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sparrow
02-06-2008, 07:59 PM
Hi people,

Question, when I have just the one access point on the outdoor coil, suction line.
I can clearly see the temp and pressure relationship on my gauges low side only how do I work out the discharge side?
Also following pump down, It is correct that i can just evacuate through this side even though it will be liquid initially.

thanks for any pointers

sparrow

sparrow
03-06-2008, 08:50 PM
ok, I just worked the expansion pressure out from temp on the pipework. I think that was the only way. I have a question however, if anyone is interested, the heat coming off the coil in my opinion was sum what lacking I was getting the right conditions in the server room downstairs but I am sure something here is not quite right.

Sparrow

Thermatech
03-06-2008, 09:49 PM
Measure the discharge pipe temperature at compressor discharge.

Use your gauge temperature pressure scale or a comparitor to convert the temperature to pressure.

Most recovery machines can pump liquid.

nike123
03-06-2008, 09:57 PM
Measure the discharge pipe temperature at compressor discharge.

Use your gauge temperature pressure scale or a comparitor to convert the temperature to pressure.

Most recovery machines can pump liquid.

Compressor discharge is superheated. You cannot convert superheated vapor temperature in to pressure without knowing how much superheat you have.
To convert temperatures in to pressures you need to take measurements in parts of system where refrigerant is saturated (condenser, receiver)

Thermatech
03-06-2008, 10:11 PM
Agreed
But a reasonable estimate of the SH should put you in the ballpark.

750 Valve
04-06-2008, 07:07 AM
Agreed
But a reasonable estimate of the SH should put you in the ballpark.

what would a reasonable guess at discharge superheat be?

unless you had an enthalpy chart I reckon you'd be clutching at straws... considering the suction pressure and superheat has an affect on the discharge temp (along with refrigerant type) you'd need to be a walking talking enthalpy chart for each refrigerant to make a "reasonable estimate" at how superheated the discharge gas is.

At the end of the day you cannot really guess at the discharge pressure/sat cond temp from just the suction guage - no design data will really help you unless the equip is running exactly at its design max, then you could take the ambient temp and knowing the design cond TD have a ballpark guess at the sat cond temp providing the gas charge was correct and the coil was clean.

Thermatech
04-06-2008, 09:28 AM
This is why I try to get young engineers to always look at the discharge SH when they are commissioning new equipment so then they will get a feel for what the SH should be when the system is working correctly & has spot on refrigerant charge.
Discharge SH can also be used as a key indicator to estimate if the system refrigerant charge is correct.

For instance
Air conditioning split systems & vrf systems with R410a will run normally with 25 to 45 degC SH with correct charge.
Less than 10 degC SH & the system is overcharged & some vrf systems will stop on overcharge fault based on the discharge SH data.
More than 45 degC SH & the system is undercharged or a refrigerant flow problem in the system & some vrf system will stop on high discharge temp fault at 110 to 120 degC. At this extreem discharge temp the discharge SH will likely be over 70 degC.
If you work alot on a/c kit with R410a & you always look at the discharge SH as a key indicator then you soon get a good feel for what the discharge SH is when the system is working correctly & what the cause might be if its too high or too low.

Discharge SH is a very usefull indicator of system performance but many engineers dont think to use it.

sparrow
04-06-2008, 02:56 PM
Hi Thermatech, I know you have put quite alot of information in your reply there, can I confirm that it is reasonable to take a discharge temp from the pipework, and convert this with a temp after the evaporator to find a specific SH value?
Thanks also to 750 valve nike 123 for your input (nike i sent you a private message about this problem did you receive it?) this system is R410a I took the ambient temp into consideration from the start then thought well what's the point? I only have one access port,I wish to know if this system is working within its correct parameters, I evacuated the refrigerant and discovered it was 100 grams over I started to question myself as to how much of an impact this would have? I replaced the refrigerant to the correct charge and monitored the situation. i was still not completely satisfied with the expansion line temp however, and would of liked to have felt more heat. It was really bad rain on the roof though yesterday so who knows.

