bad installation... sometimes its mis-matched components... sometimes its undersized. Whatever it is, I have always considered it to be my job to somehow make it work.