Handbook of air conditioning and refrigeration / Shan K

8.26 CHAPTER EIGHT
For small low-temperature hot water heating systems, an open expansion tank is usually used.
For medium-size and large systems, a diaphragm tank may be more suitable. Circulating water
pumps are often on-line pumps (circulators) with low pump head.
Part-Load Operation and Control
In a low-temperature hot water heating system, one of the basic part-load controls is the variation of
the hot water supply temperature from the boiler in response to a variation in outdoor temperature.
For instance, in the midwest, a low-temperature hot water heating system has a winter design
outdoor temperature T o � 0°F (� 17.8°C). At the winter design outdoor temperature, the hot water
supply temperature T ws is 190°F (88°C). When the outdoor temperature drops, T ws is reset as follows:
A hot water temperature sensor located at the hot water exit of the boiler, whose set point is
reset by an outdoor temperature sensor, is used to control the firing rate of the boiler by means of a
DDC unit. Zone control can be performed better by sensing the hot water temperature that returns
from each individual loop and then modulating the control valve to vary the mass flow rate of hot
water supplied to that zone through an on-line circulating pump (see Fig. 8.7).
ASHRAE/IESNA Standard 90.1-1999 specifies that a hot water system with a design heating
capacity exceeding 300,000 Btu/h (88 kW) shall include controls to automatically reset to a lower
hot water supply temperature according to building loads or outdoor temperature during part-load
for energy saving except for systems that use variable flow to reduce pumping energy.
For a low-temperature hot water heating system installed with multiple boilers, the control strategy
is to decide when to turn a boiler on or off. This optimum operation control can be accomplished
by using a microprocessor-based controller that fires a standby boiler according to a preprogrammed
software instruction. This strategy depends on not only the increase or reduction in heating demand
but also how the operating cost of such a hot water heating system can be minimized.
ASHRAE/IESNA Standard 90.1-1999 also specifies that for a boiler plant equipped with more than
one boiler, provisions shall be made so that the hot water flow in the boiler plant can be automatically
reduced when a boiler is shut down.
Example 8.1. A two-pipe individual-loop low-temperature hot water heating system is used to
heat a factory that has a layout shown in Fig. 8.7. At winter design conditions, hot water is supplied
to the heated space at a temperature of 190°F (88°C) and returns from the baseboard finned-tube
heaters at a temperature of 150°F (65°C).
1. If the space heating load for the largest room (facing north) is 90,000 Btu/h (26.4 kW) and for
the northwest corner room is 11,000 Btu/h (3.2 kW), and if steel tubes and fins are used, determine
the number of feet of finned tubing required for each of these two rooms.
2. If a pressure drop of 1 ft/100 ft (1 m/100 m) of pipe is used and the hot water system is
equipped with an open expansion tank, determine the diameter of the hot water supply main for
these two rooms.
3. Divide this hot water system into appropriate control zones, or individual loops.
Solution
Outdoor temperature To, °F (°C) Supply temperature Tws, °F (°C)
0(�17.8) 190 (88)
32 (0) 135 (57)
60 (16) 85 (29)
1. For the largest room, if a two-row finned-tube heater is used, then from Table 8.1, for an average
hot water temperature of (190 � 150)/2 � 170°F (77°C), the heat output of each foot of two-row