a. Supply and Return Temperature. A supply

temperature limit of 440 degrees F. is generally

found to be the economic limit for space and process

heating because of the high pressures (400 psig)

required. Higher supply temperatures require rapidly

increasing pressures throughout the system,

and while high pressures result in higher heat carrying

capacities in smaller pipe sizes, the saving in

pipe size is partially or fully offset by the expense

of generators, fittings, pipelines and heat exchangers

strong enough to withstand higher pressures.

Return water temperatures should be between 250

degrees F. to 275 degrees F. minimum. Primary

fuels or alternate fuels with high sulfur content

can cause generator tubes to corrode. The selection

of temperatures in the hot water generator during

all operating conditions should be given special

considerations with high sulfur content fuels.

When flue gas is cooled below the dew point, the

gas side of boiler tubes is subjected to corrosion.

Corrosion is caused by moisture in the flue gas

and acids which result from the combination of

condensed moisture and sulfur compounds in the

flue gas. The higher the percent by weight of

sulfur the higher the minimum metal temperature

must be to prevent this type of corrosion. When it

is desired to transfer very large heat quantities

over unusually long distances, an increase in the

generator pressure to 600 psig corresponding to a

supply temperature of about 480 degrees F. might

be justified. For distances up to six miles, the highest

supply temperature will be selected below 440

degrees F. The generator supply temperature is

always maintained somewhat above the temperature

of the water distributed to the system. In

steam pressurized systems this is necessary to prevent

vaporization from taking place if the pumps

should suddenly be stopped and where the heat

users are located at elevations higher than the

generator. The HTW system supply temperature is

maintained by blending the generator output

water with the system return water at the pump

suction header. The HTW generator water is proportioned

through an automatic mixing valve. Due to heat losses

from the distribution system piping,

the supply temperature at the use points will be

reduced by approximately three to five degrees per

mile, depending upon the insulation efficiency and

the amount of water being circulated. The faster

the water is circulated, the less the temperature

drops. Process equipment using either direct high

temperature water or steam produced by converters

generally requires temperatures at the use

points ranging from 250 to 400 degrees F. For example,

some laundry drying equipment requires

steam pressures of 100 psig which necessitates

high temperature water of more than 355 degrees

F. Therefore a heating installation which includes

such laundry equipment will need a supply temperature

of at least 375 degrees F. to provide for

temperature losses and use point requirements

and a generator pressure of no less than 175 to 200

psig. It often happens that while temperatures as

high as these are not needed for space heating,

they are economical supply temperatures to use

for lower temperature applications because smaller

pipe sizes are required. All HTW systems will

be designed for a minimum of 150 degrees F. differential

between supply and return water temperatures.

 

b. Operating Pressure. This pressure is directly

related to the operating temperature which is essentially

equal to saturation temperature in the

expansion drum in a steam-pressurized system and

slightly higher in an inert gas-pressurized system.

Pressure head to overcome frictional and other

losses must be added to the operating pressure

when selecting pumps, piping, fittings, and other

system components. The pumps should never be

used as part of the pressurization system.

Source: U.S. Army, Corps of Army Engineers