While chillers, or refrigerated temperature control equipment, have traditionally been viewed as the preferred choice for removing heat from process equipment or for controlling temperature, heat exchangers can represent a viable alternative, depending upon the application and the degree of control required.
The two critical variables enabling the use of heat exchangers are a facility’s water supply temperature and process fluid temperature stability. If these two variables are sufficiently low, heat exchangers can provide an optimal solution. A look at how a memory manufacturer applied the technology with positive results can demonstrate its potential.
In general, optimal temperature control is achieved by using equipment that offers stable, reliable performance as well as a low cost of ownership. When tallying ownership expenses, both equipment and operating costs such as maintenance and electricity must be considered. While chillers typically are recommended for process cooling applications, they may not always be the optimal equipment when both equipment and operating costs are considered.
Although both chillers and heat exchangers have recirculation subsystems, chillers also have refrigeration subsystems, which make them inherently more complex. In addition, refrigeration subsystems typically add more expense on a cost-per-heat-load basis. Therefore, if raw heat removal at moderate process fluid temperature stability is permissible, and if the process fluid supply temperature is higher than the facility’s water supply temperature, heat exchangers often become an attractive alternative to chillers.
Improving Process Fluid ControlBecause a facility’s water is prone to fluctuations in quality, temperature and pressure, it is sometimes desirable to isolate the application from the water supply by using a chiller or heat exchanger. This promotes better process fluid quality, higher process fluid supply pressures for higher process fluid flow rates, and better control of process fluid supply temperatures.
The following case is a specific example from the semiconductor manufacturing industry. Both a heat exchanger and two configurations of chillers were evaluated for support of a 300 mm physical vapor deposition (PVD) tool. A heat exchanger best met the requirements of the tool, including lowest cost of ownership.
PVD Tool Requirements. The PVD tool has identical left and right halves. Each half has various process chambers, and each process chamber has various components such as cathodes, stages, walls and pumps, most of which require water cooling.
The water is supplied to these various components via two manifolds, referred to as the cathode manifold and the module manifold. Both manifolds require a supply pressure between 30 psi(g) and 150 psi(g), a supply temperature between 68 and 77oF (20 and 25oC), and a supply resistivity of 1/100 MΩ-cm or 10 kΩ-cm. Also, the cathode and module manifolds require a flow rate of 18.5 gal/min and 10 gal/min, respectively. The maximum supply pressure should not be exceeded to prevent damage to the components.
Typical supply and return lines between the temperature control equipment and the cathode and module manifolds, including fittings such as elbows and valves, are estimated to introduce a pressure drop of up to 35 psi(d). Thus, the supply pressure at the chiller or heat exchanger typically has to be higher than 65 psi(g) when the return pressure to the chiller or heat exchanger is 0 psi(g). The latter typically is the case in open-loop temperature control equipment that contains reservoirs.
Chiller Performance. Two chiller options were considered as alternatives to the heat exchanger in order to meet the requirements of the PVD tool: a single HX-750 chiller and two chillers, Models HX-300 and HX-500, used in tandem. While the heat load capability of these two alternatives was sufficient, the inability to provide pressure via one unit and the need to provide flow rate via two units were undesirable. The matchup between chillers and tool is shown in table 1.
Heat Exchanger Performance. The SWX-100 unit is rated for a heat load of 100 kW with a 25 gal/min facilities flow rate and a 25 gal/min process flow rate when the temperature difference between process supply and facilities supply is 18oF (10oC). The performance can be de-rated with that difference as long as the flow rates remain equal and both remain close to the process flow rate of 25 gal/min. For example, at a temperature difference of 9oF (5oC), the heat load that can be handled by the SWX-100 is only 50 kW.
Therefore, at a typical facilities supply temperature of 60oF (about 15oC), up to 50 or 100 kW can be removed from the tool if the process supply temperature is 68 or 77oF (20 or 25oC), respectively. This performance does not change substantially as the process and facility’s flow rates are increased from 25 gal/min to 28.5 gal/min to accommodate the requirements of this particular tool.
Furthermore, these figures indicate maximum performance. A reduction from these figures is achieved automatically by facility supply valve modulation. The CP-13 pump of the SWX-100 system can deliver the required process flow rate of 28.5 gal/min at a process supply pressure of about 130 psi(g). In order to establish this flow rate at the predicted supply pressure of no more than 65 psi(g), pressure reduction is necessary. Thus, the need for external valves becomes evident; the SWX-100 unit has no internal flow control valves. For reference, the facilities pressure drop within the heat exchanger at a facilities flow rate of 25 gal/min is 12.5 psi(d). The matchup between heat exchanger and tool is shown in table 2.
Assuming a permissible temperature rise across the tool of 18oF (10oC), the tool will be generating 75.3 kW at most, which is likely to result in a process supply temperature of 72.5oF (22.5oC). However, given that a HX-750 unit with a heat load capacity of 24 kW was sufficient to handle the heat load, if not the flow rate, the actual temperature rise likely will be no greater than 5.7oF (3.2oC).
Heat Exchanger in UseThe combination of the SWX-100 heat exchanger and the 300 mm PVD tool was deployed at a major memory manufacturer on the East Coast. The SWX-100 unit was installed so that it provided a process flow rate of up to 35 gal/min at a process supply pressure of up to 85 psi(g) to the tool. The initial resistivity was 12 MΩ-cm, which went down to the desired level of less than 10 kΩ-cm as time progressed.
This application more than met the flow requirement but exceeded the resistivity limit. If simplistic scaling of process supply pressure with the square of process flow rate is assumed, the target process flow rate of 28.5 gal/min could be achieved at a process supply pressure of 56.4 psi(g), falling reasonably close to the predicted number of 65 psi(g). Furthermore, the resistivity could be reduced simply by eliminating the deionization cartridge, thereby also eliminating a maintenance need.
The heat exchanger’s pump allowed the user to exceed the tool’s flow rate requirement, whereas previous attempts to use chillers in this application had consistently fallen short. Effectively, this qualified the SWX-100 unit as support equipment on the tool. Because the tool manufacturer leaves selection of such equipment to the end user, this is particularly significant. The memory manufacturer was able to replace chillers with heat exchangers without further qualification. This option has proven to be an effective alternative for the end user’s heat removal application needs.