The Priority Is Pressurization
Is pressurization required?The first issue to address is whether or not pressurization control is required for all or parts of a facility. Must the enclosed area be kept clean and free of any contamination from outside sources? Are production and/or processing procedures producing hazardous materials or odors that must be contained to prevent migration to other areas? Other than mandated requirements, these factors are the most common reasons for requiring pressurization control.
Next, determine what specific areas are to be controlled. Would this area encompass a large, open space? Or instead a segmented (or isolated) part of a larger area? For example, if the production process creates noxious gases which are not particularly dangerous, you need to determine whether those gases can be contained by specialized exhaust equipment. In many cases, this would be a more sensible and cost-effective solution than installing equipment to maintain negative pressure in a large area just to prevent fumes from entering other parts of the building. This is obviously a critical issue and should be evaluated carefully with regard to pressurization of a large workspace vs. localized exhaust or containment.
The same rule applies for cleanroom spaces. It should be determined whether you need to maintain a Class 100 space at 1,000 sq ft or whether a clean bench within Class 100,000 workspace would be sufficient. Obviously, these decisions will affect the direction you take since they will determine the amount of airflow required to maintain those conditions.
Two Methods of PressurizationOnce it has been established that pressurization control is needed, there are a number of methods to accomplish it. The two most popular methods include airflow tracking and differential pressure control. This discussion will provide information on each of these methods, explaining their differences, advantages, and disadvantages, and provide examples to help you make a determination as to which method might be more suitable for your particular application.
Airflow Tracking vs. Differential PressureControlling room pressurization using airflow tracking methods is based on the principle of measuring and controlling airflow in and out of a confined space. This method maintains desired cfm differential (offset) between supply and exhaust air and permits precise airflow control resulting in either positive or negative pressure, depending upon requirements. Differential pressure is a technique that directly measures the pressure difference from the enclosed workspace to a reference space (usually an adjacent corridor). Variable airflow control into the pressurized workspace is used to maintain a fixed level of differential pressure between the controlled space and adjacent area.
Advanced Instrumentation Permits Precise ControlAs with most technologies, each of these offers certain advantages (as well as disadvantages), depending upon circumstances. The good news is that each of these technologies is practical and easy to achieve thanks in large part to development of new instrumentation capable of measuring extraordinarily low levels of differential pressure (typically in the region of .001 in. wc). To a large extent, lack of accurate instrumentation in the past prevented the possibility of achieving space pressurization at desired levels (typically about .02 in. to .05 in. wc). In the past, pressurization levels were almost an order of magnitude higher, mainly because of limitations in both measurement devices and display instruments.
Lower levels of differential pressure are desirable for several reasons. Safety considerations dictate reasonable levels of differential pressure be maintained so that doors may be operated properly and safely. For example, if a door opens inward and the space is at a high positive pressure, it may not be possible to open the door; conversely, if that space were under a high negative pressure, releasing the latch may cause the door to fling open possibly causing injury to the person attempting to exit.
Second, depending upon construction, it is possible that the pressurized space could implode - blasting ceiling panels downward where injury or equipment damage could occur. Third, in order to maintain accurate temperature control, it is desirable to minimize the quantity of air infiltrating or exfiltrating the space.
Basically, from a technology standpoint, either airflow tracking or differential pressure control will achieve the same objective; that is, to maintain the desired direction of airflow into or out of a space. On the other hand, there are factors that must be considered to help you determine which method is best for your application, and the most important of these are architectural details and access to/from the controlled space(s).
Traffic Flow and Building ArchitectureThe first consideration is access to and traffic flow in the workspace. You should determine how accessible the area (or series of areas) is to general spaces and means of egress to the facility. Is it located in a high- or low-traffic area, for example? Will there be limited access to the pressurized space? Will air locks be utilized? How many people will be involved in that space? Answers to these questions will help you determine the most suitable technology.
For instance, if the pressurized space is in a high-traffic area open to active movement of many people, pressurization control would not be desirable because of the multiple upsets to the airflow in the workspace and therefore too many pressurization variations.
