BTU Ratings – Estimating
Often for old gas-fired equipment, BTU input labels are long gone. Also, a lot of gas-fired equipment is custom made, and there may never have been an actual label attached to the equipment. A good estimate is better than nothing, and may be the only option available for estimating gas usage according to an input formula.
Techniques for Making Good Estimates
All of the following techniques become easier with a little experience and familiarity with equipment. It will generally be a combination of more than one of these techniques that will help make a good estimate. They are presented in no particular order of importance or order of action; use what works for the situation.
- compare the unknown equipment with known equipment in terms of physical size, production capacity, etc
- look at the physical size of the burner and compare it to burners of known sizes
- look at the size of the gas pipe that serves the burner and compare it against the size of the gas pipe that serves known size equipment; the larger the pipe, the larger the input
- consider the natural gas delivery pressure; the higher the pressure the higher the gas usage and the smaller gas pipe that can be used to serve it
- look at gas pressure regulators to see if they are stamped with an orifice size; the larger the orifice, the larger the gas load
- consider the size of the gas meter that serves the load. It can be assumed that at the time of installation, someone knew the gas rating and correctly sized the facilities to serve it; or, even if they just got lucky, if the equipment is working, it can be assumed that the load is smaller than the maximum capacity of the gas pipe, regulator and meter that serves it.
- write down the name and as much info as can be found in order to perform a web search; if the manufacturer is still in business, chances are good that a model number or serial number can be used to estimate the ORIGINAL BTU rating of the equipment
- if possible, isolate the load and clock the gas meter; this is not practical in most facilities for checking individual loads. However, a facility may have a boiler house served by a dedicated gas meter. When the boiler(s) is firing, clock the meter and note the firing rate of the boiler (High, Low, etc) and how many boilers are on.
- check inspection and repair records – boiler inspection records generally do NOT have the boiler rating on them; however, some maintenance or repair records might contain a horse power or steam rating. If possible, talk to the person who maintains the equipment; they may have a good estimate if not the actual number.
- consider the production capacity of the equipment; for example, a parts washer that heats 10,000 pounds per hour of steel parts to 180F must have a BTU input rating of at least 206,000 BTUs. (10,000 lbs steel x 0.12 specific heat of steel x 120F Temperature Rise / 70% efficiency)
Source: Text Bob Fegan 1/2009