I have a customer who wants to increase the tonnage on his walk in.currently it has a 7.5 ton & 5 ton the dimensions are 70' x 20' x 10' he s wanting to replace the 7.5 with a 10 ton. it s a meat locker they try and hold it at -10.the 5 ton is actually a back up to the 7.5. I m not experienced in sizing freezers. 10 tons = 120,000BTU/hr. From 8am to 5pm there s alot of traffic from customers so the rate of air change is triple that of normal walk ins. I guess what I m wanting to know is the formular for sizing the equipment. 14,000 ft/3 , @ 90 degree outside ambient, -10 to 0 range.any help would be appreciated.