Lower Air off temp required

[Collie]

Hi guys, just a question about modifying an existing dx system.
The customer was enquiring about lowering the supply temp to the space and was asking how major the modifications would be. Could we just increase the size of the TEV? or are we looking at increasing evaporator and/or compressor??
The current supply temp is about 15Celcius and they want to drop that to 10.
can i use the loading equation Q(kW)=m.c.dt? or is that simplifying it too much?
I dont really have much more details than this at the moment, its preliminary investigation im doing to get an idea of what will be required, any thoughts? or any books recommended on designing a/c systems?

[martinw58]

if the tevs are sized corectly then you cannot just fit bigger ones you need to look at the duty of the system and the duty required. if you have the supply air temp to low you can get condensation on the ductwork

[lawrence1]

You can slow the fan down and that will lower the supply air temp.

[RSTC]

^agree with Laurence. If all you want to do is lower the off coil temperature then drop the airflow. i.e. the 'm' in your calculation above.

BUT

you haven't told us why you want to lower the off coil temperature.

If you lower the off coil temperature you will drop off the total cooling and increase the latent cooling capacities (well, the ratio at least)

Lowering the off-coil may not give you additional cooling in the room (and it will make the ducts sweat if the insulation isn't sufficient!)

Oh, and a low supply air temperature will really irritate anybody sitting near a grill!

[Tesla]

HI
On the lo side you could use a liquid to suction heat exchanger and increase air fol with pulley sizes but need to check the rest of the system can handle it or needs beefing up to.

[Collie]

Sorry i know the info was a bit vague, it was just a passing question put to me by my boss the other day and i didnt get too much info from him. Its serving some blood transfusion place so i dont think comfort cooling is the requirement here Someone calculated that if they want to achieve their desired room setpoint, they'll need to lower the supply temp of the air to about 10c.
Regarding the slowing down of the fan, i guess the question is how low can i get that air temp to go without my coil starting to frost over?

[Collie]

If you lower the off coil temperature you will drop off the total cooling and increase the latent cooling capacities (well, the ratio at least)

Lowering the off-coil may not give you additional cooling in the room.

Hi, sorry, could you explain a bit more for me please? just trying to get my head around this

[Collie]

if the tevs are sized corectly then you cannot just fit bigger ones you need to look at the duty of the system and the duty required. if you have the supply air temp to low you can get condensation on the ductwork

Ok thats good to know, its not just a case of increasing the capacity of the valves then.
I was just trying to see if there was any way of increasing the cooling capacity without major changes i.e, evap and compressor.
But i suppose, just by having a lower supply temp dosnt necessarily mean greater cooling capacity am i right?

[martinw58]

you can fit inverter to the supply fan and slow the fan down it untiil you get near to the temp you need without making the evap coil start to ice

[RSTC]

Aha! So you want more cooling. Cold is good... so colder must be better?

The output capacity of the coil (like all plant) is best at one particular point, so if you drop off the airflow: you drop off the efficiency (that's energy efficiency not temperature).

Air conditioning capacities in catalogues are generally rated in TOTAL output, (SENSIBLE cooling + LATENT Cooling = TOTAL Cooling). The ratio of sensible to latent capacity can be anywhere from 50/50 to 95/5 but for any coil it will drop off as you drop the airflow.

Sensible cooling being the TEMPERATURE drop and latent cooling is the HUMIDITY drop.

EG: Say your room is 100m2 and you've calculated 14kW load. The AC is a 14kW unit (non inverter for this example) ...All seems ok... but you get the tech data and the actual capacities for Low/medium/high fan speed are 12.5/13.3/14kw and the ratio of sensible cooling to latent cooling is .65/.75/.85.

In Ireland we have relatively low latent heat (low humidity compared to some climates) and your room load is, say, 13kW Sensible + 1kW latent = 14kW total (it's all PC's and medical machines and such).

So back to your machine:
@high fan speed: 14kW x .85 = 11.9 kw sensible (we need 13kW thats why it's struggling)
"Hey didn't somebody on RE forum suggest lowering the fan speed!"
@low fan speed: 12.5kW x .65 = 8.1kW sensible (now we're really in trouble)

Sure, put your hand infront of the grill; it will Feel colder, but the room will still struggle to be comfortable. In the example above you have only 8.1kw of the 13kW of SENSIBLE cooling you need.

[Collie]

Hahah, yes unfortunately our irish climate isnt much for humidity!
Ok, that all makes sense, thanks for the explanation. I guess now im wondering whats the simplest way that i can increase the cooling capacity??

[RSTC]

...Or lower the load.

If the room is too hot (>25oC?) and off coil is 15oC, then I'm guessing the plant is running OK. What is it? AHU, Split, VRF, etc.

might be easiest just to add in an extra room unit (unless you designed it and only fitted it last week and now you're being sued - in which case put in the extra unit and shut up it'll be cheaper than senior council!)

[Collie]

Oh its an ahu with dx coil in it. I guess that adding say a split ac wasnt an option as they must feel the existing plant is capable of extra duty?? unfortunately i dont have any figures on what the load is and what the cooling capacity is so i cant really be halped too much! haha, I just wanted to get a feel for what would be involved. So to summarise, lower temp off coil doesnt mean more cooling capacity, that makes sense to me