Remember, every change in the room or reference pressure causes the control system to respond and vary the airflow to or from the controlled space. You may also look at some of the general construction details in the facility to help determine whether the walls that define the space are contiguous, for example, from structure to structure, slab to slab, or floor to floor.
The logic here is that there should be no possibility of air migration from room to room above the ceiling. Essentially, the more you can isolate the controlled space, the easier it will be to successfully implement differential pressure control methods vs. airflow tracking (Table 1).
Cascaded Pressure ControlA variation that uses elements of both tracking and pressure control is known as "cascaded pressure control." This technique measures all supply and exhaust flows in and out of a workspace and maintains a fixed cfm differential between supply and exhaust air. Cascaded control adds the element of measuring differential pressure in the space as well, using that measurement as a reset point to the cfm offset. This allows the cfm differential to be varied between minimum and maximum values to respond to any influences that might affect the pressure.
The advantage of this technology is that it provides the stability of airflow tracking with the flexibility of allowing variable cfm differentials to meet temporary external conditions without sending the space out of control.
Differential Pressure MonitoringAnother variation that has application in certain areas as well, is tracking with differential pressure monitoring. In this application, airflow tracking is used as the control method, and differential pressure monitoring is overlaid to function as an alarm setpoint and as a maintenance management point throughout the building's ventilation system.
For example, if a differential pressure measurement changes over time (other than when a door is opened or closed), it usually indicates that one of two events occurred - either airflow was degraded on one side of the system (thus eliminating desired differential pressure), or there has been a change in the envelope. Someone may have opened a hole in a confining wall to install a new piece of ductwork without proper sealing, for example; or perhaps a pipe was installed through a floor and the resultant gap was not sealed. Cascaded pressure control techniques can be handy in these applications since they don't particularly add complexity to an overall control scheme.
Since we've discussed comparisons for differential pressure vs. airflow tracking technologies, we will now look into overall environmental control. In a critical environment, there is generally a sequential hierarchy for most control requirements starting with pressurization, then temperature, then humidity.
System Performance ConsiderationsThe method of pressurization control selected obviously has a direct bearing on managing these other parameters. For example, with airflow tracking you can achieve the desired temperature in the workspace to a very high degree of tolerance; however, if you are using differential pressure, you may lose temperature control because the volume of air is being dictated by pressure requirements as opposed to temperature requirements. In essence, if pressurization control is the driving variable for the quantity of airflow into the space, all other control parameters may suffer as a result, with temperature usually being the first.
For example, if you wanted to maintain a space under a negative pressure and were using differential pressure control, the supply airflow would vary to maintain pressurization. If a door were opened to that space, the first response from the system would be to reduce the amount of supply air into the room so as to maintain a negative pressure.
The system response would likely include closing or reducing the supply airflow to zero in order to maintain the space at a negative pressure. When that occurs, temperature control is lost immediately. And if supply air is used to maintain humidity control (and it usually always is), you would also lose control over humidity in the space as well.
If airflow tracking is the control method, the system will not respond to a door being opened. While pressurization is not maintained, the cfm differential remains the same. Consequently you will still maintain direction of airflow for pressurization purposes while maintaining temperature and humidity control - or overall environmental control. This is basically the only way to accomplish this level of control if those parameters are critical.
Temperature/Humidity ManagementWe should discuss tolerance levels for temperature and humidity management here as well. In most hospital, laboratory, or even manufacturing spaces, you can expect to control temperatures to tolerances of ± 1° to 2°F. Humidity is more difficult to control and usually has a wider tolerance parameter, depending upon environmental factors within the facility.
Typically, a hospital or laboratory would ideally maintain humidity at 50% rh but that can easily range between 40% to 60% rh which is usually an acceptable deviation. On the other hand, for many manufacturing processes (such as tablet production), humidity control must be much tighter, particularly if you are involved with materials such as powders or coatings, which are highly sensitive to ambient moisture content.