Front of store return air

[Beerpmguru]

I am evaluating an hvac design for a grocery store and comparing the energy usage to a typical design.

Test store:
Main air handler with supply air fan at the rear of store
UtiliZes duct socks for all supply air
Return air is at the front of store near the sliding entry way doors


Typical store:
Air handler with supply air fan at rear of store
Ducted supply air With diffusers
Return air is located at the rear of store more near the air handler


Both stores have about 130' of refrigerated cases in the center of the store. When comparing the energy usage between the two designs what are the key considerations and calculations that should be done? My gut is telling me the test store will have higher fan power due to longer run for ducted return air but it may be more efficient overall because less infiltration air will be pulled across the refrigerated case coils which have a lower eer then the hvac coil?

Thoughts?

 

[ChrisConley]

Why would locating the return at the entrance reduce infiltration? If the building is negative, it will draw air into the building.

I typically see return air ducted directly from the refrigerated cases... but in my climate the heat recovery is what we're looking for. In a hotter climate, direct exhaust from the refrigerated cases would have an argument.

 

[Beerpmguru]

The store is typically neutral pressure or slightly negative. I didn't mean it would actually decrease the amount of infiltration, but rather it would capture most of the infiltration air from the front doors opening and closing just after it enters the building and return it to the HVAC system for dehumidification.

With rear store return the infiltration through the front doors then mixes with the conditioned space air and the latent load will likely cross the case coils before seeing the HVAC coils.

The HVAC has a higher EER then cases typically so my question is does this have any energy impact of we are in fact able to capture a higher percentage of the infiltration through the front doors, if so what would be the best method to model the savings?

 

[marcoh]

There are a couple of other items to consider:

1 - Where is the temp sensor located that controls the system? The system will control to the setpoint at the sensor location. which may/may-not be influenced by the infiltration

2 - Do you haver RH control in the store? If so where the the RH sensor located as per the temperature sensor above.

3 - Why do you have slight negative pressure? System should be designed to be slightly positive

Typically the grocery stores I have done have had return through the refrigerated cases with the balance of RA through a RA grille at the rear of the store (where the plantroom usually is). RH is often controlled as well to around 45%-50% to maximise fruit&vege shelf life and reduce frost build up is refrigerated cases. RH control is typically by sub-cooling and using the refrigeration system condensing coils as a reheat source.

 

[Beerpmguru]

Thanks for the feedback guys! I am new to this forum and it has exceeded my expectations so far...

Marcoh

1 - Temp Sensor / RH sensor is in the center of the store at the Grocery Aisles (not near refrigerated cases). There are also some other localized zone temp sensors used for a few PTAC's that take care of smaller areas like pharmacy and back room, which im not too concerned with.

2 - Yes Located near the Temp Sensor

3 - The stores are designed for slight postive pressure, but reality is the store is generally neutral because of door openings (front entry ways, and loading docks etc)


I agree the typical design is for 70deg 50RH due to needing this to meet health code on refrigerated cases (this is the environmental rating on the cases so that they can maintain product temps). Typically RA under case and RA Grille Backroom is our typical design as well.

We have moved away from refrigerant hot gas reheat /reclaim from the refrigeration condensing system to minimize refrigerant charges, typically it's natural gas re-heat for us.

I am just evaluating a radically different design from one store I've seen and trying to make sense of any energy advantage it might have. I am having a hard time quantifying anything really.

My next steps I think will be to just actually electrical sub-meter the refrigeration and HVAC loads on this store and then sub-meter a typical store and measure any differences....

 

[walkes]

Seems like a CO2 sensor would help in this case. You may be able to modulate your outside air damper based on CO2 levels. The return will have a higher percentage of outside air periodically and will act as a mixing plenum. I don't believe you will have energy savings from this method since you will just be switching locations of your required outside air intake.