Environmental control systems and their use in cleanroom applications
02/01/2007
Selecting the right control system depends on the performance requirements and the need for additional features and functions
By Robert A. Adams, Controlled Environment Structures, Inc.
Every cleanroom or special environment utilizes some type of control system to operate the mechanical equipment that maintains the environment. Whether it’s a cleanroom in a semiconductor, biopharmaceutical, or food application, there are three basic types of controls hardware. In order of degree of sophistication, they are:
- electromechanical controls such as thermostats, humidistats, relays, timers, and switches (no digital communications are available with this type of control);
- electronic setpoint controllers that combine a sensor or other device with an electronic device that reads the sensor input and electronically performs a control action (some models offer basic digital communications in addition to their control functions);
- digital programmable controls (also known as direct digital controllers [DDC], programmable logic controllers [PLC], or “smart”), which combine a series of input and output signals with a microprocessor and memory to run a user-programmable control strategy. Such controls exist in two programming forms-menu-driven and fully programmable-and include a broad range of digital communications, such as serial (RS232/485), Ethernet, or CAN.1
There are two types of control signals: digital and analog. A digital signal is either on or off, true or false, open or closed, set or clear. Examples of digital signals are switches, relays, contacts, and push buttons. Analog signals vary over a range of values. Examples of analog signals are sensors (temperature, humidity, pressure, etc.), voltage signals, current signals, and pressure signals. Control signals that modulate are analog signals. Most electronic analog control devices are actually digital devices that convert digital signals to analog signals and vice versa by using multiple digital steps to simulate the analog signal range.
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Control signals can exist as either an input or an output, and are referred to as I/O. Combined with the signal types, a control system thus consists of digital inputs, digital outputs, analog inputs, and analog outputs. Control systems may also contain a communications system (bus) that allows the system to connect to external devices such as a PC or alarm/monitoring system. The communications bus can allow the control system to be used for data acquisition and for creating a graphical user interface for monitoring system status, trending, alarms, and operator adjustment (see Fig. 1). Finally, the control system may also contain a real-time clock that allows scheduling functions and time/date stamping of the data.
A control strategy consists of an I/O points list and a sequence of operation of the mechanical equipment to be controlled. Generally speaking, a higher level of sophistication of the controls hardware allows the mechanical system designer greater flexibility in the sequence of operation, as well as more options for alarming, monitoring, and data acquisition.
Basic cleanroom control functions
Cleanroom mechanical systems that need to be controlled consist of some or all of the following:
- fans and air handlers
- chillers
- boilers
- pumps
- cooling systems
- heating systems
- humidifiers and dehumidifiers
- dampers
- valves
The above mechanical systems and devices are controlled as per the sequence of operation to maintain a specified range of environmental conditions such as temperature, relative humidity, differential pressure, and airflow. The design of the control system is driven by the complexity of the mechanical system, the sequence of operation, the tolerances of the environmental conditions, and the need for external communication.
Control requirements for semi and electronics cleanrooms
Semiconductor and electronics cleanrooms can represent some of the most challenging environments for the controls designer. The processes that take place inside these cleanrooms, such as photolithography operations and steppers, typically require accurate temperature, relative humidity and airflow control, and the mechanical systems must operate over a wide range of outdoor air conditions. Precise humidity control cannot occur without precise temperature control (see Fig. 2). The continuous heat load from the tools and air handling equipment requires the cooling equipment to run year-round, and the high process exhaust air volume requires a correspondingly large outdoor make-up-air system.
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The constant operation and the high revenue per day generated by semiconductor fabs demand a high level of performance and reliability from the control system. Temperature and relative humidity uniformity of +/- 0.5°C and +/- 2 percent RH are typical requirements. Furthermore, the operating cost of the mechanical system is significant, so control strategies that can reduce operating costs are highly desirable. Thus, it is common to see a high-end digital programmable system with a complex sequence of operation running these cleanrooms. It is also common to see alarm and system status monitoring and even redundant systems to maximize uptime and reliability.
The control system selection for small-scale, research, and single-shift semiconductor and electronics cleanrooms depends on the process environmental requirements, the value of the process or product, and the complexity of the mechanical system. It is actually more difficult to accurately control the environment in a small cleanroom, as the impact of local disruptions is greater due to the lower air volumes used. Unlike in the biopharmaceutical industry, there is usually no regulatory requirement for data acquisition and record keeping, although internal QA may require it. For undemanding environments, simple electromechanical controls can be used, with the added benefit that any HVAC service tech can install and troubleshoot the system. Electronic setpoint controllers add more functionality and features such as local display and built-in alarms, but they are less service-tech-friendly as they typically require a menu-driven setup and diagnostic routine that varies by manufacturer. Digital programmable controls make sense in demanding applications and where scheduling, data acquisition, and alarm monitoring are desired. Service of digital programmable controls is typically provided by the vendor or installing contractor.
Control requirements for biopharm cleanrooms
Generally speaking, the environmental and mechanical system requirements for biopharm cleanrooms are less demanding than those in semiconductor cleanrooms. Temperature and relative humidity uniformity requirements are less stringent, with +/- 1°C and +/- 10 percent RH typical, and the overall airflow volumes are less. However, isolation of various zones requires separate air handling systems, and stepped differential pressure to prevent unwanted infiltration between zones is critical.
