Cooling System Renovation

Q:  My facility was recently purchased so we’re demolishing one building and constructing a new one down the street. My cooling system will be in pandemonium, what should I do?

Engineers are always being challenged with flexing systems. One key analysis that must be done is to ensure all the equipment or buildings are getting the flow rate requirements during the construction and final commissioning phases of a project. This article demonstrates how PIPE-FLO is an excellent tool for flow rate analysis, providing the confidence you need that your system will give top performance during all phases of the project.

The heart of the flow rate analysis is the pumping station and it will be important to have all pump curves available for the model. The hydraulic performance for other piping system devices should be obtained as well. Details such as control valve flow coefficients, orifice sizes, and any other device performance curves (i.e. heat exchanger flow rate verse differential pressure). In the absence of good manufacturer data, PIPE-FLO has sizing tools and methods to approximate device performance.

Step 1: Model what you have today.

This in itself can be problematic. One method of attacking this challenge is to slice up your overall system into sections, and we have listed examples listed below. 

  • Pumping station
  • Main Supply and Return headers
  • Individual loops and users

An important aspect of modeling with PIPE-FLO is to establish the boundary conditions of your piping system based on what is known about the system. The entire system can be modeled or just portions of the system between known boundary conditions. The key to slicing up your model is to use a combination of Pressure Boundary Devices or the combination of a Flow Demand Device and a Pressure Boundary Device. These devices are used in the model where field data has been verified. Once all the individual sections are modeled and verified, they can be connected together into one large system.

Step 2:  Simulate what will happen when the building is demolished.

This is easy to simulate in PIPE-FLO because all you need to do is "close" the pipes to the demolished buildings using the Close Pipe function . Typically, the pump performance is reviewed to make sure no damaging effects to the pump will occur. Since the flow rate will be reduced, the pump will operate further back on its curve shown in the example below.

This pump graph shows the pump’s operating point (red knee mark) is very close to the minimum flow rate (100 gpm) and the operating efficiency is 56.1 %. Knowing where the pump will be operating during the site demolition phase could save you unexpected maintenance cost.

Step 3:  Model the new building requirements.

We recommend you model the new building first as an independent section. One trick is to use one Pressure Boundary to represent the supply tie in location and a second Pressure Boundary, set to zero (0), to represent the return tie-in location. Next, you increase or decrease the Supply Pressure Boundary until you get the desired flow rate for the section. The section differential pressure is easy to calculate since the Return Pressure Boundary is set to zero (0).  Below is a sample section where two Pressure Boundaries are being used.

Note the Return Pressure Boundary is set to zero (0), so the differential pressure for this new building is 22 psid.

Another trick is to use Inlet and Outlet Flow Demands at the original model’s tie-in location to see how much differential pressure is available.

This screen shot shows two Flow Demands being used to simulate the tie-in location of the new building in the original model. When a Flow Demands are used, PIPE-FLO will calculate the pressure at the location. In the above example, the supply pressure with a 70 gpm Flow Demand Out is 46.85 psig and the return pressure with a 70 gpm Flow Demand In at 19.15 psig. The differential pressure is 46.85 - 19.15 = 27.7 psid. With this proactive analysis, it can be seen that the new building has a required differential pressure of 22 can easily be meet with the 27.7 available differential pressure.

Of course, not all projects are this simple, so PIPE-FLO has built-in messages to alert you of unrealistic configurations. For example, if you get a (-) dP or “flow cannot be achieved” message, that means you do not have enough energy available at that location in the system to accommodate the flow rate. This may mean you need to install additional pumps, up size the existing pumps, or evaluate some other way to increase the energy in your system. PIPE-FLO can also be used to evaluate these additional changes  to your overall system.