(C)
(F)
(lbm/s)
(kg/s)
0.20
100
Design Conditions
0.4
203
0.179
95
0.396
200
90
0.15
0.3
180
80
0.10
0.2
160
70
0.05
0.1
Flow Rate
Temperature
140
60
Modulation
Modulation
0
7.49 10
0
20
30 (kW)
103 (Btu/hr)
20 26
0
40
60
80
100
Used Load (kW)
Figure 1. An example of temperature and flow rate modulation.
achieve temperature differences of 55C (100F), just
can take place as portions of the distribution system
are replaced, thus making the economics more at-
like medium and high temperature water systems.
tractive.
Low temperatures and pressures make it easier to
Despite their widespread acceptance in Europe
control the building space heating and domestic hot
and several successful recent installations in the
water heating systems; thus, it is easier to maintain
United States, low temperature hot water systems
significant temperature differences between supply
are still associated with a number of myths. Some of
and return even in times of lower load. In larger
the more commonly held misconceptions are de-
systems, the flow rate is usually modulated along
scribed below.
with the temperature in order to meet the varying
Low temperatures are not suitable for large systems.
load conditions while keeping the cost of pumping
One only needs to look at some of the enormous
and heat losses as low as possible. The temperature
systems in Europe to be convinced that this is clear-
modulation is done in a manner similar to the way
ly not the case. For example, all of Copenhagen and
the temperature of hydronic heating systems in
the surrounding towns are tied into one big low
buildings are reset based on outdoor air temper-
temperature system. St. Paul, Minnesota, has a large
ature. An example of temperature and flow rate
system and several other towns in the United States
modulation is given in Figure 1 (from Phetteplace
have fairly large systems. Because of the high effi-
and Labbe 1978).
ciency of urethane insulation and the lower tem-
Even when temperature differences between sup-
peratures, heat losses are low (approximately 5% of
ply and return are somewhat smaller, pipe sizes do
system capacity) and significant temperature drops
not need to be increased significantly. For example,
during transport are simply not a problem. For ex-
consider the Ft. Irwin LTHW system and the Ft.
ample, consider the 150-mm (6-in.) LTHW system
Jackson MTHW system, which will be discussed in
at Ft. Irwin, discussed in more detail later. The aver-
more detail later. Assume a flow velocity of 1.5 m/s
age heat loss from the supply pipe is only 20 W/m
(5 ft/s) in each system and a temperature difference
of 36 C (65F) for the LTHW system and
(21 Btu/hr-ft) (Phetteplace 1992). If a modest flow
55C(100F) for the MTHW system. The heat con-
velocity of 1.5 m/s (5 ft/s) is assumed the water in
this supply pipe would experience a temperature
veyed by the MTHW system with 125-mm (5-in.)
drop of only about 0.3C (0.6F) over a transport
piping is about 3.81 MW (13.0 MBtu/hr). For the
distance of one mile.
LTHW system with 150-mm (6-in.) piping the heat
Low temperature systems require much larger piping.
conveyed is only slightly lower at 3.72 MW (12.7
As stated earlier, for a given flow rate, the amount
MBtu/hr). Despite the fact that the flow rate in the
of heat conveyed depends only on the temperature
LTHW system would need to be about 50% higher
difference between supply and return, not the abso-
than for the MTHW system, the pumping energy
lute value of the supply temperature. With proper
costs would only be about 25% higher as a result of
design, low temperature water systems can and do
the larger pipe size. Pumping energy is usually a
4
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