A chemical plant in Cheswold, Delaware required chilled water for various processes and air-conditioning applications. During evening time the water is diverted to a 174,370 m3 tank, an integral part of a Thermal Storage System (TES) with the goal to pull down the tank temperature to 3.3°C. With the existing chillers, two (2) shell & tube flooded ammonia tied to 500 hp screw compressors and one R-11 centrifugal compressor, the lowest water temperature achieved was 4.4ºC. Therefore, the minimum storage tank temperature attained was never lower than 6.1ºC.

A comparative analysis between a spray evaporator and a conventional flooded evaporator was undertaken and presented to the end user. Spray evaporator was selected to be the best option because it has certain inherent characteristics that make it an equipment of choice for such an application.

• First, it has higher heat transfer coefficient. Hence, for the same size and load the operational approach temperature is lower.

• Second, due to these characteristics the refrigerant suction temperature could be raised therefore, resulting in compressor capacity enhancement.

• Third, due to low approach temperature the suction temperature could be maintained at close to or higher than the freezing point of the process fluid.

• Fourth, the refrigerant charge is order of magnitude lower than the same capacity flooded evaporator, hence, making it environmentally attractive option.

• Fifth, because of the low charge and the shell being devoid of liquid refrigerant it is well guarded against freezing in the event of process pumps or refrigerant control failure.

• Sixth, there is no hydrostatic head penalty, hence, no adverse affect on the LMTD.

Special design aspects were considered in order to achieve an optimized evaporator. Spray evaporators work effectively if the feed ratio is higher than 2:1 so that no section of the bundle is starved of liquid refrigerant. Appropriate refrigerant distribution system was designed utilizing full-cone spray nozzles. The final design was a half bundle shell and tube configuration. Commercially available full cone spray nozzles were selected. To improve the efficiency of the evaporator, doubly enhanced surface carbon steel tubes were incorporated. The unit was completed on site and put to operation as shown in Fig. 1. It has exceeded all expectations under different seasonal conditions. Water outlet temperature as low as 0.6ºC has been recorded and there are confirmed reports of storage tank temperature as low as 1.7°C.

Table 1 shows comparative features of the present unit and a conventional flooded evaporator. Beside the physical and economical advantages it is apparent that the refrigerant charge was order of magnitude less than the flooded evaporator. It was recently reported that the two old flooded chillers developed tube ruptures due to freeze-up and were therefore taken out of the loop. Since then this single spray evaporator has been handling the entire load in the plant. Early this year (2004) a second duplicate spray evaporator was fabricated and supplied to the said plant and has been put in line. All physical features were similar to the existing unit except two different types of enhanced tubes were used in the newer version. The top half has tube geometry similar to the existing unit, however, in view of the maldistribution concerns the lower half has tube structure favorable for longitudinal flow of refrigerant that would therefore, minimize any chance of dry spots in the lower section of the bundle.