Hi guys & girls. Apologies for the length of this one but pls bare with me.

So far no manufacturer has been able to give me any definitive answers or data on this so I thought I’d open it up to a wider audience for your opinions and theory’s.
OK we all know that comfort cooling DX mini splits get installed in server rooms and the like (small to medium size rooms up to 30-35m2 or 30-40kW of cooling).

We also know that mini splits are not the right equipment to use for this application – low sensible / high latent heat capacities for what is a high sensible application. However, like it or not, splits do get used for server rooms because:-


  • ·Mini splits are available ex-stock (close control systems are normally built to order and 6 weeks is normally the minimum availability).


  • ·They have a wide range of compact light weight indoor unit types that don’t take up valuable floor space, high wall units being the most popular (close control units are much larger / heavier & more difficult to locate in small rooms. Let’s face it some of the close control units are larger than a couple of server racks).


  • They are cheaper to buy / install than close control systems.


  • They are available up to 14kW capacity in 1ph power supplies and have low starting currents (some as low as 5amps). Most close control systems are 3ph above 5kW and have DOL start.


  • They are capable of long pipe-runs (up to 70m on larger sizes) and need no special oil traps, double suction risers or oil separators as do some close control systems.

As we know the big draw back with comfort cooling splits (when applied to server rooms) is their low sensible heat capacity which is 0.7 at best. This is fine for cooling people but not equipment and causes low room humidity, static issues and can lead to indoor unit freeze ups if the set point is set too low with a low indoor fan speed. The other area that gets overlooked is that cooling kW for cooling KW splits have a much lower air volume than close control units. Close control units need the higher air volume to achieve the high sensible cooling ratios and temperature sampling / air change rates that allows them to control to ±1°c of set point, which of course, most comfort cooling systems cannot do.
So consider the following application:-

  • Server room.


  • Cooling Load , say 21kW sensible, 23.5kW Total (SHF of 0.9)


  • Room design 22-23°c / 50%RH


  • Design external ambient 30°c


  • Proposed Equipment 4 off R410a High wall split systems capable of 10.6kW Total & 7.5kw sensible cooling each at the above conditions.


  • Indoor fans on manual high speed only.


  • Installed Sensible capacity 4 x 7.5kW = 30kW (N+1 if you will).


  • Installed Latent capacity = 3.1kW per system x 4 = 12.4kW against required 2.5kW.

The plan is to run all 4 split systems all the time i.e. no run & standby control strategy. So, in theory with all 4 systems running (30kW sensible cooling available against a 21kW max load) the Inverter controlled compressors will reduce their capacity proportionately to approximately 70% each on a design day and probably even lower at other times of the year.

Question – what is the SHF of a 10.6kW (Total) Inverter split system when it’s running at 70% part load capacity?

You can probably see where I’m going with this. If we installed extra Inverter split systems for additional capacity / redundancy reasons and then run them all together the SHF of each unit at a part load condition should be higher than the normal 0.7. In which case, they will not extract as much moisture as they do at full capacity. This partially over comes the low humidity complaint normally associated with mini splits in server rooms.

Variable speed Inverter systems can control to quite close tolerances – close enough for small server rooms so this should not be an issue either.

Has anyone ever seen part load SHF data published by any split system manufacturer or have you got any thoughts on this theoretical approach?

Enjoy>>