Question about Sizing HVAC split 1

[illuzionb18c1]

Lets assume the following in a single room:
Sensible gain: 20662
latent gain: 1662
total: 22324

I am looking at a mr slim type rated at 24,000 btu unit with the following:
sensible capacity: 14800 btuh
latent capacity: 9400 btuh

I see the total for the unit is greater than the total heat gain in the room. But, the sensible capacity for the unit is far less.

1) In this case, what will happen to the room? (ie. is the unit undersized?)
2.) If the unit is undersized, that means i have find a unit that can handle my sensible gain in the room? wont this in turn cause my humidity to go crazy?

thanks in advance

 

[trashcanman]

What are you trying to control, the dry bulb temperature or the RH%?
If the dry bulb, pick a unit that can handle the sensible load.
If the RH, pick a unit that handles the latent heat.
If close (within 10%) to latent heat, pick that one. The unit will run longer, drying out the air.
Double check your calculations too. Where is the latent heat coming from - people, cooking, ?

 

[walz]

The Unit is not undersized.

What matters is the latent load and the total load. Your selection meets the total load requirement. Though instead of being capable of removing more sensible, your unit is capable of removing more of the latent. But, if you dont have that much latent inside the room, offcourse it can handle that much (or more ) of sensible. Coil never knows which load it is going to take care. It is the enthalpy that matters.

 

[illuzionb18c1]

Oh ok. So basically given my 24,000 btu unit, if there is less than 9400 btu of latent heat (which is the number the unit is rated at for latent), say there is 2000 btuh.

Does that mean that the unit will actually be 'transfering' the rest of its capacity to sensible cooling? (ie. the unit could actually do 22000 btu sensible and 2000 btu latent?) even though it says the sensible for the unit is 14,800?

Thanks again!

 

[EnergyProfessional]

what happens is:
- if air is humid, more vapor will condense on the cooling col, hence more latent removal but less sensible removal (i.e. air is not cooled enough)
- if air is dry, more sensible removal (because the less condensation process " steals less cool" )

The numbers they give you is for standard test conditions (likely ARI). All standard AC units for normal spaces will have about 25% latent, 75% sensible and should work for normal spaces. Since humidity varies, the system is very forgiving. In daily life you never will have the exact sensible/latent heat load that you calculate anyway.

If you have a specific application for dehumidification there are specific units, (they probably sub-cool the evaporator coil, and have lower air flow, or something like that). they might have a 50%/50% ratio under standard test conditions (but will vary as well). If you have sensible heat only (IT server room), there are units that have higher sensible ratio. I assume they cool the evaporator less and increase air flow, or something like that. As long as you pick the right type of unit, I wouldn't worry about more than the total load and capacity.

Even in non-canned systems (i.e. air handler where you regulate the flow) this works out pretty well.

 

[willard3]

The sensible/latent ratio can be controlled by the depth (number of rows) of the coil. If the cataloged coil is 4-row, change to a 2-row and see what the sensible ratio is.

 

[walz]

Yes indeed illuzion.

The latent is more likely to get "transferred" to sensible. But it could not be the other way around. I mean sensible is not likely to get "transferred" to the latent.

the coil properties mentioned by willard (no. of rows and FPI (fins per inch)) are varied by the manufacturer to handle different sensible/latent ratios.

but latent load is the one which is more stringer and cost effective. and that determines no. of rows. Condensation (latent removal) needs more no. of rows. But if the latent is less in amount these no. of rows help to handle sensible load more effectively.

on the other hand if there is a coil which is designed to handle more of the sensible and less latent. (Lesser rows and more FPI), it cannot take more of latent, cuz latent requires more no. of rows.

 

[EnergyProfessional]

I'd think the situation is a bit different for DX and chilled water coils. In DX the temperature of the coil is the same in each row (hence same saturation) and in chilled water there are parts with higher water temperature. I could think a coil for a higher dT has more sensible removal. Or maybe it doesn't matter.

but the start chilled water temperature (or refrigerant temperature/pressure) matter. The colder, the less humid it can get with the very same coil.

I know, the thread starter asked about split systems which are DX.

 

[Hydronican]

Coils are manufactured for differentr design concepts.

If a coil is mainly for sensible load, it will have high S/T ratio, less rows, higher air flow, high EER, higher approach (12 ~ 15oF typical), an higher leaving air temperature limit (such as 47 oF).

A coil mainly for latent load will have a low S/T ratio, more rows, lower air flow, high MRE (moisture recovery efficiency), and lower approach, and lower leaving air temperature.

Of course you can select a coil with high latent load for a application with high sensible load/low latent load situation, but it will be less economic; On the other side, a coil of high S/T ratio will not work for a high humid environment, because the demumidification is a water vapor transport (convection and diffusion) controlled process, it needs more rows and low air flow to achieve the goal.

____________________
Comfort has a price.

 

[walz]

Hydronican..

You have good arguments for different design conditions. But they may be good for Chilled Water Air Handler Coils. Where Manufacturers vary these conditions to achieve desired S/T Ratio.

For DX Split Units, i havent seen any different configurations to achieve different S/T Ratios. They are just made to take care of total load. (With some capability of latent removal within).

Living in a perfect world, your argument might be right, if illuzon is able get two or more selections which fulfill the requirement of total load, and latent is no more than required, and the no. of rows are lesser than the curramt selection and he saves the money out of this.

 

[weaver101]

With such as this equipment, you can not reach your design conditions. You can reach more dry bulb temperature and less RH. For example, if your design conditions is 24C-%50RH, your end conditions may be 27C-%35RH.

 

[walz]

weaver .... how come you arrived at this figure without knowing the airflow...??

 

[VEEKRISH]

Hi,

I assume that the Split Unit would be a DX Coil, and, that the Coil depth will not probably be more than 3 row deep. In your case since he total Enthalpy of the Room is lower than the Unit Capacity, it should do the job, albeit at a slightly elevated DB with lowered RH. You will still be within the comfort zone.

 

[weaver101]

walz...for design airflow, of course.

 

[walz]

weaver ....

ok but you did not mention that airflow rate....

whats the airflow rate you used to arrive at 27C-35%

 

[toughguy1]

My understanding is a little different: If you don't cover your sensible load with the air conditioner, you will have problems with reaching the proper temperature. Yes, it will be dryer because you have more than enough latent, but the temperature will be higher at design conditions. You really need to look at the manufacturer's capacity table and find the proper sensible/latent capacities based on the wet bulb/dry bulb mixed air temperatures. If you have no fresh air, your mixed air temperature would be something like 74/63. This is a lot lower than the nominal 80/67, and your equipment latent capacity would be lower. But my feeling is that you won't have enough sensible capacity. I would upsize. What this means is that your unit will short cycle a little, but with your small latent load, you will not have a problem with humidity. The next size unit will have plenty of latent capacity even with short run times to handle 1662 Btu/hr.