Hvac system in hot and humid climate 1

[leo2011]

I'm designing Hvac system for restaurant application in hot and humid climate (Miami, FL) My load calculations came up 15 tons of cooling. I have the option to use split Dx system or VRF system. What would be the best choice based on energy efficiency and operation wise. Any recommendations!

 

[Drazen]

it's more important to match latent loads with calculated cooling loads that to decide on split/vrf.

anyhow, your load is a bit too large for multisplit system if you don't want to put local units everywhere around. though it also depends on building geometry, number of spaces etc. i assume that restaurant has few larger spaces with high load, and enough space in suspended ceilings to install lot of accessible tees, so it generally points toward vrf.

 

[EnergyProfessional]

keep in mind you need a DOAS (ERV + cooling coil) for the VRF to treat OA unless you don't care about humidity.

 

[dbill74]

NOT VRF. These systems do not handle high ventilation rates very well at all. A Dx system deigned to handle a high amount of OA (25-30%) is the better bet. Aaon units are popular in Florida.
Due to differences in load profile of the seating area vs. the kitchen area I trust you are planning separate systems for each part of the building.
I trust also you are not conditioning hood make-up air, would be an awful waste of energy to do so.

 

[cry22]

DOAS with enthalpy wheel and VRF is my choice. Evaluate if you need reheat at the DOAS unit depending on your load. You may be able to use DOAS air directly into the space without reheat and downsize your VRF units.
DCV should be used too per new ASHRAE 90.1, Single zone VAV must be used too per latest ASHRAE 90.1
VRF (and single zone VAV) gives you better humidity control at part load conditions (most of the time, your restaurant does not see full load). Regular DX will need to be undersized to work continuously so it can dehumidify properly.

Check with your VRF vendor such as Daikin, they have good literature on the subject. also, check Trane ENL (Engineering News letter) youtube videos on the subjects (VRF, single zone VAV, DOAS with or without reheat...), you will come out informed, this is not a small subject that anyone can answer in one post.

ASHRAE has a document called "HVAC in hot and humid climate"

 

[marcoh]

No issue with using VRF but you must use one of their outside air preconditioning units to mainly remove moisture from the outside air. If you do not use an outside air preconditioned you will have RH issues as the standard VRF FCU's cannot handle significant latent loads.

If you have sufficient spill air you should also use energy recovery systems which the VRF suppliers also supply.

 

[willard3]

Most mfgs make VRF with two-row coils maximum, not so good for removing moisture.

A DX system will at least allow a coil choice that will deal better with high wet bulb in the OA.

 

[dbill74]

If you go VRF, a DOAS would be needed I agree. But why add more equipment than absolutely necessary? It will increase the upfront cost of construction and because you've doubled the amount of equipment that has to be maintained, annual maintenance costs are going to go up too. Owner will not like you for that.

I was taught as a young boy and again in engineering school to K.I.S.S. (Keep It Simple Silly).

 

[HoustonTradeTraining]

The ASHRAE guide does help although I live in Houston, Texas and not many people actually go the extra mile here to make them more efficient.
But if you search for "ASHRAE Guide for Buildings in Hot Humid Climates" you can download the .pdf

HTT Houston Trade Training LLC

http://houstontradetraining.com

 

[cry22]

dbill74

in a hot and humid climate, the single DX system for high occupancy is always way oversized, meaning that during low occupancy, the DX will overcool the space too quickly and then has to sit there 10 to 20 minutes without cooling (i.e without dehumidification). This kind of situation creates a stuffy space with damp smell, check many old single family homes' basements, you will smell it. The DX being off, and the kitchen exhaust ON, lots of infiltration of untreated air, resulting in mold build-up inside the walls, air trapped and condenses when DX is back on. The bad smell will drive customers away faster than you think.
Two (or more) smaller DX units, one controlled with room humidity (other with thermostat) can be used satisfactorily, but one large DX unit controlled by thermostat only (the KISS Method) will not get you the comfort you need.

AND economics can be explained to an owner, you want to pay for it now or lose customers and go bankrupt later.

There is Simple and there is Cheap - Big difference.

 

[dbill74]

Cry22,
I agree there is a difference between Simple and Cheap.
Run of the mill Trane/Carrier/Daiken units will result in the musty smell you talk about, because like VRF fan coils there is a limit to the amount of OA they can handle, generally 15-20%.
That is why the specified and installed units needs to be designed to handle the extra OA. I mention Aaon earlier, they have numerous products specifically designed for high OA situations easily handling up to 40% OA. (Aaon units do tend to cost 50-60% more than a run of the mill Trane unit.) It may also be appropriate to have multiple units for the dining area or perhaps a small VAV system. These high OA capacity units are used all the time. When is the last time you've walked into a McDonald's, Wendy's, Taco Bell, Subway, etc. and it smelled like grandma's basement? Walk up on their roofs in Florida, and I'll bet you $10 more often than not you will find Aaon packaged RTUs.

Given the choice between VRF and packaged Dx in a restaurant, I would go packaged Dx. At least 2 units, one for kitchen, one for dining. For large seating areas, 5,000+ SF, additional units may be prudent.

 

[RandomUserName]

cry22,

Understanding your point of view, would you not agree that one could select some of the more modern RTUs that have modulating compressors/ condenser fans and humidity sensors to not only maintain (and match) sensible loads but latent loads as well? Would this marginal cost (from say a Trane precedent to a Trane precedent with eFlex) be smaller than the cost of a DOAS? These units, even the small 3-5 ton range, now have the ability to modulate OA based on schedule / occupancy sensors.

What are your thoughts?