heating water

Figure 3-5
Figure 3-6
internal coil heat exchanger
circulator
heat
source
indirect
water
heater
Forced convection
(inside coil surface)
Natural convection
(outside coil surface)
Forced convection
(inside tube surface)
Natural convection
(outsside tube surface and fin surface)
warm air out
hot
water
in
cool air in
finned-tube baseboard
warm
water
out
aluminum
fins
at higher flows. This demonstrates that there are practical
limits on how much the heat output of a hydronic heat
emitter can be increased based on higher flow rates.
NATURAL vs. FORCED CONVECTION
When fluid motion is caused by a circulator, a blower, a fan
or any other powered device, the convective heat transfer
is called “forced convection.” When the fluid motion is
strictly the result of buoyancy differences within the fluid,
the convective heat transfer is called “natural convection.”
There are many types of heat exchangers that operate with
natural convection on one surface and forced convection
on the other side of that surface. One example is at the
external surface of an internal coil heat exchanger within
an indirect water heater. Another is the external surface of
finned-tube baseboard, as shown in Figure 3-5.
Natural convection is typically a much “weaker” form of
heat transfer compared to forced convection. This is the
result of much slower fluid motion created by buoyancy
differences in the fluid versus faster fluid motion created
by a circulator or blower. ⎛ Slower fluid motion increases
boundary layer thickness, q = A
k ⎞
which creates greater resistance
to heat transfer between ⎝
⎜
∆the x ⎠
⎟ (∆T )
bulk of the fluid and the
surface.
The difference between forced convection and natural
convection explains why a small wall-mounted fan-coil
that’s only 18 inches wide can provide the equivalent
q = A heat output of 10+ feet
R
of (∆T finned-tube ) baseboard when
both are operating at the same water supply temperature
and flow rate. The rate of heat transfer from the finnedtube
element in the baseboard is limited by natural
convection heat transfer between its outer surfaces and
the surrounding air.
The rate of convective heat transfer can be estimated using
Formula 3-4.
Formula 3-4:
i29 formulas
R = ∆ x
k
q = hA(∆T )
Where:
q = rate of heat transfer by convection (Btu/hr)
h = convection coefficient (Btu/hr•ft 2 •ºF)
A = area over which fluid contacts a surface with which it
exchanges heat (ft 2 )
∆T = temperature difference q = sAF T 4 4
12 ( − T
1
between bulk 2 )
fluid stream and
surface (ºF)
⎡ 1
+ 1 ⎤
⎢ −1⎥
⎣ e 1
e 2 ⎦
Although Formula 3-4 is relatively simple, determining the
value of the convection coefficient (h) is often a complex
30
Re# = vdD
µ