Ps @ Low Flow

Ps @ Design Flow

Pmax

Pr @ Low Flow

Pr @ Design Flow

P min Supply

Pmin Return

Distance from Plant

All the calculations made to check for absolute pressure constraint satisfaction

should use the maximum (design) mass flow rate. This will assure that constraint

satisfaction will be possible at all flow rate conditions. Since the pressure losses in

the piping and the consumer's heat exchanger will be greatest under this load

condition, the difference between supply and return pressure at the heating plant

will also be greatest under this load condition. Thus, under this condition the least

flexibility exists to adjust the supply or return pressure at the plant without violating

either the maximum pressure constraint in the supply, eq 4-11, or one of the

minimum return line pressure constraints at the plant, eq 4-24 and 4-25. If the

various maximum and minimum pressure constraints are satisfied for all points in

the network at the higher flow rate condition, it will always be possible to satisfy

them at the lower flow rates. This is easily shown graphically by considering the

pressures in the system along the piping route out to a consumer and back, as shown

for a hypothetical consumer in Figure 8.

In Figure 8 the horizontal lines are the constraints on the absolute pressures that

must be satisfied at all points along the route to the consumer. The solid lines that

have both positive and negative slopes are the supply and return pressures under

maximum load conditions. The magnitude and the sign of the slope of these lines are

determined by the sum of the hydrodynamic and hydrostatic pressure gradients as

given by eq 4-14. The dotted lines that behave in a similar fashion are the supply and

return pressures under some mass flow condition that is lower than the maximum.

In the extreme case where there is no flow, the pressure losses in the piping and

consumer equipment all vanish and the absolute pressure level is identical in the

supply and return lines for any point along the route. Also notice that we have

shown the pressure drop at the consumer as being lower at the reduced flow

condition. This results from lower pressure losses in the consumer's heat exchanger

at the reduced flow rate (see eq 4-6) as well as lower losses in the consumer's control

valve. If the network were ideally balanced and this consumer were the critical

consumer, his control valve would be completely open at all levels of load (i.e., flow

rate) and the pressure losses would always be the minimum possible.

By studying Figure 8, we can see that if we are able to "fit" the supply and return

absolute pressure lines within the constraints at the maximum flow condition, then

we can always do so for any lower flow condition simply by adjusting the absolute

pressure of either the supply or return at the heating plant. This results from the

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