Chilled DI Water Loop Study 03/01/2014

SUMMARY - The purpose of the study was to characterize the flow in the “Making area” Chilled/Ambient Deionized Water Loop. The study involved three activities:

Measuring the elevations and distances of the constituent piping segments in the system. Making a compilation of the flow resistances associated with each segment. Incorporating the data in a visual basic routine that generates flow rates and pressures at the

terminal ends of each segment.

The calculations were repeated varying the flow rates in the branched segments until the terminal pressures at all the segment junctions converged. Two operating modes for the loop were studied: 1) the recirculation/purge mode, where DI water is drawn from the Main DI Loop then a portion is recirculated through the cooling sub-loop, while the remainder is returned to the main loop via a slipstream segment; and 2) the Mix Tank filling mode, where all of the water drawn from the main loop is routed into Mix Tank B.

The flow rates predicted for the recirculation/purge mode are: 72 gpm from the sub-loop recirculation pump; and 20 gpm through the return leg of the sub-loop. The remaining 52 gpm is returned to the Main DI Loop through the slipstream leg where it is subsequently exposed to a UV sanitizer.

The straight-through flow rate predicted in the Mix Tank filling mode is 130 gpm which varies slightly with the liquid head in the Mix Tank.

RESULTS and DISCUSSION – The pressure and flow rate results for both operating modes are tabulated below. Accompanying each table is a visual depiction of that mode’s individual piping segments superimposed on a Process & Instrumentation Diagram (P&ID) of the Chilled/Ambient DI Water Loop (these drawings are proprietary and are not shown).

TABLE 1

Recirculation/Purge Mode

Segment

segment flow rate

gpmsegment

velocity fps

dynamic press drop

psi

segment exit press

psig commentsMain DI Loop in 61.3 network entry pressure

Segment 0 52 5.87 0.83 60.2 sub-loop isolation valveSegment 1 72 7.86 1.57 88.45 sub-loop recirc pumpSegment 2 72 14.33 14.29 73.9 pump discharge lineSegment 3 72 1.98 0.6 73.52 sub-loop heat exchangerSegment 4 72 7.86 10.62 62.47 return line before purgeSegment 5 20 2.18 0.83 61.21 return line after purgeSlipstream 52 5.67 2.49 55.21 purge line to Main DI loop

Main DI Loop out 208 5.54 1.67 54.83 network exit pressure

In recirculation/purge mode, table 1 shows there is an ample volume of water (52 gal) purged from the chilled water system every minute through the Slipstream segment. The purge is only interrupted when the system is filling one of the Mix tanks in the Filling mode. This offers little opportunity for unsanitary water to accumulate in the sub loop. The flow velocity in the 1.5” line between the pump discharge and the heat exchanger is 14 feet/second which is undesirable but not problematic.

TABLE 2

MMT Filling Mode

Segment

segment flow rate

gpmsegment

velocity fps

dynamic press drop

psi

segment exit press

psig commentsMain DI Loop in 61.3 network entry pressure

Segment 1 130 14.19 9.68 75.78 sub-loop recirc pumpSegment 2 130 25.79 43.78 31.78 pump discharge lineSegment 3 130 6.17 1.81 29.75 sub-loop heat exchangerSegment 4 130 14.19 6.74 22.13 heat exch to tank filling dropSegment 5 0 0 0 22.13 return line after filling dropSlipstream 130 14.19 19.58 3.16 drop to MMT recirc loop

MMT Recirc Loop 130 3.46 0.47 0 network exit press (no recirc)

In filling mode the flow rate through the sub loop nearly doubles owing to the elimination of flow through the purge and recycle segments which removes these line resistances from the circuit. The check valve in Segment 5 blocks pressurized water from the Main DI header from short circuiting the heat exchanger while the tank is being filled. Velocities in the sub loop piping are 14 feet/second except through the line from the recirculation pump to the heat exchanger (Segment 2) where the velocity is 25 feet/second. The pressure drop through this relatively short segment exceeds the total dynamic head of the pump by nearly two times. The pressure recovered by replacing this segment with a 2” line could replace the pump in the filling mode, and allow the tank to be filled using line pressure in the Main DI Loop.

RECOMMENDATIONS – The 1.5” tubing and valves between the recirculation pump and the heat exchanger should be increased to 2 inches in order to obtain a reasonable velocity in that segment. That change will increase the available pressure and flow rate in the system in both recirculation/purge mode and filling mode. The increased flow will be well tolerated by the former, but will result in velocities well above accepted design practices in the filling mode, and may require the installation of pressure or flow control.

REMARKS – Flow calculations were carried out using software written by K. Hunter. The network inlet pressure was estimated in a previous calculation. Its accuracy is not critical because all calculated pressures are relative. Vendor flow coefficients (Cv values) were used for pressure drop calculations

through valves. However, at high fluid velocities small deviations in Cv can generate large errors. This is especially true for the valves in Segments 2. Pump calculations were programmed using the vendor capacity curve (Waukesha Purity “C” Series w/4.25” impeller).

TABLE 1

Recirculation/Purge Mode

Segment

segment flow rate

gpmsegment

velocity fps

dynamic press drop

psi

segment exit press

psig commentsMain DI Loop in 61.3 network entry pressure

Segment 0 52 5.87 0.83 60.2 sub-loop isolation valveSegment 1 72 7.86 1.57 88.45 sub-loop recirc pumpSegment 2 72 14.33 14.29 73.9 pump discharge lineSegment 3 72 1.98 0.6 73.52 sub-loop heat exchangerSegment 4 72 7.86 10.62 62.47 return line before purgeSegment 5 20 2.18 0.83 61.21 return line after purgeSlipstream 52 5.67 2.49 55.21 purge line to Main DI loop

Main DI Loop out 208 5.54 1.67 54.83 network exit pressure