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CENTRAL PLANT PIPING DESIGN


With the advent of less expensive and more reliable variable frequency drives, primary pumped constant volume chilled water and hot water systems designs evolved into primary-secondary pumped systems that use variable frequency drives to vary the speed and thereby HP of the secondary (distribution) loop pumps according to demand. In theory, pump energy is reduced as only the required water is distributed in lieu of 100% of the design water as was the case in constant volume systems.

The vast majority of primary-secondary systems designs share these common characteristics:

  • Full size, usually front, decoupler
  • Typically 1.5 ft. maximum pressure drop in the decoupler
  • Decoupler 3 pipe diameters minimum length
  • Decoupler typically same size as distribution piping
  • Matched primary/secondary flow rates
  • Differential pressure controls the secondary flow rates


This piping configuration can produce three possible flow conditions:

Primary (production) flow equal to secondary (distribution) flow. While ideal, it begs the question why utilize this piping design? Why wouldn't a variable flow primary only system be cheaper to install and more energy efficient to operate? The simple answer is yes it is, and Energy-Environment-Economics has converted existing constant volume systems to variable primary, has designed new variable primary systems to replace existing constant volume systems, and has commissioned newly installed variable primary systems.

The practical answer is that prior to widespread use of Direct Digital Controls (DDC) on water chillers, chiller manufacturers were uneasy about varying flow rates thru their machines. Hence the constant volume primary pumps. Today's chillers are comfortable with varying flow rates, as long as the rate of change is not extreme, so variable primary systems are becoming more popular. Another possible hydronic condition......

In this flow condition the building's load requires more water than a single chiller system can deliver, and the excess water is decoupled to the return loop, depressing the chillers' entering water temperature and reducing load. Note that the secondary pump variable frequency drives have slowed to provide only the necessary GPM. While acceptable but not optimal, it certainly is preferable to....

Known as "reverse" decoupling, this flow condition raises the delivered chilled water temperature, causing the chilled water coils to require more water and thereby causing the pumps to increase flow rates and consumed HP. The obvious solution is to start another chiller system and return to flow condition #2 described above. The real problems arise when the system temperature differential (DT) is less than design; the chillers are unloaded but are required to run because the loads are requiring more water.

For example, the formula for coil capacity is QBtu/H = (GPM*DT)*500. Obviously for the same capacity the GPM would have to be doubled if the DT was halved. Most of today's systems are operating with less than design DT's, necessitating additional flows usually resulting in starting additional chiller systems to provide the required flow rates. For example, from Figure 4 above and for a constant cooling load, a five-degree system DT would require 3000 GPM in lieu of 1500 GPM, so an additional chiller system - chiller, primary pump, condenser water pump, and cooling tower - would be forced to run. And, of course, the secondary pump variable frequency drives must speed up to increase to the required flow rate. Low system DT's are an energy efficiency cancer. Energy-Environment-Economics can diagnose why your system is suffering and can devise solutions to alleviate the condition and save large amounts of energy and money.

 

ComPipe

 

      Copyright Energy Environment Economics - 2003