Consulting-Specifying Engineer - April 2008 - (Page 36) pumps were moving more water than required because the head required by the installed system was significantly below what was anticipated by the designer. The other was that the adjustment of the level control valves needed to be fine tuned. After these issues were addressed the level control problems disappeared and the team moved on to their next challenge. 6” PT Plug Chiller 2 Dr PT Plug Chiller tube bundle pressure drop It is good practice to provide a pressure gauge or gauges on the condenser and evaporator tube bundles of the chillers, and this is quite 6” common on the chillers installed on most campus systems, such as during a commissioning project at South East Missouri State University. Figure 5: A typical schematic drawing showing pressure measureBecause flow and pressure drop through the tube bundles are related ment provisions in the form of PT plugs. to each other and cataloged by the manufacturer, the information provided by the gauges often is used by the chiller’s factory startup technician and the test and balance team to set up and/or verify flow. The gauges also allow the operating team to monitor the tube A2 D2 bundle pressure drop and compare it to the original setup. A change may indicate an emerging problem in the form of a change in flow through the evaporator or fouling of the tubes or both. Contract documents frequently indicate the requirement for pressure B2 C2 gauges or some means of measuring tube bundle pressure drop via a D1 A1 C1 schematic detail (see Figure 5). On the plus side the detail clearly shows that a means of measuring pressure is required on each side of the tube bundle. Negatively, the detail is subject to interpretation in the field B1 because the piping connection to the mains frequently requires elbows and other components not indicated on the schematic (see Figure 6). Most field personnel have seen chillers with gauges installed at all of the locations illustrated in Figure 6—although they are seldom installed Figure 6: Pressure readings and associated flow rates based on the at all of these locations on the same project. Most manufacturers cata- pressure readings taken at different points in an evaporator piping log the pressure drop for their tube bundles based on gauges installed circuit. in the piping immediately ahead of the water box— illustrated in location C1 and C2 in Figure 6. Thus, if the shop drawing for the chiller Flow based on differential pressure indicated that at design flow, the evaporator pressure drop should be Location Pressure difference Flow based on pressure difference 17 ft water column, the start-up technician would adjust the system (ft/water column) gpm % until the reading at C1 and C2 matched the requirements. This is the D1-D2 14.48 1,363 85% base case documented in Table 2, associated with Figure 6. Notice how measuring the pressure difference with one or more C1-C2 17.00 1,600 100% elbows between the gauge and the location used by the manufacturer B1-B2 18.26 1,718 107% to catalog the chillers performance can result in overestimating the A1-A2 19.78 1,862 16% system flow rate, even though the loss through the elbow would be considered relatively minor and would be difficult to actually measure Table 2: Note that the manufacturer’s performance is based on the using the gauges provided in the field. pressure reading taken at C1 and C2. A the flow condition under Also note that gauges installed directly on the vent and drain consideration, the pressure drop through the elbows serving the connections on the waterbox (points D1 and D2 on Figure 6) will chiller is about 0.52 ft water column or 0.23 psi. indicate a different pressure from the gauges on the piping only Source: David Sellers inches away. This is because there are losses associated with the change in velocity when the water exits the pipe into the water not been provided in the piping and were frantically trying to figure box and then re-enters the pipe from the water box. I learned this out why we were short of flow. When in desperation, I called an older gentleman I knew who the hard way on a project at the Anheuser-Busch campus where had been a representative of the chiller manufacturer for years, he we had installed gauges directly on the waterbox because taps had 36 Consulting-Specifying Engineer • APRIL 2008
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.