Refrigeration – Automatic control – Refrigeration producer
Reexamination Certificate
2000-07-18
2001-04-03
Wayner, William (Department: 2857)
Refrigeration
Automatic control
Refrigeration producer
C062S513000
Reexamination Certificate
active
06209334
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to systems and methods for ensuring that a single refrigeration unit supplying refrigerant for a number of individual control channels is not overtaxed by the demands of one or more of the channels.
In a number of situations in which controllable refrigeration capacity is needed, it is advantageous to utilize a single refrigeration unit for providing pressurized refrigerant to two or more control channels, each of which is separately regulated to maintain the temperature of an existing fluid circuit or device. An example is found in the above referenced prior filed application, which is particularly described in terms of an example for controlling the temperature of a number of different tools in a cluster tool system for fabricating semiconductors. In this context, minimal floor space and high reliability are of paramount importance, so that a compact physical configuration, long term operation, and precise and stable temperature control are significant operative goals. The system described and claimed in the referenced application achieves these objectives by utilizing a single large refrigeration unit having adequate nominal capacity for total demand and providing pressurized liquid refrigerant into separate control channels, each including a control loop for adjusting the temperature of heat transfer fluid circulated through the associated tool. The flow rate of the refrigerant in each channel is separately controlled in accordance with the target temperature needed at the tool at particular point in time. In each channel, the refrigerant is fed at its chosen rate, and in a two phase state, into an evaporator/heat exchanger thus being at the evaporation temperature. In the heat exchanger heat of evaporation is given up in cooling a thermal transfer fluid also passing through the evaporator/heat exchanger at a constant rate. This occurs independently for each of the channels, which can have different refrigeration demands at any particular time. For example, a maximum demand can be imposed in one channel which is required to drop the tool temperature as rapidly as possible, as to effect a process changeover, while one or more other channels are, temporarily at least, in a relatively stable mode.
In this system, the temperature level that is to be regulated is that of the semiconductor fabrication tool, which is outside the closed refrigeration loop. Thus the superheat (the difference between refrigerant temperature before and after heat exchange) cannot be used for control and limiting of demand.
Under these circumstances, an unstable condition can arise when any channel or the refrigeration unit itself is temporarily overtaxed, since the flow rate to each evaporator is servoed to the target temperature of the remote tool. The instability arises when the return refrigerant is not entirely in the gas phase, so that liquid is present in the refrigerant returned to the compressor from one or more channels. In consequence, a well known but complex reaction can occur within the refrigeration system in which the compressor drops in efficiency (and can be damaged). When this “flooding” occurs, the greater the demand for cooling capacity the lower will be the performance (the opposite of what is wanted) and, as flow is increased the more liquid will be returned, increasing the likelihood of damage to the compressor system. It is not feasible in this type of system to place arbitrary limits on the refrigeration capacity of each channel, because this would unduly extend the costly process times involved in semiconductor fabrication, even though adequate refrigeration capacity may be available for each channel.
Although conventional proportional flow valves, including temperature responsive control valves using bimetallic elements, can be utilized in the separate channels, it is preferred, primarily because of higher reliability, but also for reasons of linearity, resolution and freedom from hysteresis to employ thermal expansion valves which are responsive to the pressure in a closed gas circuit. These closed circuits include a bulb containing a pressurized gas that is positioned proximate a temperature source at a given temperature level, such as a chilled refrigerant conduit. A tube from the bulb communicates with a volume, within the valve, that is bounded by a flexible diaphragm, which then flexes in response to the gas pressure. The diaphragm in turn controls the position of a valve element which occludes a flow orifice to a selected degree. The internal pressure within the bulb and the closed pressure circuit can be changed by command signals applied to a heater in thermal interchange relation with the bulb, and regulated by a command servo circuit As described in a previously filed application of Richard Petrulio, et al. Expansion Valve Unit, Ser. No. 08/953,101, filed Oct. 17, 1997, assigned to the assignee of the present invention, this system can incorporate a thermal insulation between the heater and the bulb in order to integrate temperature fluctuations. It can be used so as to control refrigerant flow in response to a remotely detected temperature (i.e. a tool, controlled unit, or refrigerated compartment).
Whether referred to as regulation of capacity or avoidance of flooding of the compressor, the system must avoid initiating the astable condition, while at the same time it must be able to utilize the available refrigeration capacity in the most efficient manner for each of the two or more independently operable channels that may be used in the system. Since the control temperatures and the refrigeration demands in individual channels vary independently and since total refrigeration rate also changes, these objectives present unique problems if overtaxing the refrigeration unit is to be avoided.
SUMMARY OF THE INVENTION
These and other objectives of the invention are met by a system utilizing a temperature reference that is responsive to refrigeration unit output to compensate flow rate control signals before an astable condition arises in any channel. The system imposes very low demand on refrigeration capacity, and enables each control channel to utilize available refrigeration capacity to the fullest extent consistent with proper operation of that channel. To these ends, a small reference evaporator is disposed in parallel with parallel evaporator/heat exchanger combinations in each of two or more channels. The reference evaporator is supplied refrigerant through a constant expansion device, such as a capillary, to establish a low reference temperature at the evaporator that varies only with the available refrigeration unit output. The thermal expansion control valves in each of the channels are made responsive to this reference temperature as well as command signals provided to heaters associated with each sensor bulb. Each of the heaters is driven by its command signal to a selectable temperature determined by the servo circuits in the controller so as to balance the valve setting at the needed position for flow control. In addition, the boil off rate at each separate evaporator is indirectly measured by determining the difference between each evaporator output temperature and the reference evaporator output. In one example, a multi-channel controller monitors the temperature differences to insure that a certain minimum differential is maintained, assuring adequate boil off to avoid the return of liquid to the refrigeration unit. When the temperature differential reduces to or below a threshold level, the controller reduces the heater control signal for that channel, thus reducing the orifice size and the amount of refrigeration capacity used in that channel.
If, for example, an initial temperature correction signal represents a transitional cooldown phase, the valve can be opened substantially fully, with the expectation that there will be adequate initial evaporation. The servo command is however subordinated to the compensator, which reduces the flow rate and the demand for refrigeration capacity if the chann
Cowans Kenneth W.
Zubillaga Glenn
B E Aerospace
Bogucki Raymond A.
Jones Tullar & Cooper PC
Wayner William
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