Boiler Rating Method
The construction of low-pressure steel and cast-iron heating boilers is governed by the requirements of the ASME Boiler and Pressure Vessel Code. This is a nationally recognized code used by boiler manufacturers, and any boiler used in a heating installation should clearly display the ASME stamp. State and local codes are usually patterned after the ASME Code.
The location of the identification symbols used by the ASME is specified by the code and determined by the type of boiler. For example, on a water-tube boiler, it appears on a head of the steam- outlet drum near and above the manhole opening. On vertical fire- tube boilers, the stamp bearing the identification symbol should appear on the shell above the fire door and handhole opening. Other types of boilers (e.g., Scotch marine and superheaters) have their own specified location for the identification symbol stamp.
The ASME Boiler and Pressure Vessel Code applies only to boiler construction, specifically to maximum allowable working pressures, not to its heating capacity. A number of different methods are used to rate the heating or operating capacity of a boiler. The boiler manufacturers have developed their own ratings, but these are generally used along with rating methods available from several professional and trade associations.
The Steel Boilers Institute no longer exists, but its SBI rating is still found on many existing steel boilers. The I=B=R (or IBR) logo was created by the now defunct Institute of Boiler and Radiator Manufacturers to indicate the gross output(s) at 100 percent firing rate for most sectional cast-iron boilers. The I=B=R rating logo is now used by the Hydronics Institute Division of the Gas Appliance Manufacturers Association (GAMA).
The Mechanical Contractors Association of America has devised a method for rating boilers not covered by either the SBI or I=B=R codes. Finally, gas-fired boilers are rated in accordance with meth- ods developed by the American Gas Association.
Other rating logos appearing on boilers and in their installation and operation manuals are the Underwriters Laboratories, Inc. (UL) and the Underwriters’ Laboratories of Canada logos.
In terms of its heating capacity, the rating of a boiler can be expressed in square feet of equivalent direct radiation (EDR) or thousands of Btu/h. Sometimes a boiler horsepower rating is also given, but this has proven to be misleading.
For steam boilers, 1 square foot of equivalent direct radiation (EDR) is equal to the emission of 240 Btu/h. For a water boiler, 1 square foot of EDR is considered equal to the emission of 150 Btu/h.
A boiler horsepower (bhp) is the evaporation of 34.5 lb of water into dry steam from and at 212°F. For rating purposes, 1 bhp is considered as the heat equivalent of 140 ft2 of steam radiation per hour. In some cases bhp ratings are obtained by dividing steam SBI ratings by 140.
A boiler is rated according to its operating or heating capacity, but this rating will vary in accordance with the type of load used as the basis for the rating. The three types of connected loads used to determine the rating of a boiler are:
1. Net load
2. Design load
3. Gross load
Net load refers to the actual connected load of the heat-emitting units in the steam or hot-water heating system. Design load includes the net-load rating plus an allowance for piping heat loss. Finally, gross load will equal the net load and the piping heat loss, plus an additional allowance for the pickup load.