15 May 2003
BOILERS. This section is intended for gas- and oil-fired low-temperature hot
water boilers and low-pressure steam boilers that are used primarily for single building
Multiple Boiler Installations. Provide adequate room, connections, piping,
etc., in boiler installations where future expansion is likely. Determine the number and
size of boilers to efficiently serve both the maximum winter design load and the
minimum summer load. With the largest boiler off line, the remaining boiler(s) will be
capable of carrying not less than 65 percent of the maximum winter design load. Where
the smallest boiler installed has a capacity of more than twice the minimum summer
load, consider adding an additional boiler or hot water heater sizes for the anticipated
5-3.2.1 Provide heaters for the de-aeration of feedwater for all boiler installations with
steam capacities in excess of 20,000 MBtuh (6,000kW). Evaluate the benefits for
heaters for the de-aeration of feedwater for all boiler installations of 15,000 MBtuh
(4,500 kW) to 20,000 MBtuh (6,000 kW) capacity where estimated makeup rates are 15
percent or more or where the plant serves a number of buildings. Installations using
either hot lime-soda or hot lime plus hot sodium zeolite softeners to de-aerate treated
water and condensate require no additional de-aeration.
5-3.2.2 Provide steel surge tanks for the storage of condensate. Install the surge
tanks upstream of the feedwater heaters where the space-heating load predominates,
where large quantities of condensate are returned by condensate pumps, and where
steam-driven auxiliaries are used. Size surge tanks for 20 minutes of condensate
storage based on boiler steaming capacity.
5-3.2.3 Feedwater flow rate to the heater should equal the boiler demand. Size
feedwater pumps 10 percent larger than the capacity calculated to allow for pump
5-3.2.4 Install feedwater heaters above the boiler feed pump suction at a height
sufficient to prevent flashing at the pump inlet at the design feedwater temperature.
With a feedwater temperature of 212 degrees F (100 degrees C) and a back pressure
on the feedwater heater of approximately 2 psi (14 kPa), the hydrostatic head on the
feed pumps should be approximately 10 feet (3 meters). Where the boiler feed
operates at temperatures above 212 degrees F (100 degrees C), increase the
hydrostatic head in proportion to the increase in the vapor pressure of the liquid.
Provide a bypass and isolation valves for each feedwater heater to permit operation of
the boilers at times when the heater is being serviced.
of combustion equipment,
including burners and draft fans, shall be in accordance with ASHRAE Handbooks,
Underwriters Laboratory (UL), National Fire Protection Association (NFPA), and the
recommendations of equipment manufacturers. Note that ASHRAE 15 prohibits open
flame in machinery rooms where refrigerants are used.