Mitsi Heavy short cycling Issue!

[Daikin=Overated]

Hello all.

I wondered if any one had encountered similar issues with Mitsi Heavy Ind SRK fixed speed range from 2006 on wards. I've installed this kit in 2 high heat load areas (server room and motor room) and both appear to be short cycling. I searched the posts and have found some great info on the subject, the importance of sizing (was correctly sized) and humidity issues (most likely the cause) and my question is this.

Is it possible to place an overrun timer on the control circuit to allow the comp to run say a minimum 20mins each time? Otherwise the outdoor unit just kicks in/out far too often which concerns me, not to mention that this is effecting the cooling situation no end.

Failing this is their some sort of kit high ambient etc available for splits of this type??

Many thanks in advance.

J.

[sinewave]

Whats the RH in the rooms?

[Thermatech]

If the unit is cycling on coil frost prevention then the unit may have some liquid flood back & compressor can be damaged due to liquid flood back & excessive restarts & all sorts of pipe fractures & lost refrigerant problems.
Is the suction temp below zero ?
It will be lower than coil frost prevention temp if the humidity in the room is lower than about 40 % RH & 15 deg C WB.
Try reducing the outside unit air flow & monitor discharge & suction pressure. You will find that as you push up the discharge pressure the suction pressure / temp will improve. If you can get the system to run with a suction temp say 2 or 3 deg C then the indoor unit will not cycle on coil frost prevention because you are maitaining coil temp just above coil frost conditions.
Most split systems from Japan have oversized outdoor coil for our ambient conditions because they are sized for 40 to 45 deg C. Overcondensing can be a trial to overcome. I have seen twin outdoor fan units running good with one fan & motor removed. HP control with pressure switch & contractor to switch fan on /off is crude but works.
You can use any low ambient control / HP control set up which allows the system to run with coil temp above coil frost prevention condition.
Cooling performance will be slightly reduced & the compressor will run at higher amps but you can get the system to run without coil frost prevention untill the room set temp is reached so the system does not constantly cycle on/off, so overall the cooling performance is improved.

Air circulation in small computer room is another issue which can make the system cycle on/off because the cold supply air short circuits straight back to the unit & it stops before the room temperature gets down to set temp.
Look carefully at the air circulation in the room & monitor the return air temp at the indoor unit. Some times a remote temp sensor can be deployed to ensure all the room get down to set temp.

Ideally a high sensible type cooling system should be installed & this would work better & last many years.
Using the standard comfort cooling split system for this application makes a number of problems & the life time of the system is much reduced. So after the enduser had paid for a number of new compressors & then a new replacement system he actually would have been better off spending the extra on a proper high sensible type system in the first place.

Although some of the inverter units are a bit better for this application because they slow down the compressor to maintain target coil temperature but cooling performance is much reduced so you have to estimate how much to oversize the system to cope in the low humidity application.

[sinewave]

Which manufacture's make dedicated units for applications such as this?


I've never noticed any in the catalogues!

[The Viking]

Airdale
Liebert
Stultz

To name but a few and in no particular order.

[Greengrocer]

Yep those manufacturers & others but there is one big drawback to High Sensible / Precision cooling systems - their physical size and capacity.
The original poster mensioned MHI SRK fixed speed spits the largest of which is only approx 3.6kW Total (2.66kW sensible). Close control systems don't come much smaller than 10kW, most are floor mounted and the smallest are the size of a server rack. Most smaller server rooms (broom cupboards) don't have the space for floor mounted kit this, plus a 415v/3ph power supply, are the main reasons why compact wall mounted splits get used in small capacity server / comms rooms.
However, there are a few exceptions - I believe Airedale do make a wall mounted close control system. Only snag is that the indoor unit weights 40kG which rules out any lightweight stud walls. It's also built to order (6-8weeks) which is another reason std splits are used over CC systems.

The only other issue with the earlier MHI fixed speed units is that they didn't have any head pressure control fitted - because they were aimed at the residential market. Consiquently they didn't operate properly in ambients of 21c or less - not much good for a server room. I believe the latest fixed speed models now have HP control fitted as std.
These systems are cheap. My advise is if you have to use std splits for server rooms go for the inverter models which can at least reduce their capacity to prevent short cycling.
There is a niche market out there for a high sensible cooling only inverter high wall split up to 10kW. Problem is the market is not big enough for the big boys to bother with.

