Pizza Shop Load Calc - Heat Gain From Oven?
Anybody have any good info or tried'n'true rules of thumb for estimating heat gains from gas convection ovens?
I work with an MEP design firm and we have opportunities to do design work for a pizza chain. The overall design is pretty straight forward otherwise, but we're having trouble finding any good information on how to size the RTU to adequately cover the oven's heat gains. We've met with the hood manufacturer and have good methods for compensating for make-up/exhaust air, now we just need to make sure to cover the oven adequately. From running test numbers it looks like it would be very easy to greatly under/over-size the system without the proper method.
I've looked through the commercial kitchen section in ASHRAE but didn't find it to be much help...seemed more like it just said "keep this in mind when designing" rather than providing any sort of data or equations. Maybe I just haven't found the correct chapter...
The oven is an Impinger dual oven, 120,000 BTU/Hr each, conveyorized convection setup. I called Impringer and their engineering department couldn't tell me squat, said they've never tested the heat put off by their ovens.
Obviously we need to account for the gain, but don't quite know how much to account for. Most of the heat will go through the exhaust but some of it is going to affect the space. I worked in a pizza shop back in high school...there is definitely a significant sensible gain coming off that oven!
Any info, ideas, insights, etc are hugely appreciated.
Last edited by theWilly; 09-29-2010 at 04:33 PM.
Reason: typos, added more info...
In my experience a properly sized and balanced hood/makeup air system will exhaust 99% of the heat produced by these ovens, or any configuration of equipment. The trend toward total fresh air return in kitchens and restaurants in new installations affects the calculations much more. The roof temps in the summer can approach 150F and the new restaurants I was associated with in Atl. had more issues with controling this fresh air return flow and the heat associated with it. The other issue is employee or managerial ajustments to the systems once their installed; especially during the warranty period, which is when you'll get calls.
They use a hood system that supplies 80% make-up air through an unconditioned supply fan that blows through the perforated hood supply. The other 20% is then supplied through the RTU outside air supplies.
We plan to provide slightly more supply air than exhaust air (total exhaust approx. 1900 CFM, total supply approx 2000 CFM).
So....if you're saying we can essentially remove the oven from the equation (or at least provide a nominal % amount BTU when other equipment is added up, refrigerator compressors, lighting, etc)...then, how much of that introduced unconditioned supply air is affecting the system? It has to be dumping large amounts of latent heat into the space....and while I'm sure some amount of that air is sucked directly back through the exhaust there's some interaction with the conditioned space.
Anybody know of studies or anything done on this?
These guys ditched their last designed after receiving inconsistent HVAC performance...without some way to calc this stuff and backup my design with numbers I don't know that my firm could do any better...or at least defend ourselves should something not perform to their expectations. I could cover my butt and just tell them drop a pair of 10 tons on the roof (typical store is 1100 sq ft) but they're not going to be very happy with that.
Captive Air is a hood supplier that has done a lot of studies and is involved with several large corp accounts. I would try searching their website and then contacting them for info. Typically balance means just that, and not over suppling unconditioned air. Cracker Barrels for example supply conditioned makeup air in an oversupply and their kitchen stays cooler than customer spaces. On the other habd Golden Corrals are using fresh air returns and make up air and their kitchens are hot in the summer and cold in the winter and their mgrs. turn off makeup sir when it gets too cold and then wonder why the heat is sucked out of the seating areas. There is info out there. I am on the practical end and deal with real world conditions, and was just trying to give you some insight from experience.
I had some dealings ith a pizza resturant one time .... I think seperating the dine in customers the kitchen is a big problem .... The oven hood can really screw with your conditioned space ...
My avatar is a picture of a Goodman Silencer .....These were commonly used in Goodman country ....Photos by hvac tech ( PaysonHVAC )
Hey, thank you! CaptiveAire is actually who they use already. I went to one of their hood seminars a while back and learned some good stuff.
What still eludes me though is coming up with numbers for how much unconditioned makeup air interacts with the conditioned air, and how much extra load to add in for it. I've run several Manual J load calcs on a proposed job and that makeup air really messes with things. I know I have to account for some portion of it but I just don't know how much, and if I go safe on it and account for something like 80% of it I end up requiring 18 tons in an 1100 sq ft space. I know they're not using that much cooling in any of their other stores.
