There's a grocery store that I go to often, and I know the owner. He knows I do HVAC, but I'm more of a resi / light commercial guy. He asked me the other day if:
#1 Do the fans on his rooftop units need to run all day while the store is open?
#2 What the thermostat temps should be set at? (Summer/Winter)
He has them set at around 67-68 degrees.
This is in the Chicagoland area, and during the winter people wear their coats while shopping, being it's cold outside.
He says his electric bills for running all the compressors for refrigeration and cooling is costing him around $15000 a month. This isn't a very large grocery store, more or less a neighborhood grocery store. My guess would be around 50 -75000 sq ft.
My thoughts to the first question was yes the fans need to run all day for MUA.
No, the fans do not need to run all day. I set all but the largest unit to "auto" fan and leave the big one run to help to control stratification. It's an individual thing, there are no real hard and fast rules. Tell him to try it and see what happens. The worst is that he winds up with a slightly cold store for a day or so.
I'll set 68 day and drop back 2-4 degrees at night. Does he have setback control? If not he should invest in setback stats. It will save him a good chunk. Again, the customers won't really notice the lower temps, the problem that you will have is the employees. They'll scream much below 67, remember, they aren't wearing coats.
Another avenue to pursue for him is his refrig equipment. Is it running as efficiently as possible? All superheats need to be set correctly. Cases set to the correct temp, not too cold. If he is using programmable rack controls, he can use a floating suction strategy to help to save a few bucks.
Also, tell him that $15k for a 50-75,000 square foot store isn't all that far off. I've got stores running $20-25K bills for the same square footage of buildings.
If the unit is doing dehumidification duty it needs the fans on continuously, otherwise you can auto them. Be aware that some areas of the store may get colder when you do this. ie the refrigerated aisles.
depends where you're from. in my state (Minnesote) code for commercial buildings states that the RTU'S or air handler fans must run continuously during business hours
If roof top units used for outside make-up air, dehumidification, or heat reclaim the fans should run continuously.
In supermarket systems the enviromental system is not just for customer comfort the primary purpose is controlling conditions for the display cases.
If you cycle the HVAC equipment off you will have product loss and increased operating cost due to extended defrost.
While you are correct, using a good control system to monitor and manage those same store conditions, you can still cycle the HVAC equipment, keep a comfortable store, manage energy consumption, and maintain conditions favorable for the cases to operate in.
The reason for rules like that is to assure minimum fresh air. Don't they allow for alternative means of assuring fresh air like CO2 sensors and demand ventilation up there? Most supermarkets have exhaust fans and a lot of infiltration from traffic through the doors. They would seem to be ideal candidates for CO2 sensors and other means of reducing infiltration in the winter months.
Originally Posted by superfittertech
I agree tight control is important.
My point is if temps go too low it may create defrost problems. Many display cases in our area use air for defrost (Even low temp) so enough heat has to be available in the air for defrost.
I think we both agree that the store conditions are primarly governed by the display case requirements.
I do see your point that many times things are just running wild with no control and no purpose.
How cold does the store need to get to negatively affect off cycle defrost performance? I have seen humidity isssues in summertime cause that, obviosly, but the reverse never occurred to me.
Originally Posted by steve wright
With gas and energy prices, I have several stores that are pushing the comfort envelope at 64 degree daytime temps and 62 or less at night. If this will create other problems, I'd like to nip it now before we get embarrassed.
Most say max 75F 55%RH and a min number I have seen specified is 68F.
I,m talking about "Air Defrost" which is a Kysor trademark. We still have a lot of it around here, you all may not have it in your area so much.
I think what you are doing with off cycle, electric, or gas defrost is OK.
On another note a supermarket's utilities here are about $15K for a 50K sqft store in our area. We still have realativly low energy rates.
You are right it takes a lot of energy to heat a supermarket and even that fan power cost a lot. Most people don't realize that it is a double whammy. I completly agree if you are meeting design conditions turn the device off if practical.
Speaking of fan power I have heard of a new trend of cycling off the unit cooler fans when there is no call for refrigeration. JP have you heard of that or tried it?
You need to read an Ashrae book because the air handlers have to run when the building is occupied.
Haven't heard of that one.
Originally Posted by steve wright
An interesting concept. I can see it making an impact, kind of like the motion sensitive LED lighting that WM uses. Lights are off when nobody is in aisle. When you are there, lights are on low. Open the door and light kick to high.
Energy conservation is rapidly becoming my favorite part of this job because the solutions are so creative.
What JP and I are saying is you may be able to cycle some equipment off to save fan HP as long as you meet code and ASHRAE requirements for outside air and space CFM. Other refrigeration issues have to be considered as well.
Before anything was done an assesment would have to be done on the building.