Handbook of air conditioning and refrigeration / Shan K

7.28 CHAPTER SEVEN
Changeover
Two Pipe or Four Pipe. In a dual-temperature water system, the water piping from the boiler or
chiller to the coils and the terminals, or to various zones in a building, can be either a two-pipe system,
with a supply main and a return main, as shown in Fig. 7.10a; or a four-pipe system, with a hot
water supply main, a hot water return main, a chilled water supply main, and a chilled water return
main, as shown in Fig. 7.10b. For a two-pipe system, it is impossible to heat and cool two different
coils or terminals in the same zone simultaneously. Changeover from summer cooling mode operation
to winter heating mode operation is required. A four-pipe system does not need changeover operation.
Chilled and hot water can be supplied to the coils or terminals simultaneously. However, a
four-pipe system requires a greater installation cost.
Several decades earlier, there was also a three-pipe system with a hot water supply main, a chilled
water supply main, and a common return main. ASHRAE/IESNA Standard 90.1-1999 clearly specifies
that hydronic systems that use a common return system for both hot water and chilled water shall
not be used. This is because of the energy loss during the mixing of the hot and chilled water.
In a dual-temperature two-pipe system, changeover refers to when the operation of one zone or the
entire water system in a building changes from heating mode to cooling mode, or vice versa. During
changeover, the water supplied to the terminals is changed from hot water to chilled water, or
vice versa. The changeover temperature T co, °F (°C), is the outdoor temperature at which the space
sensible cooling load can be absorbed and removed by the combined effect of the conditioned
outdoor air, the primary air, and the space transmission and infiltration loss. Such a relationship can
be expressed as:
K � 60� soc pa
T co � T r � �Q ris ��Q res � KV˙ so(T r � T so)
q tl
where Tr � space temperature, °F (°C)
sum of internal sensible loads from electric lights, occupants, and appliances,
Btu/h (W)
sum of external sensible loads through building shell, Btu/h (W)
volume flow rate and density of conditioned outdoor air, cfm (m3 /min) and lb/ft 3
(kg/m 3 �Qris �
�Qres �
V˙
so, �so �
)
cpa � specific heat of air, Btu/lb�°F (J/kg�°C)
Tso � supply temperature of outdoor air or primary air, °F (°C)
qtl � transmission and infiltration losses per 1°F of outdoor-indoor temperature difference,
Btu/h�°F (W/°C)
Changeover usually takes from 3 to 8 h to complete. The greater the size of the water system, the
longer the changeover period. To prevent more than one changeover per day, the changeover temperature
T co may have a tolerance of � 2°F (1.1°C).
Changeover may cause a sudden flow of a large amount of hot water into the chiller or of chilled
water into the boiler. Such a rapid change in temperature imposes a thermal shock on the chiller or
boiler and may damage the equipment. For chillers, the temperature of water entering the chiller
should be no higher than 80°F (26.7°C) to prevent excessive refrigerant pressure in the evaporator.
For boilers, a temperature control system bypasses most of the low-temperature water until the water
temperature can be gradually increased.
Changeover may be performed either manually or automatically. Manual changeover is simple
but may be inconvenient during periods when daily changeover is required. With sufficient
safety controls, automatic changeover reduces the operating duties significantly. A compromise is
(7.5)