Owner's Manual

33
3.4 TEMPERATURE DROP—Selection of temperature
drop has received attention in recent years as a means
of reducing piping or pumping costs. Over several
previous decades the 20°F water temperature drop had
been standard for the hydronics industry. Recently,
temperature drops of 30°F, 40°F and even 50°F have
been used successfully when the distribution system
and terminal units are properly sized for these larger
temperature drops. In new construction it is advisable
to consider the savings in materials that can be made
by designing with a temperature drop larger than 20°F.
3.4.1 Figure 3-5 is a typical friction-velocity-ow diagram
used by most designers of large systems. The lower
scale, Heat Conveyed, is based on a 20°F temperature
drop. However, the ow rate in gpm is shown on
the upper scale, and can be used to size pipe at other
temperature drops by converting heat conveyed to
ow rate in gpm.
Example: Find the pipe size required to convey
1,000,000 Btuh in iron pipe at a friction loss of 500
mill inches/ft. and temperature drops of 20°F, 30°F
or 40°F.
Solution: The gpm ow rate for 1,000,000 Btuh is
found by dividing:
1,000,000 by (500 x 20) = 100 gpm for 20° drop
1,000,000 by (500 x 30) = 66.7 gpm for 30° drop
1,000,000 by (500 x 40) = 50 gpm or 40° drop
Enter Figure 3-5 on the horizontal line for 500 mill
inches per ft. and read across to the right to the
vertical lines for 100 gpm, 66.7 gpm and 50 gpm.
On the slanted lines read the corresponding pipe
sizes (use the larger if between two pipe sizes)
100 gpm = 3” Pipe
66.7 gpm = 2½” Pipe
50 gpm = 2½” Pipe
Figure 3-6 can be used in a similar manner for
copper pipe.
3.4.2 The size of the terminal units (baseboard, convectors,
fan coils, etc.) must be adjusted according to the
actual temperature of water owing in those units. In
general, the rst terminal unit on a circuit will receive
hotter than average water and should be undersized,
and the last terminal unit will receive cooler than
average water and should be oversized. The designer
should consult a sizing procedure such as that
contained in the ASHRAE Guide or I=B=R Guide
#250.
3.4.3 It should be noted that the selection of system
temperature drop has no effect on the sizing of the
boiler.
3.4.4 On remodeling jobs it is generally too expensive to
modify the terminal units for temperature drops other
than that used by the original system designer. It
is not “safe” to assume that the original design was
based on 20°F drop and thus the owner’s records
should be consulted.
3.5 MAIN PIPING—Selection of Main Size and the
system pump must go together. The system designer
can select the pump and size the pipe accordingly,
but more often the best economics of pipe and pump
causes the system designer to select the minimum
pipe size based on a maximum pressure drop and
then select a pump(s) to meet ow and pressure drop
requirements of the total system. It is recommended
that pipe sizes be selected in the unshaded portions of
Figure 3-5 or 3-6. The minimum pipe size will occur
on or close to the upper limit of the unshaded areas.
Example: Find the minimum main size and
corresponding friction for three 808HE modules using
iron pipe and a 20°F temperature drop.
Solution:
1) The output of three 808HE modules is 3 x 410 x
.80 = 984 MBH. Refer to Figure 2-3 for module
input and Figure 2-1 for input to output multiplier.
2) Enter Figure 3-5 on the lower horizontal scale at
984 MBH and move vertically to the upper limit of
the unshaded area.
3) On the lines that slant upward to the right, read the
pipe size. In this case, the pipe size is greater than
2½” but less than 3”. Select the larger of 3”.
4) From the point in 2) above move down vertically
to the 3” pipe line and horizontally to the left hand
scale. Pick off 300 mill inches per foot friction.
3.5.1 In calculating the total equivalent length of pipe it
is necessary to consider the additional resistance of
elbows. Figure 3-7 shows the equivalent lengths.
The total equivalent length of pipe in a circuit is the
measured length plus the equivalent length of all
elbows in that circuit. The total equivalent length
of the longest circuit in the system is useful in
determining the head requirement of the system pump.
3.6 COMPRESSION TANK—Selection of the
compression tank must be based on the following
items:
a) volume of water in the system
b) initial ll pressure of the system
c) maximum operating pressure of the system
d) maximum operating temperature of the
system
3.6.1 It is necessary to calculate the volume of water
contained in the total system including piping,
modules and terminal units. Figure 3-8 can be used
to determine the volume of the piping by measuring
the length of each size of pipe and multiplying by the
appropriate factor from Figure 3-8.
Example: Find the water volume in the piping of a
system having 40’ of 3” pipe, 72’ of 2” pipe, and 52’
of 1¼” pipe.
Solution: From Figure 3-8 obtain the gallons/ft.
from each size of pipe and multiply by the length of
that size of pipe.
1¼” Copper = .065 gal/ft x 52 Ft = 3.4 Gal.
+ 2” Copper = .161 gal/ft x 72 Ft = 11.6 Gal.
+ 3” Copper = .357 gal/ft x 40 Ft = 14.3 Gal.
Total volume in piping = 29.3 Gal.