PipeNet-Cooling Water Systems Part B

Balancing the Flow

The requirements of the heat exchanger and the pressure availability are shown in the following tale:

The Heat exchanger “DE01” works exactly as it should. The flow rate is 250 US gpm and the pressure loss is 5 psi. This is because the flowrate in that leg of the system is 250 USgpm and DE01 will give a pressure loss of 5 psi at that flowrate. So no change from a flow balancing point of view needs to be made.

The Heat exchanger “DE02” deviates very slightly from the required performance. The flow rate is 300 USgpm by virtue of the specification but the pressure difference available is 5.682 psi (64.408 – 58.726). This is only slightly more than the required pressure loss. So it is a matter of engineering judgement as to whether or not a restriction orifice plate is needed in this line. In order to achieve an ideal system we will place a restriction orifice plate in our case.

The Heat exchanger “DE03” on the other hand has requires a pressure loss device. The pressure difference available is 6.696 psi as opposed to a requirement of 4.6 psi. The reason it is important to place this pressure loss device in this line is the following. If this pressure loss is not used then this heat exchanger is liable to drain the system starving the other heat exchangers of water. (An alternative way of rectifying this problem is to get a bigger pump. In addition to the additional cost, it is also possible that heat exchanger “DE03” now gets more water than desired). One option is to place an orifice plate in front of heat exchanger “DE03” which creates a pressure loss of 1.096 psi.

Two Discussion Points:

· Although we are planning to place restriction orifice plates in the required parts of the system, what we are really trying to do is to optimise the system so that we are able to determine the smallest pipe sizes and the smallest pump. So in the actual detailed design of the system we may in fact use throttle valves instead of orifice plates. What the calculation shows is the actual design performance of the pump which would exactly meet the requirements of the heat exchangers, if the throttle valves were set to achieve exactly the required flowrates through the heat exchangers.

· The question arises as what it means if the available pressure drop for a heat exchanger is less than the pressure drop the heat exchanger would produce. For argument’s sake let us suppose that DE02 had an available pressure drop of 5 psi instead of the required value of 5.5. psi. This just mean that the connecting link in the system would be DE02, rather than DE01. The network would look as shown below. This would ensure that the requirements DE02 are exactly met.

Now we go back to placing orifice plates to balance the flow.

We place two orifice plates.

The Properties dialog box for the two orifice plates are shown below.

We note that the required diameters of the orifice plates are 90.0811 mm and 72.1387 mm. We enter this as the diameter of the orifice plate instead of the required pressure drop. (If we do not do this, the diameter could change between calculations).

As a check, we make sure that the results are the same with the orifice plate diameter fixed.

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