High Temperature Hot Water (HTW) Systems. HTW systems are designed
to operate at export temperatures of 350 to 420 degrees F (177 to 215.5
degrees C). Generally 400 degrees F (204.5 degrees C) is considered as a
maximum design temperature due to high pressures and the relative costs to
achieve pressurization. The system pressure must be at least 25 psi (172 kPa)
above the saturation pressure of the HTW maximum temperature to prevent pump
cavitation and flashing of superheated water to steam. A system operating at
a maximum temperature of 350 degrees F (177 degrees C) requires at least 160
psia (1104 kPa) pressure and a system operating at a maximum temperature of
400 degrees F (204.5 degrees C) requires a minimum pressure of 275 psia (1898
kPa). The maximum system temperature drop from export temperature to return
temperature should be 100 to 150 degrees F (55.5 to 83.33 degrees C). High
temperature differentials in the HTW system can reduce distribution piping
sizes and distribution pumping costs by requiring less water flow for the heat
required. However, as the temperature differential is increased the system
heat storage capacity is reduced.
Medium Temperature Hot Water (MTHW) Systems. MTHW systems are
designed to operate at an export temperature between 250 and 350 degrees F
(121 to 177 degrees C), with a maximum temperature drop of 75 to 100 degrees F
(41.7 to 55.6 degrees C). The system pressure must be pressurized at least 25
psig (172.5 kPa) above the saturation pressure to prevent pump cavitation and
flashing to steam.
Low Temperature Hot Water (LTHW) Systems. LTHW systems are
designed to operate at an export temperature between 150 and 250 degrees F
(65.6 and 121 degrees C). Systems above 180 degrees F (82.2 degrees C) must
be pressurized to prevent pump cavitation.
Types of Systems
a) Forced circulation hot water generators, separate expansion
b) Forced circulation hot water generators, separate expansion
vessel, steam pressurization, double pump. See Figure 3, at low loads,
pressurization is greatly effected by increased temperatures.
c) Forced circulation hot water generators, separate expansion
d) Forced circulation hot water generators, separate expansion