Trane TRG-TRC016-EN manual Critical VPF System Requirements

period three

System Variations

notes

Notice also that the VPF system must continue to include a bypass. Although a control valve prevents flow in the bypass for most system operating conditions, the modulating valve and bypass are required to ensure that the water flow through the system remains above the minimum flow limit of the operating chillers. This bypass may be in the same location as in the primary- secondary system, or it may be a three-way valve on a few of the cooling coils.

Critical VPF System Requirements

▲Chillers must handle variable evaporator flow

▲System must include a bypass

▲System-level controls must limit the rate-of- flow change

▲Adequate time to design and commission controls

▲Operator must understand the system

Figure 73

Although VPF systems have been successfully installed and operated, they are more complex both to design and to operate when compared to a primary- secondary system. The sequencing of chillers and pumps requires a thorough understanding of system dynamics because flow rates will vary through every operating chiller. The control system needs to avoid cycling (restarting the chiller too soon) and maintain the rate-of-flow variation through the chiller evaporators within the allowable limits. This becomes very complicated as the number of chillers increases.

Another important consideration when investigating VPF systems is the fact that they take more time and planning to design and commission properly than other systems. The system design engineer must thoroughly define the control sequence early in the design process, and clearly communicate it to the controls provider. Also, the system operators must understand how the VPF system works; therefore, training is mandatory. The success of a system design is directly related to the ability of the operator to carry out the design intent.

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