independently for the constraints given by eq 4-11, 4-22 and 4-23. Since verification
of satisfaction for all of these constraints requires either directly or indirectly the
calculation of the pressure losses in the supply and return pipes, we begin by doing
so for each of the pipe segments. The pressure loss in either the supply or return line
is calculated by modifying eq 2-15 slightly so that it applies to each pipe indepen-
dently. The results are
(∆Pd,s )j = a / 2 εb (4 / π)2+c (ρ1 c )d,s md+c L d (5+b+c)
(∆Pd,r )j = a / 2 εb (4 / π)2+c (ρ1 c )d,r md+c L d (5+b+c) .
Once the piping pressure losses are known, we can calculate the non-control-
valve pressure losses ∆Pncv,i for each consumer using eq 5-7 and sum this with the
minimum control valve pressure loss ∆Pcvm,i to find the consumer with the highest
value of this sum, our critical consumer. The sum of the pressure losses for this will
become our pressure increase across the pump at the heating plant ∆Php, as given by
eq 4-2. For this consumer the constraint of eq 4-5 will be active, as shown earlier.
Using the value of ∆Php calculated for the critical consumer, we can then calculate
the control valve pressure losses for all of the other consumers using eq 4-2.
With the piping and consumer pressure losses known, we can calculate the abso-
lute pressure level at all nodes in the pipe network with either a maximum absolute
pressure assigned to the supply pipe at the heating plant, or a minimum absolute
pressure assigned to the return pipe at the heating plant. If we set the minimum
pressure level in the return pipe at the heating plant, we can use the constraints of
eq 4-23, 4-24 and 4-25 to guide our choice. Note that when eq 4-23 is evaluated at the
heating plant, the entire left-hand side of the equation reduces to the value ∆Php,r.
Dependent on the particular parameter values for the problem at hand, one of these
constraints will "dominate" (see Papalambros and Wilde  for concept of
constraint dominance). The cases for constraint dominance are simply as follows. If
Pa + Pasa ≤ PNPSH ≤ Px,sat + Psaf
eq 4-23 dominates. If
Pa + Pasa ≤ Px,sat + Psaf ≤ PNPSH
eq 4-24 dominates. If
Px,sat + Psaf ≤ PNPSH ≤ Pa + Pasa
eq 4-25 dominates.
Alternately, as noted above, we can also assign the maximum absolute pressure
in the supply pipe at the heating plant and use that value to find the other absolute
pressures in the network. The logical choice for the maximum absolute pressure
value in the supply pipe at the heating plant would be the maximum absolute
pressure allowable for the piping system being used Pmax. In most cases the
maximum absolute pressure in the system will occur at the heating plant in the
supply pipe; thus, this is a logical choice. It is possible that this will not be the case,
however. Using eq 4-15 we have shown earlier that the maximum pressure must be
at a nodal point location. In the discussion after eq 4-15, we also developed a
procedure that can be used to minimize the number of nodes at which the absolute
pressure must be calculated. If this procedure is used, we can quickly determine if
the heating plant will be the location of the maximum absolute pressure.