Sparrow

nike123
05-06-2008, 05:19 AM
(nike i sent you a private message about this problem did you receive it?)

Sparrow

Nope, I did not!

750 Valve
05-06-2008, 11:36 AM
This is why I try to get young engineers to always look at the discharge SH when they are commissioning new equipment so then they will get a feel for what the SH should be when the system is working correctly & has spot on refrigerant charge.
Discharge SH can also be used as a key indicator to estimate if the system refrigerant charge is correct.

For instance
Air conditioning split systems & vrf systems with R410a will run normally with 25 to 45 degC SH with correct charge.
Less than 10 degC SH & the system is overcharged & some vrf systems will stop on overcharge fault based on the discharge SH data.
More than 45 degC SH & the system is undercharged or a refrigerant flow problem in the system & some vrf system will stop on high discharge temp fault at 110 to 120 degC. At this extreem discharge temp the discharge SH will likely be over 70 degC.
If you work alot on a/c kit with R410a & you always look at the discharge SH as a key indicator then you soon get a good feel for what the discharge SH is when the system is working correctly & what the cause might be if its too high or too low.

Discharge SH is a very usefull indicator of system performance but many engineers dont think to use it.

25C to 45C is a big variation... what would cause this variation - ambient temps, suction superheat, design condenser TD differences for different brands. So if its say 30C air on, what would you expect as discharge SH? And then for say a 20C air on would would you expect? (without checking enthalpy charts I reckon it'd be no more than a stab in the dark)

So next you get called to a compound 2 stage compressor on R22 running an old freezer at -20C storage temp and in a 30C ambient.... The discharge SH off the top of your head would be?

Or a 2dr upright medium temp bottle cabinet on R404a in a 20C kitchen?

Or a domestic fridge/freezer on R134a in a 18C house?


I'm sorry but I really think this is taking a guess at system conditions, I cannot really justify teaching anyone to "guess" at the system conditions, turning up at a new service call you cannot even be sure the system is correctly charged by just the suction pressure and superheat so taking a leap of faith and guessing cond pressures and SCT from the discharge pipe temp is just a little far flung for my liking.

Thermatech
05-06-2008, 12:47 PM
Sparrow

You say the indoor unit is in a server room.
We assume you have some sort of R410a split system.

There are some specific issues with using standard split comfort cooling systems in very low air humidity conditions as found in most computer/server rooms.

Most manufactures design comfort cooling split systems for about 0.75 sensible heat ratio & main target market is domestic applications.. So the unit needs some latent heat to work properly & for comfort cooling in a domestic application this is appropriate because moisture is generated in kitchen, bathroom & people so there is plenty of latent heat load.
But in a server room with no fresh air & no ocupants there is no latent load.
So if you measure the relative humidity in typical server room you often find its down at 25 to 30 %RH & depending on the exact dry bulb temperature you will find the wet bulb temp around 11 to 14 deg C.
Almost all split systems are made for minimum wet bulb on coil temp on 14 or 15 deg C WB.
So the unit is installaed in an application for which it was not designed & it will not operate at temperature & pressure you might normally expect to see.
The indoor unit will have very low coil temperature & refrigerant evaporating temp due to the very low WB on coil temp / low humidity / no latent heat load. The indoor coil is effectively under sized for the high sensible load.

A lot depends on the exact model split system you have as to how it will cope with this extreem operating condition.

Some fixed speed systems with coil frost prevention mode will constantly stop every few mins due to very low indoor coil temp & need to prevent freeze up.
These systems will only do about 50% duty because the unit runs for 3 mins & stops for 3 mins on coil frost prevention.
Some Inverter systems look at the coil temperature data & see the low coil temperature & slow down the compressor untill the indoor coil temp is within target & above coil frost prevention conditions.
Again the system works well below max capacity.

If your unit is an inverter then it is possibly running at low compressor speed low discharge pressure which results in the reduced heat discharge at the outdoor unit which you have noticed.

Is this why you want to estimate the discharge pressure ?

If you further reduce the outside air flow the discharge pressure will be increased & the evaporating temp at the indoor unit will increase a small amount & the system will speed up the compressor.