[Daikin=Overated]

Thank you very much to all those that have taken time to reply to my question above.

Sorry for the delay....It took forever to get out of Glastonbury haha!:cool:

The replies have made for a very interesting read, and I'm much clearer on the limitations of such systems in these enviroments. The splits were installed a couple of years ago and are still operartional in every respect albeit with the short cycle issue raised. I can confirm that minimum piperun has been exceeded, so liquid floodback should be minimal-though I will be checking suction temps at ccu.

Just my luck MHI revised the design and implemented HP control on the latest systems.Doh!:rolleyes:

My priority is now to gather readings (DB/WB etc) and proceed from there. I have service calls to attend on both and will update on my findings as soon as I get the results.

Thanks again.

J.

[Greengrocer]

Make sure the controller setpoints are no lower than 21c - preferably 23c and on high fan speed. If not fitted you may be able to get a HPC kit from HRP/3D for the condensing units. Lowest cost retrofit HPC kits usually sense the outdoor coil temp via a sensor strapped to the outdoor coil and modulate the outdoor fan accorrdingly. Others sense pressure via a capilliary line. Check with your MHI supplier that the outdoor fan can be speed regulated - normally depends on what type of bearings the motor has.
Where there is no other option we have succesfully used single and multiple high wall splits in small server rooms. However, we always go for the Inverter type, never fixed speed equipment. Not only can these vary their capacity to prevent short cycling, when at part load the indoor coil sensible heat ratio increases which helps to prevent freeze ups and reduces the amount of moisture removal (reduced refrigerant flow at part load into indoor coil = higher evap temp).
Using comfort cooling splits on small server / comms rooms is not ideal but circumstances are such that on most small jobs compromises have to be made since the pukka equipment cannot normally be used. Providing care is taken selecting the right equipment splits can work ok.

Glastonbury? How the hell did you get time off for a camping holiday during the silly season?

[Daikin=Overated]

Cheers Greengrocer!

I take that on board. I'm going to contact HRP tomorrow to see what they can come up with for the series 1 SRK splits. Hopefully a kit of some description hey!

Totally agree with the sentiment regarding system selection versus budget. Low cost split a/c alsways seems to prevail.

Glastonbury....Well I notified well in advance, all my clients on contract that I was on annual leave for the week, put my collegue in place on standby, and managed in the end to remind myself that I work to live, not live to work (which seems to take some convincing these days lol). Twas well worth the effort!

All the best.

J.

[BritCit_Juve]

H i all sorry for delay
Fixed speed SRK are limited to 20°C ambient in cooling but have a straightforward 230V fan motor so a marstair head pressure control kit will work and prevent this problem. (stap on temp sensor, to be mounted at about 2 oclock on liquid line, and modulates the neutral from the motor. be sure to adjust the minimum speed to just above stall speed)

HRP have started doing some 2kw to 8kw wall mounted close control splits for comms rooms and manufactured by a company called compact UK

http://www.compactaircon.co.uk

The units are available cool only, cool + heat or cool heat, hum and dehum control.
Try quoting one though and the clients just laugh as its a hell of a lot more expensive than a standard high wall.

they look uglier too much more functional!

Brit

[carlfoster]

Just browsing through and noticed this thread and felt I needed to reply to the last post.

Yes, we at Compact do manufacture a range of wall mounted close control units designed specifically for data rooms.

As to the price, well, close control kit will always be more expensive than your regular high wall splits for a number of reasons:

1) They contain much more functionality and adaptibility to maintain required temperature and humidity in data rooms.
2) They usually need more air flow.
3) They can humidify
4) they're designed to run for 24 hours a day non stop
5) They designed to last

Your run of the mill high wall split would not be able to do this to the degree that most data rooms need and they certainly don't humidify.

As to the comment that they are ugly, well, that depends on your perspective. They don't have the same rounded shape as your modern splits, true, but they are made for data rooms, not offices.