I like the idea of using conditioned makeup air...but the client hasn't been too receptive to it, saying they've never used that before and don't think it's necessary.
Would it make sense to you guys figure that if the CaptiveAire makeup air DX conditioner is rated at 5 tons, then I could run a load calc including the 20% difference between make up and exhaust as ventilation (forgetting about interaction between the unconditioned makeup and conditioned inside air) then take the outputted figure (roughly 7 - 10 tons depending on glass, orientation, and location) and add 5 tons to it (to cover the CaptiveAir 5 ton makeup conditioner unit) ?
I don't "like" that scenario, because it makes more sense to me to condition that makeup air at it's source and the standard RTU is going to have to work much harder to compensate....but using the conditioned makeup system hasn't received a warm reception with these guys.
thanks for all the help!
My answer to them not goiung for conditioned air is to take them to C.barrel and then to Golden C and let them decide. Also, if their not satisfied with what they have isn't your job to change things? So 18 tons or conditioned air. Captive Aire's makeup unit is a box with one filoter wall on the roof sucking in 130F in the summer and 20F in the winter. Got o a couple of their existing stores and take some measurmeny=ts to plug into your calcs.
Ok, I talked to by brother who does these calculations all the time. He says the exhaust should be 10% more than the make up. You always have a slightly positivwe air flow from dining to the kitchen, you add 25% after you calculate the load as a regular hvav space. We've been in the business for 30 years and this is the way Mom& Pops are calculated by us. It works. Hopes this helps
The numbers from CaptiveAire are supply air @ 20% less than exhaust....and there's not much I can do to change them on that. They've engineered ththat hood system...
When you say to "add 25% after you calculate the load as a regular hvac space" you mean to add an additional 25% to the final cooling BTU / tonnage?
How much ventilation is being considered there? Is it only the 20% that is being supplied through the RTU fresh air?
Depending on how much of the makeup air is calc'ed as ventilation it makes a HUGE difference in the result
thanks for all the help man! You've got me thinkin through this stuff pretty thoroughly
The exhaust and makeupair fan speeds are both adjustable. If you use the 20% number the flow from dining will be stronger unless you are offsetting it with a source you haven't mentioned. The makeup air is not calced as ventilation as the hood/makeup air is considered a closed system and the makeup air is negative as it pertains to the air being exhausted. The extra fresh air through the RTU should be considered as to load on A/C and heat. 130F in summer and 20F in winter. These temps are for N. Alabama and will change with location in the country. The summer won't change much, but the winter cold air load can change considerably the farther north you go.
Originally Posted by theWilly
That makes good sense.
These stores are primarily takeout and rarely have a dining area. They try to use a single RTU where possible to save build costs...granted the operation costs will be higher. When there is a dining area they've used 2 RTUs
The 20% difference between exhaust and makeup is covered by the RTU outside air supply.
The 130F summer temp is surprising to me though...but makes sense. Its like a built in worst case scenario / safety factor? And also covers for high roof top temps for that fresh air.
So...it sounds like through this method the exhaust air and oven heat is covered in the calcs.
Do you guys still add heat load for refrigerators and heating racks, etc?
thank you very much for all the info! Its very helpful
Originally Posted by theWilly
I have installed quite a few exhaust/makeup systems both Mom and Pop and Corp[orate sometimes with hvac sometimes not. if there are specs for in and out pressures or equalization we use them to set the systems up, but they are usually altered during the punch lists. 130F is an average that might even be low; the worst case is 150F July/August and aparapet roof can add 10 to 20 degrees as can roof color. !30F starts the last ten days of May to end of June. It goes down quicker in the fall although this year add tendegrees for the near 100F for weeks. I am this accurate because I have thermometers out while setting walkins and hvac pressures and I've done alot of sweating and measuring over 30 yrs.
Yes we add heat load for any unit that exhausts or creates heat and quote those numbers in our bids to show reasons for tonnage. For some reason most engineers overlook the electronics heat in the office ( computers, camera systems Muzak. These little offices have a large amount of heat source and while there is usually a supply register there is rarely a return.
Sorry about all this info but I've developed a few pet peeves about building design over the years; enginerrs cutting corners for end price or just not thinking the design throughI'm not sure, but if you cover all the factors include the data or your bid may be out of line. or me considering all factors and showing it would put your bid out front as they weren't satisfied with what they had and you will be showing why. Good Luck!