GMP pharmaceutical manufacturing often requires validated systems, and regulations require data acquisition and record keeping. In the past, this record keeping was done by manual logs or by chart recorders. The FDA recognized the benefits of digital data acquisition over manual systems and mandated the implementation of digital record keeping for GMP pharmaceutical manufacturing in 21CFR Part 11. Full implementation of 21CFR Part 11 has been delayed, however, due to the cost and complexity of conforming to the regulations. GMP protocols and validation are not generally required in medical device manufacturing and USP <797> cleanrooms, thus the control system requirements can be less demanding.
Design of modern control systems for GMP pharmaceutical manufacturing is thus influenced both by the need for basic control functions and by the ease of integration into automated data acquisition and system validation. Digital programmable controls with communications to data acquisition and alarm monitoring systems can provide an automated solution, with the understanding that the entire control system, including software, can be validated if required. There are software packages available in the market today that tie into most DDC systems and offer validated solutions for 21CFR Part 11 record keeping. Some setpoint controllers have an optional communication bus that can be integrated into a digital record-keeping system, but this function is seldom used, as custom software is generally required to gather the information and convert it into a functional system.
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Another GMP requirement is regular calibration of sensing devices. In any level of control system, proper system design and installation can make the calibration task easier and less costly. By providing calibration-friendly sensing devices, wiring taps, and adjustable control offsets, a well designed control system can simplify the calibration process and reduce the ongoing time and cost of calibration as well (see Fig. 3).
Control requirements for cleanrooms in food applications
The use of cleanrooms in food manufacturing and handling is less common than in the semiconductor and biopharm industries, and even the word “cleanroom” can have a different meaning. In food, cleanrooms refer to space that is constructed and maintained to strict sanitation requirements, although there may not be a requirement for ISO certification of particle counts. Also, many food applications have the additional design requirement of refrigerated space.
There are guidelines for the different types of food manufacturing and handling applications from the FDA, USDA, and food safety organizations. For example, the American Meat Institute (AMI) has 11 principles for sanitary design of facilities, which include room temperature, humidity, airflow and air quality. Record-keeping and monitoring requirements are spelled out in the Hazard Analysis and Critical Control Point (HACCP) guidelines, and in the Code of Federal Regulations Title 9.
Control system selection for food-related cleanrooms is similar to biopharm in that the environmental requirements are less demanding than those in semiconductor cleanrooms. However, refrigerated and high-humidity/wash-down environments require the selection of special sensors that can operate reliably under these conditions. Today, simple electromechanical controls are typically seen on refrigeration equipment, as the market has been somewhat slow to embrace more sophisticated systems. With the push to reduce facility energy and other operating costs, it makes sense to use smart controls that will run the equipment efficiently, allow for scheduling, send alarms when something is wrong, and periodically record the room conditions as well.
Alarm/monitoring systems
If environmental conditions such as temperature, relative humidity, and room pressure must be maintained to protect the product or process, then an alarm/monitoring system can provide real-time monitoring of critical parameters and notify personnel when a condition goes out of limits (see Fig. 4). Some common reasons for monitoring and alarming include product or process requirements; quality assurance; regulatory requirements; and insurance requirements.
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Before digital communications with control systems were available, alarms and monitoring came from add-on, stand-alone systems such as chart recorders and setpoint alarm relays. With the advent of digital control systems and digital communications, control system designers gained the ability to alarm, monitor and collect data directly from the control system and to store and display the information on a PC. Modern systems now use the Internet and wireless communications to share data in real time with any location that has access. Some advanced alarm notification strategies include dial out, pager, text messaging to cell phones, and e-mail alarms. These strategies make sense in facilities that contain a high-value product or process or where downtime is costly.
Since all points in a digital control system are available to the communications bus, the mechanical system parameters can also be monitored and alarmed so that facility personnel can quickly find the source of problems. Rolling trends are an especially effective method of displaying the data for monitoring. These parameters and alarms can also be recorded in a historic database so that users can view the records to pinpoint when failures occurred. A PC-based graphical interface can be designed to display these functions both locally and across a network. If real-time, remote service monitoring is available, it helps service techs prepare for a call by identifying the source of problems before traveling to the site. In addition, most cleanroom particle counters include a digital communication bus that can be read into a control system for real-time particle monitoring, data acquisition, and alarming.
Conclusion
The selection of the right control system for cleanrooms and other special environments depends on the performance requirements and the need for additional features and functions such as alarms, monitoring, scheduling, energy management (operating cost reduction), and data acquisition. The advantages of custom sequence-of-operation programming and the availability of the additional features and functions mentioned above make a compelling case for the selection of a digital programmable system. The continuous decline in the price of digital hardware has made the cost of these systems similar per point to electromechanical and setpoint controllers, so it makes good economic sense to start with a digital system even if the advanced functions are not initially required. One disadvantage of a digital system, however, is that many maintenance and service technicians are not familiar with the technology and are thus unable to effectively troubleshoot and solve problems. This problem can be reduced by incorporating into the control system design tech-familiar interfaces to the mechanical equipment, such as control relays with HOA (hand/off/auto) function, manual valve position indicators, and well written troubleshooting documentation.
Robert A. Adams is president and co-founder of Controlled Environment Structures, Inc. (Mansfield, MA), a manufacturer of insulated panels, cleanroom wall systems, integrated environmental control systems, fans and fan filter units, and doors for refrigerated environments. He has a BS in chemistry from the University of Houston and his background includes many years as a sales engineer, designing and selling cleanrooms, environmental rooms, and computerized control systems. He can be contacted via e-mail at [email protected].
Notes
- It is important to note that the cost of digital programmable controls hardware has fallen to the point that it is comparable to the less sophisticated controls hardware. However, there is an additional software cost to implement the additional features.