District Cooling Plants Require Careful System Design in High Heat

The Pearl's Integrated District Cooling Plant (IDCP), owned and operated by the Qatar District Cooling Company, provides air-conditioning services to the Pearl of Qatar off the eastern coast of the peninsula. The plant stores water tanks in the basement, chillers and chilled water pumps on the ground floor, the reverse osmosis system and electrical switchgear on the first floor, and the cooling towers and condenser water pumps on the roof. The facility generates chilled water for energy transfer stations, heat exchangers, valve pits and plant cooling systems, including air-handling and fan coil units.

System Design

At 120,000 tons, the IDCP is the world's largest district cooling plant. The facility's capacity is at least twice the capacity of the largest chiller plant in the Middle East at the time of construction. The IDCP was designed with a guaranteed capacity of 115,000 tons expandable to 120,000 tons. Twenty-three pairs of 2,500-ton electric motors drive the centrifugal chillers. Each chiller unit is approximately 7.9 meters long, 3.4 meters wide and 4.2 meters tall. The chillers are in a series. Each pair's counter-flow arrangement provides a zero-tolerance capacity of 4,850 tons. Two independent facilities are connected to an underground chilled water distribution system to enable hydraulic operation at the plant. The plant design incorporated plans for a future expansion using two additional 2,500-ton electric motor-driven chillers. In any district cooling system where chilled water must be transferred through piping, fittings or heat exchangers, a pump is required. These factors help determine system friction losses or pressure drops at critical points throughout the facility. Unplanned head loss can damage equipment and lead to higher maintenance and life cycle costs. Most designers use hydraulic modeling programs to calculate pump head requirements. These head loss calculations determine equipment selection and pressure class for any chiller system design. Every component in the system will affect the pressure and fluid flow rate, and the design must be able to overcome any potential head losses.

Regional Challenges

Harsh temperatures are a consistent problem for engineers and system designers in any industry in the Middle East—especially cooling applications. The chillers operate at extreme condenser water entering temperatures that could reach up to 36 degrees Celsius at 50 C ambient. Some chillers can shut down under such extreme conditions. Because water is scarce in the Middle East, the plant was designed with makeup from three sources: potable water from the local water utility, treated sewage effluent from the nearby sewage treatment plant and permeate (lower salinity water) from a 14,000 cubic meters per day desalination plant. The IDCP is one of the first district chiller plants in the region to be equipped with a desalination plant. Raw saltwater is pumped from the Arabian Gulf to saltwater storage tanks in the IDCP. The raw saltwater is pumped through reverse osmosis membranes, and the permeate is stored in the permeate storage tank within the IDCP. The system is configured with a control algorithm that controls the ratio of the various water sources to maintain water quality. The four constant-speed raw water pumps are horizontal split-case centrifugal, direct-connected, base-mounted pumps, each selected for about 375 cubic meters per hour with total developed head of about 20 meters. Each pump driver is a 30 kW motor. The pumps are capable of running in series to reach a design capacity of 750 cubic meters per hour.