ASHRAE Journal - December 2009 - 44

The equipment is site assembled and requires a minimum of one to two months for installation and commissioning, depending on site location. Refrigeration Cycle of Water Chilling Plant Temperature (°C) 35 33 31 29 27 25 23 21 19 17 15 45 40 35 30 Ambient Temperature (°C) 25 1°C 4°C Refrigeration Load (kW) The compressor sucks low-pressure refrigerant from the shell side of the vertical chiller and discharges to the condenser. In the condenser, high-pressure superheated refrigerant is liquefied to high-pressure liquid refrigerant. Later, this refrigerant is collected by gravity flow in the receiver. High-pressure liquid from the receiver is fed to the vertical chiller through an expansion device. Low-pressure liquid inside the vertical chiller absorbs heat from the water and converts to vapor form. On the tube side of the vertical chiller, water is cooled to the desired temperature and later supplied for concrete cooling. The compressor sucks low-pressure vapors from the chiller. Because the water outlet temperature is required to be in the range of 1°C to 2°C (34°F to 36°F), a vertical chiller is used for cooling water. A vertical chiller is installed above the cold well and water flows from top to bottom. The tube side is open to the atmosphere. The freezing of water will not lead to fabrication failure of the vertical chiller. Chilled Water Circuit Figure 1: Freshly mixed concrete temperature versus ambient temperature when mixed with water at 1°C and 4°C. 0.31 0.30 0.29 0.28 0.27 0.26 0.25 0.24 0.23 0.22 32 33 34 35 36 Water Temperature (°C) 37 38 1°C 4°C The chilled water circuit is an open circuit. The chilled water is mixed with concrete and consumed. Therefore, water is cooled from 38°C to 1°C (100°F to 34°F). Generally, a vertical chiller is designed to reduce chilled water temperature from 10°C to 1°C (212°F to 34°F) for optimization. Nearly four times as much of the required chilled water flow rate is fed into the vertical chiller to cool the water from 38°C to 1°C (100°F to 34°F). Due to the mixing of chilled water, at 1°C (34°F), and hot water, at 38°C (100°F), the final water temperature reaches around 10°C (50°F) and later the vertical chiller further cools it to 1°C (34°F). Disadvantages of conventional water chilling plant for concrete cooling include: • A bulky design because a flooded chiller is used. • Installation of equipment at the site requires a minimum of one to two months to erect and commission. • A foundation is required for installation. • Cold and hot water tanks (usually made of concrete) are required, increasing cost. • Large installation area is required. • When concrete cooling is no longer needed, the plant must be reinstalled at other sites. Reinstallation requires complete dismantling of the plant and erecting/commissioning at its new location. This activity requires a minimum of three to four months. • Since single-stage cooling is used, the COP for this plant is 2.59 (–4°C/43°C). Multiple-stage cooling can increase COP. • The flow rate of a primary chilled water pump is four times that of required flow rate. Therefore, the pump44 ASHRAE Journal Figure 2: Refrigeration load per cubed meter of concrete per day versus water temperature. 190 Refrigeration Capacity (kW) 180 170 160 150 140 130 120 110 100 90 320 350 400 450 500 Concrete (m3/day) 550 600 4°C Water Figure 3: Refrigeration capacity versus concrete capacity at 38°C water supply. ing cost adversely affects the running cost. A secondary chilled water pump also is required. • The overall cost increases due to the previously mentioned factors. A typical 2.94 L/s (6.24 cfm) plant requires 152 kW with a total connected electrical load of 172 kW for cooling water from 35°C to 1°C (95°F to 34°F). To overcome these disadvantages of a conventional water chilling plant, you need to use a containerized water chilling plant. ashrae.org December 2009

ASHRAE Journal - December 2009

Table of Contents for the Digital Edition of ASHRAE Journal - December 2009