Plate-and-frame free cooling

Transcription

1 Plate-and-frame free cooling Plate-and-frame heat exchangers are sometimes referred to as waterside economizers. One way to reduce the energy consumption of a chilled-water plant is to precool the water in the chilled-water loop before it enters the evaporator. This can be accomplished by piping a plate-and-frame heat exchanger into the chilled-water and condenser-water loops. Using the piping arrangement shown in Figure 3 9, free cooling and mechanical cooling occur simultaneously. (Plate-and-frame free cooling can be achieved with other piping arrangements, depending on the operating characteristics desired.) Figure 3 9 Chilled-water plant with plate-and-frame free cooling When the ambient wet-bulb temperature is low enough, the heat exchanger transfers heat from the chilled water returning to the evaporator to the condenser water returning from the cooling tower. Precooling the chilled water before it enters the evaporator lessens the cooling burden, reducing the energy that the chiller uses. Application considerations Adding a heat exchanger to provide free cooling increases the initial cost of the system. The additional pressure loss also raises pumping costs. The ambient wet-bulb temperature determines the free-cooling capacity of the heat exchanger cooling capacity diminishes as the ambient wet-bulb temperature rises. Free cooling is only Plate-and-frame free cooling 3 43

2 available when the ambient wet-bulb (AWB) temperature is less than the design return chilled-water (DRCW) temperature plus the heat exchanger approach (HEA) temperature, that is, AWB DRCW + HEA The cooling tower must be designed for winter operation. Water entering the condenser must be maintained within the temperature limits specified by the chiller manufacturer. Related reading Multiple-Chiller-System Design and Control Applications Engineering Manual (Trane literature number SYS-APM001-EN) Chilled Water Systems Air Conditioning Clinic, one of the systems series (Trane literature number TRG-TRC016-EN) A New Era of Free Cooling, Engineers Newsletter (volume 20, number 3) Sample scenario A chilled-water plant includes a three-stage centrifugal chiller and a plate-and-frame heat exchanger that has a 3 F approach. The following illustrations demonstrate how to model this plant. Cooling-equipment definitions in the TRACE 700 library include an approach temperature for a plate-andframe heat exchanger the default is 3 F. To model a different approach temperature, use the Library/ Template Editors program to copy and modify an existing piece of cooling equipment. Note: TRACE 700 assumes that the capacity of the heat exchanger equals the cooling capacity of the chiller to which it is assigned. then type the new approach temperature here. To change the approach temperature, choose a plateand-frame type here 3 44 TRACE 700 User s Manual CDS-PRM001-EN Plate-and-frame free cooling

3 To model a cooling plant that includes a plate-andframe heat exchanger: 1 Select the desired equipment type. For this example, pick one of the centrifugal chillers from the water-cooled chiller category. 2 Enter the full-load consumption for each pump. 3 Click Controls and choose one of the plate-and-frame options as the free cooling type. For a cooling plant with multiple chillers, be sure to specify the free-cooling option for each chiller. Plate & frame series uses the heat exchanger to supplement mechanical cooling. Plate & frame uses the heat exchanger instead of mechanical cooling, but only if ambient conditions permit. The heat exchanger only operates if it can satisfy the entire cooling load. Plate-and-frame free cooling 3 45

4 Refrigerant-migration free cooling As the name implies, refrigerant-migration free cooling is based on the principle that refrigerant migrates to the coldest point in a refrigeration circuit. Under favorable conditions, this energysaving option for centrifugal chillers can provide up to 40 percent of the design cooling capacity of the chiller. When condenser water returning from the cooling tower is colder than the water in the chilled-water loop, as depicted in Figure 3 10, the refrigerant pressure in the condenser is less than that in the evaporator. This difference in pressure drives refrigerant vapor from the evaporator to the condenser, where it liquefies. With the help of gravity, the liquid refrigerant flows back to the evaporator. The non-mechanical refrigeration cycle is sustained, providing free cooling, as long as the differential refrigerant pressure is sufficient to drive vapor from the evaporator to the condenser. Figure 3 10 Refrigerant-migration free cooling Application considerations Mechanical cooling and refrigerant-migration free cooling cannot occur simultaneously. This type of free cooling can only be used if the cooling capacity of the tower water is sufficient to meet the entire building load TRACE 700 User s Manual CDS-PRM001-EN Refrigerant-migration free cooling

5 Little, if any, free-cooling capacity is available when the ambient wet-bulb temperature exceeds 50 F. Unlike the compressor (which is off), accessories such as pumps and cooling-tower fans continue to operate in the conventional manner during the free-cooling mode. The cooling tower must be designed for winter operation. Related reading Chilled Water Systems Air Conditioning Clinic, one of the systems series (Trane literature number TRG-TRC016-EN) Sample scenario A water-cooled centrifugal chiller is equipped for refrigerantmigration free cooling. When ambient conditions permit, free cooling satisfies up to 40 percent of the design capacity of the chiller. To model a chiller with refrigerant-migration free cooling: 1 Add the appropriate equipment category to the cooling plant. For this example, select watercooled chiller. (This is equivalent to dragging an equipment icon to a plant on the Configuration tab.) 2 Replace the default equipment tag with a more descriptive name, if desired. 3 Select the desired equipment type and fullload consumption for each pump. 4 Click Controls and choose the appropriate option from the list of free cooling types (in this case, refrigerant migration). If refrigerant-migration free cooling cannot satisfy the entire load, then mechanical cooling is used instead. Refrigerant-migration free cooling 3 47

6 Strainer-cycle free cooling Like other methods of free cooling for chillers, strainer-cycle free cooling acts as a waterside economizer that reduces the amount of energy consumed to produce chilled water. In this case, the source of cooling is condenser water returning from the cooling tower. Figure 3 11 illustrates a typical strainer-cycle piping arrangement that provides free cooling. When the ambient wet-bulb temperature is cold enough, the tower water is valved around the chiller and directly into the chilled-water loop. A filter is positioned upstream of the valve to strain the condenser water before it enters the chilled-water loop, resulting in the name given to this method of free cooling. Figure 3 11 Piping arrangement for strainer-cycle free cooling Application considerations Pumping cooling-tower water throughout the entire chilled-water loop increases the risk of corrosion and fouling. Water treatment, though costly, can help mitigate this risk. Free cooling is only available if the leaving-tower water can satisfy the entire cooling load, limiting the effectiveness of this option. Ambient wet-bulb temperature determines the amount of freecooling capacity available from the strainer cycle cooling capacity diminishes as the ambient wet-bulb temperature rises TRACE 700 User s Manual CDS-PRM001-EN Strainer-cycle free cooling

7 The cooling tower must be designed for winter operation. Sample scenario A chilled-water plant, which includes a water-cooled centrifugal chiller, is piped to provide strainer-cycle free cooling. After adding a watercooled chiller to the cooling plant: 1 Choose the appropriate equipment type (a centrifugal chiller for this example). 2 Specify the full-load consumption for each pump. 3 Click Controls and choose strainer cycle from the list of free cooling types. Strainer-cycle free cooling occurs at the plant level. If the cooling plant includes multiple chillers, repeat this step for each chiller. If strainer-cycle free cooling cannot satisfy the entire load, then mechanical cooling is used instead. Strainer-cycle free cooling 3 49