Sensible Cooling

[R1976]

Morning all,

I have advised someone that their classroom needs 14kW of cooling. 56 m (now, where's that button) square is the room size which gives us about 250 per square metre, 30 kids and 30 computers.

I know that in server rooms its best to take the sensible value which is about 10kW, but does this matter in a room with standard computers?

Any thoughts?

[chemi-cool]

Calculate the computers as 500 w each.
if they use old big screens, add extra 250 w per unit.

[Thermatech]

I know that schools dont have much to spend so options can be limited.
But some contractors have run into lots of problems with this application when the cheapest option has been selected.
If you work out the sensible & latent heat load for the room & then look at the SHR of standard splits you might see a potential problem.
If the room wetbulb temperature falls to less than 15 degC WB then you will be into all the same problems as computer rooms applications.
A lot depends on the fresh air arrangements so if the budget allows suggest consider using a VAM / Lossnay heat recovery ventilation system interlocked with split system.
This will help keep humidity level up & provide free cooling for much of the year.
You might be able to sell it as an environmentaly frendly option.

[R1976]

Thanks peeps,

If the room wetbulb temperature falls to less than 15 degC WB then you will be into all the same problems as computer rooms applications.

What sort of problems have you encountered?

[Thermatech]

Low humidity / high sensible load makes lots of problems for standard split systems.It depends on the type of system but common problems include.
1/ indoor units iceing up if no appropriate frost prevention mode.
2/ Fixed speed outdoor units stop & restart every 6 mins due to frost prevention mode resulting in only 50% cooling at 3 mins on 3 mins off.
3/ Premature compresor failure due to liquid flood back & constant restart every 6 mins 24/7.
4/ Inadiquate low ambient control & overcondensing problems.
5/ Inverter units poor cooling performance due to low indoor coil temperature & target coil temp / SH achieved & compressor reduced speed although room temp excessive.

You can hold up a thum & take a guess at how much to oversize the split system to allow for these problems or take care of the humidity issue.
BTW building regs call for 10L/sec per person so to comply you would be looking at 300L/sec fresh air.

[GXMPLX]

They all sound like the split system is already oversized! 1=low suction pressure. 2=Short cycling. 3? Floodback could be the result of short cycling too but the cause is either floodback XOR short cycling Which one? 4= typical of oversizes system to reduce short cycling. 5=QED. You should try a smaller unit or install capacity control to this one.

[R1976]

Hi,

Does the ventilation need to be installed even if the windows open/have trickle vents?

I spoke to Mitsubishi today and have decided on a 20 kW twin split. They say that this system (PUHZ200YHA outdoor unit) is fine for server rooms, etc.

What do you think?

[Billy Ray]

20kW in small room!!

That will blow the teacher's wig off.

12-14kW should suffice (twin split good idea). Lossnay unit is a very good idea, if not essential.

The system only needs to take the edge off, say set controller to 24deg.

Another angle would be to look at air change rate produced by the A/C unit. 10 per hour should suffice to avoid drafts.

Note. Air change rate is not fresh air or extract rate. Its room air volume passing through the a/c fan coils unit in 1 hour.

Billy Ray

[coolments]

Hi just want to touch on the issue of units sensilble and total duty, lets not forget the priciples of it, any reading of my posts will see i put little tech jargon in and say it as it is, the sensible duty is the ability to change the room temperature the rest is its ability to remove moisture with out a change in temperature, the problem being that in sealed server room or any room with no ventilation if the unit is not correctly sized the humidity drops as the unit is working constantly hard, when this happens and there is little latent heat due to low humidity left this causes the evap coil temperature to drop which is why the units start to freeze, with inverters they see this drop in evap temp (ignoring room temp) and ramp down to compensate so as not to freeze up which results in reduced cooling duty.
Looking at your install I doubt low humidity becoming a problem as the unit would not be working hard constantly only maybe an hour at the most before they all leave and the room can drop to temp and cut out on stat, people would also be in and out of the room letting moist air back in and they would also be adding latent heat just by being in there.
So if you install a system with a sensible duty to match the heat load calculated you will be fine.
if you install a unit with a total duty matching your heat load it will struggle when working at full load for sure but will provide comfort, lets face it in uk how many times a year will this be required if you design to a 22c set point with a 28c ambient.
You should not have the same issues faced with server rooms as it does not require 24/7 cooling, if the classroom has opening windows there is no legal requirement for vent as i doubt it being large enough to fall in the specified measurement to the furthest point (which I cant happen to remeber at the minute, doh sorry). Hope this helps.

[R1976]

Coolments,

Great post.

[Greengrocer]

Theres also the small matter of 30 sweaty kids! - you don't get them in your average server room. They'll do a great job of keeping the humidity up.
Do the load calcs and see what the Sensible to Total heat ratio comes out to. If its around 0.8 sensible or less you should be OK on a split providing it's set to run on high fan speed and for a room temp of 23/24c - no lower.
Also note that all manufacturers data is based on room temps of 27db/19wb and 35c ambient. Uk conditions are lower as is the capacity of the systems in UK conditions.

