heating water

Figure 2-5
to / from load
internal coil
(copper or
stainless steel)
∆P valve
ASSE 1017
thermostatic
mixing valve
DHW
thermal
storage
tank
expansion
tank
cold
water
loading
unit
pellet boiler
CW
DOUBLE WALL COIL HEAT EXCHANGERS
Some mechanical codes require a special type of coil
heat exchanger when the application requires transferring
heat from an antifreeze solution to potable water. That
coil must be “double walled.” There must be a partial air
gap between the metal wall that receives heat from the
antifreeze solution and the metal wall that transfers heat
to potable water. This air gap creates a “leakage path”
that would allow a leak in either metal wall to exit the heat
exchanger outside of the tank, and thus provide visible
evidence of the leak. Figure 2-7 illustrates the concept.
Double wall heat exchangers create lower heat transfer
rates compared to single wall coils of equivalent surface
area. They also tend to have higher pressure drops at a
given flow rate relative to single wall heat exchangers.
Opinions vary on their use, as do mechanical code
requirements on when and where they must be used.
Internal coil heat exchangers, typically made of copper
tubing, are also used for heat input and heat extraction
from large non-pressurized thermal storage tanks often
used in systems supplied by cordwood gasification
or pellet-fired boilers. Figure 2-8 shows a typical
configuration.
In some systems, one suspended coil is used for heat
input while another is used for heat extraction. This is
the scenario shown in Figure 2-8. Notice that the flow
direction in each coil is opposite the direction in which
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