Re: Frozen A/C, hot room
> A common mistake is to get the aircon spec for a server room wrong
Yes, but an "on/off" unit can cope with variable loads - it just doesn't run the compressor all the time. That is in fact how pretty well all systems worked until relatively recently when the power electronics to do variable load working because "cheap". Alternatively, if you correctly design the system, the compressor works full time sucking but can only suck down to a specific pressures, which sets a specific evaporator temperature. On part load, the compressor starts with it's cylinder partly evacuated - and the effective load on it reduces. In extreme, the vacuum at the inlet is such that at full compression it doesn't expel any gas - and the actual load on the motor is low.
A much much bigger problem is speccing the wrong sort of unit - typically one with too cold an evaporator. An AC unit intended for "comfort cooling" will expect a certain amount of the heat removed to be due to condensing water - and as long as there is enough of this heat then it doesn't freeze the condensate which then runs off the evaporator. If the air is "too dry", then it cools the air much cooler, the evaporator fins get very cold, and what moisture there is will freeze - blocking the airflow and allowing that part of the evaporator to get even colder and thus ensuring that the ice cannot melt.
So you need a unit designed for dry air - it'll have a larger evaporator so as to compensate for the higher evaporator temperature, and a different refrigerant cycle designed to work at a higher temperature so as not to freeze the condensate.
Well that's the over-simplified version at least !