[Thermatech]

When the unit has been selected for the sensible duty & adjustments made for uk temperature conditions ( dont forget pipe length correction if using extended pipe length ) you end up with a larger system than might be expected.

Schools have been well known for selecting the lowest cost option in the past this has been undersized fixed speed cooling only splits which often dont have basic safety controls or low ambient control. So the install contractors have run into lots of problems.

In this case the contractor is considering a quality inverter system which will be much better to cope with this application. A usefull feature of the standard hard wired remote controller on Mr Slim splits is that you can select a lower & higher limit for the set temp & lock any function like fan speed.
This allows the install engineer to lock the remote controller into high fan speed only & lock the remote controller for a suitable lower limit for set point.
This will make sure teachers & kids dont change the settings.

[Makeit go Right]

“I have been advised by someone that their classroom needs 14kW of cooling. Room size is 56m2, which gives us about 250 per square metre, 30 kids and 30 computers.”

“Calculate the computers as 500w each; if they use old big screens, add extra 250 w per unit.”



Apart from starting off with the wrong (guessed) room load, other normal mistakes are selecting a comfort cooling unit that will not operate properly during the winter low ambients (all year around cooling requirement, normally), or using the equipment nominal duty for direct selection of equipment needed.

It is important to start off correctly with a survey of the room and actually check what the computers are. LCDs or monitors. They are normally all the same manufacturer with a specific ‘heat dissipation’ figure on the unit or on their website (normally in Btu/hr). Multiplying by 30 makes a significant difference to the total sensible load.

CALCULATE the sensible and latent heat load…..not wet finger-m2 guesses.

Now the kit: The Daikin RZQ is good for Server rooms and will do well in an IT Classroom. Some other kit will do adequately, if able to COOL during low ambients.

The Selection: A 10kW Total Nominal Cooling unit will not give 10kW of sensible cooling, of course. Not even, “Well, let’s say 8kW.”

You have to convert the nominal 10kW to UK figures (roughly multiply by 0.92), and then get the Sensible duty out of that (normally multiply by 0.7) and……gosh, the unit only does 6.44kW UK Sensible!

That’s right. Nothing like 10kW, or 8kW, is it?

Now you can get going with the costs and quote the job.

[R1976]

Hi all,

Rang the customer to see how the quote did.

Another quote has come in 2K cheaper

I found out that this company are offering 2 Fujitsu ASY18 systems which are 5.2 total cooling and 3.64 sensible cooling each.

Also only 12 months warranty as opposed to our 3 years.

I give up!

[Makanic]

Hi all,

Rang the customer to see how the quote did.

Another quote has come in 2K cheaper

I found out that this company are offering 2 Fujitsu ASY18 systems which are 5.2 total cooling and 3.64 sensible cooling each.

Also only 12 months warranty as opposed to our 3 years.

I give up!

mmm The Company I work for would of opted for something simular ,maybe not them sizes but
2 units instead of 1 large unit = less chance of short cycling problems in low out door temps

[Makeit go Right]

Rang the customer to see how the quote did. Another quote has come in 2K cheaper. I found out that this company are offering 2 Fujitsu ASY18 systems which are 5.2 total cooling and 3.64 sensible cooling each, with only 12 months warranty as opposed to our 3 years. I give up!

Don't give up! Two units with a combined sensible output of 7kW is too small for that room.

Look, just with a couple of guess loads:
a) 30 LCD computers at 180w =5.4kW {sensible}; and
b) 30 kids +teacher @75w =2.3kW {sensible}.
So, before you add on fabric, lights and fresh air/infiltration, your sensible load is already over 7kW(all year around). If it were me, I would go in there and trash the other quote, showing them up as someone who does not know what they are doing (even if it isn't completely true). The customer doesn't want to install something too small and then wind up with egg on his own face. I would urge you to chase after that order, mate.

ps: Actually, something looks a bit odd with that quoted Fujitsu sensible kW figure............that looks like a nominal sensible kW figure, not UK sensible. Worth checking that too. I will bet that the actual UKsensible cooling output will be only about 3.35kW. And x2= 6.7kW, which is looking a bit worse. The client needs your help before he messes up.

[Slim R410a]

Don't give up! Two units with a combined sensible output of 7kW is too small for that room.

Look, just with a couple of guess loads:
a) 30 LCD computers at 180w =5.4kW {sensible}; and
b) 30 kids +teacher @75w =2.3kW {sensible}.
So, before you add on fabric, lights and fresh air/infiltration, your sensible load is already over 7kW(all year around). If it were me, I would go in there and trash the other quote, showing them up as someone who does not know what they are doing (even if it isn't completely true). The customer doesn't want to install something too small and then wind up with egg on his own face. I would urge you to chase after that order, mate.

ps: Actually, something looks a bit odd with that quoted Fujitsu sensible kW figure............that looks like a nominal sensible kW figure, not UK sensible. Worth checking that too. I will bet that the actual UKsensible cooling output will be only about 3.35kW. And x2= 6.7kW, which is looking a bit worse. The client needs your help before he messes up.

Well said, I agree.
Maybe e-mail the customer some figures, if you use an AC calc you could attach your calcs too. (I use Dir calc which shows the total sensible and latent heat of the room)