Thermal energy plant efficiency starts with reducing high pH in cold water loop

The City of Markham, situated north of Toronto, is among the fastest growing municipalities in Ontario. To help the City meet the energy challenges posed by rapid growth, an integrated and efficient energy supply network, Markham District Energy, was developed in 1999.

This entity was designed to facilitate the production of power and supply thermal energy for use in heating and cooling applications in the City’s business district.

Markham District Energy currently operates three facilities that provide electricity and temperature regulation to over six million square feet of mixed use development. One of these facilities is the Warden Energy Centre; a 5.0 MW heat and power plant that began commercial operations on June 4th, 2008. The centre utilizes two low-emission reciprocating engines fueled by natural gas to produce electricity and thermal energy.

A closed, four-pipe system with cold and hot water loops is used to deliver the thermal energy to the Markham District Energy Grid for use in temperature control applications.

In order to generate cold water for use in air conditioning units and cooling systems, the liquid in the closed cold-water loop is passed through chillers to extract heat and lower water temperature. This chilled water is then transferred to cooling systems supplied by the Markham District Grid where the cool thermal energy is extracted.

As the water makes its way through the district system, it gradually picks up heat which must be released before it can be transferred back to the chillers. This heat extraction is accomplished through the use of cooling towers which release hot thermal energy into the air. To ensure the most efficient operation of the cold-water loop, the fluid in the system requires a consistent pH balance. The alkalinity level of the water must be carefully monitored as mineral content steadily rises due to water loss from evaporation.

To meet this challenge, three metering pumps and an inline static mixer are used to periodically inject sulfuric acid into the cold-water loop. This acid reduces the high pH caused by the repeated cycling of water through the cooling towers, increasing the lifespan of the liquid within the system.

The conductivity varies from a low set point of 1,200 µs and ranges up to 3,000 µs as the number of water cycles increases and concentrates the dissolved solids. The facility’s existing injection system had been problematic since the centre began operations — the solenoid pumps used to feed the sulfuric acid to the cold-water loop were unable to maintain prime. To find a solution to this issue, facility mangers began researching metering pumps. They discovered that Grundfos offered a wide range of chemical feed products and contacted the company for a potential solution.

“The challenge that they faced was intermittent low chemical injection rate and accessing the outdoor storage tank from an overhead suction pipe,” said Michael Presement, Grundos business development specialist for Dosing Products. “We felt that the high lift rating and exceptional low feed turndown range of our Smart Digital Dosing Pumps could solve both issues.”

Grundfos DDE pumps were selected on a trial basis to ensure they could meet the difficult requirements of the application. After this period was complete, Grundfos model DDE 6-10 P-PVDF/Teflon pumps with multifunction valves were chosen as a permanent solution for the following reasons:

• Diaphragm pump design provides smooth continuous dosing strokes and prevents cavitation;
• Stepper motor delivers reliable high lift performance;

• Pump performance range easily accommodates application requirements of > 1/LPH feed rate; and,

• Simple, single variable turndown feature provides convenience for dosing adjustments

Three pumps now deliver the precise chemical quantities required for the Warden Energy Centre’s cold-water loop. Facility managers are pleased with the pumps’ reliable operation and the reduction in system maintenance and trouble shooting. The pumps’ simple turndown function also provides equipment operators with a convenient way to adjust injection amounts for periodical events such as blow downs.

As a result of the pumps’ reliable operation, facility managers at the Warden Energy Centre have chosen to expand their use of Grundfos metering pumps to other Markham District Energy Plants. The pumps will help other facilities within this supply grid to reduce their water consumption by accurately treating the cold water closed loop system, thereby increasing water cycles and reducing overall water use.

This article was contributed by SPD Sales Ltd.