General
This section of the Design Intent Report includes the narrative description of the chilled water plant process, heating, ventilating, air-conditioning, plumbing, and fire suppression systems for the Integrated District Cooling Plant (IDCP) at The Pearl, Qatar. Discussions between the Owner, Qatar Cool, PMC, the Contractor, C.A.T. International (CAT), and the Design-Build Engineer, Stanley Consultants are detailed in Exhibit A – Project Correspondence.
Compliance Statement
The Mechanical design will comply with all portions of the Tender Documents prepared for Qatar Cool and issued by DAR during the tender phase. These documents include the general requirements, tender specifications, room data sheets, tender drawings, and the appropriate addendums and tender circulars issued for clarification, except as noted below:
• Plant Arrangement: The IDCP will be based upon the plant arrangement from the Alternate 2 bid proposed by CAT and accepted by Qatar Cool prior to contract award. • Thermal Storage: Thermal storage was eliminated from the project with Alternate 2 bid
proposed by CAT and accepted by Qatar Cool.
• Number of Chillers: Thermal storage was removed with the Alternate 2 bid proposed by CAT and accepted by Qatar Cool. To make up the difference in capacity the number of chillers was increased over what was shown on the Tender Drawings and listed in Section 60, Paragraph 15.1.4.2.b of the Tender Specifications.
• Chiller Arrangement: The chillers will be arranged in pairs using a series-series, counter- flow arrangement instead of the series-parallel counter-flow arrangement indicated on the Tender Drawings and Section 9, Paragraph1.1.17.1 of the Tender Specifications.
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• Chiller Water Boxes: The chiller refrigerant condenser and refrigerant evaporator will be provided with non-marine type water boxes instead of the marine-type water boxes indicated in Section 60, Paragraph 15.2.7.4.a of the Tender Specifications. In the absence of the marine water boxes, the chiller tubes can be accessed by removal of the connecting pipes at the chiller by means of grooved-end lock (Victaulic) connections followed by removal of the water box cover.
• Chiller Connections: The chiller refrigerant condenser and refrigerant evaporator will be provided with stub-out type connections and mechanical coupling grooves suitable for bolted clamp-type coupling with grooved-end lock instead of flanged connections indicated in Section 60, Paragraph 15.2.7.4.b of the Tender Specifications. See previous exception for a discussion on access to the tubes.
• Chiller Condenser Tube Cleaning: Each chiller condenser will include a tube brush cleaning system that will scour the tubes clean by reversing the flow through the condenser using automatic control valves as shown in the piping configuration on the Drawings (Q06024-IDCP-M-I-614). The Tender Drawings show the use of four-way valves for this operation; however, four-way valves are not available in the sizes needed. • Number of Cooling Towers: Thermal storage was removed with the Alternate 2 bid
proposed by CAT and accepted by Qatar Cool. To make up the difference in capacity the number of cooling towers was increased over what was shown on the Tender Drawings and listed in Section 60, Paragraph 16.1.4.1.a of the Tender Specifications.
• Cooling Tower Design Conditions: The cooling tower flow rate to each cell will be 11,000 gpm and the condenser water temperature differential will be 13°F. The flow rate and range differ from those specified in Section 60, Paragraph 16.3.9 (3 gpm per ton at a 10°F range). The design conditions selected allow for a higher cooling tower capacity per square foot of building and provides the guaranteed capacity and efficiency at the chiller. Alternate design conditions to the traditional 3 gpm per ton are becoming more prevalent for this reason. Reference “Optimizing Condenser Water Flow Rates” presented by Bill Liegois at the 2006 IDEA Campus Energy Conference.
• Number of Basins: Each cooling tower will be divided into three basins at the building control joint locations. This differs from the Tender Drawings which shows two independent basins for each cooling tower. This modification is necessary to accommodate the structural design of the building. Each of the three basins will be connected by equalization lines for common level control.
• Condenser Water Filtration: The condenser water system will include three side-stream centrifugal filter separators for each 12-cell cooling tower connected to the respective supply headers. The side-stream flow rate will be equal to 10 percent of the total condenser water flow rate. The Tender Drawings show smaller units both on the headers and at each chiller.
• Chilled Water Pumping Scheme: The chilled water pumping scheme will be a direct primary (constant speed) system that varies flow to the distribution system by staging chillers and pumps on and off. This differs from the Tender Drawings and Section 60, Paragraph 9.3.5 of the Tender Specifications, which calls for a primary-secondary system.
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This modification will provide a more energy efficient system by eliminating the neutral bridge bypass, which can lower chilled water return temperatures and contribute to the phenomenon known as “Low ∆T Syndrome.”
• Air Separation: DAR requested Stanley Consultants to provide technical justification for the elimination of the air separators from the IDCP. ASHRAE is not specific regarding the use of air separators within chilled water systems, except in regards to smaller building type systems. Roy Ahlegren of ITT Industries gave a presentation for the University of Wisconsin's Chilled Water Plants for Central District Cooling Seminar in April 2006. He identified the following advantages of closed loop systems as they relate to air entrapment:
- Oxygen additions are limited due to the lack of interface between air and water during normal operation.
- Pressures and temperatures minimize air coming out of solution within systems. This is not the case for heating systems, which typically require air separation.
- The use of bladder-type expansion tanks further reduces the interface between air and water.
- Permanent energy penalty due to pressure drop of separators.
• Chilled Water Strainers: The Tender Specifications (Section 60, Paragraph 14.2.4.2) indicate that basket strainers are to be provided for the chilled water system. However, the vertical dimensions of the basket strainers are not conducive to making the equipment connections. Stanley Consultants recommends using Tee Strainers in lieu of the basket type to resolve this conflict. The tee strainers will be in compliance with the performance requirements of the basket strainers as specified (i.e. 5/32” perforations).