Airside system selection for cleanrooms
By Joseph DeLaurentis, Kinetics Biopharm
Table of Contents
Overview
Cooling requirements for dummies
Primary/secondary system definition
Conventional airside system definition
Fixed return air bypass system definition
Which system should I use?
Overview
Should we go with the ol' Primary/Secondary system, or utilize a conventional HVAC system (see definitions below)? What about investigating other system types? These dilemmas seem to come up for the HVAC engineer on every project associated with clean rooms. With engineering and design budgets being cut like a hot knife through melting butter, there is usually not a lot of time to provide proper studies and evaluations for each system. Many times, engineers are set in their ways, and usually design systems in the same manner as the last similar system they engineered. The problem with this, as you'll see later in this article, is that the optimum system design varies with the cleanliness classification. This article provides a general overview for system selections.
Cooling requirements for dummies
The first step is to calculate the clean room cooling and dehumidification load and the number of air changes required to meet the respective cleanliness classification. Usually, the cooling and dehumidification load required air changes fall short of the minimum air changes required for the clean room.
Example: A 10' x 10' x 9' office may require, say, 150 Cubic Feet (of airflow) per Minute (CFM) to provide proper cooling and dehumidification. If that office is a class 100,000 room, 300 CFM is required to meet the minimum 20 air changes per hour. This provides the engineer with a dilemma. A large fan is required to meet the cleanliness classification, but the cooling and dehumidification coil doesn't have to be as large to match the air handling unit. What is the best way to maintain the required room temperature and humidity requirements? How can I design a cost-effective system to maintain these conditions without over-sizing the central air handling unit cooling coil, and provide operating and capital cost savings at the same time, while satisfying the maintenance aspect?
Primary/secondary system definition
This system is a favorite among many engineers (not me), and is extensively used by many companies. This system provides most, if not all of cooling and dehumidification from a "primary" air handling unit. This primary unit feeds the conditioned airflow into what is referred to as "secondary" systems.
Each secondary system's main function is to provide the proper amount of airflow into the clean spaces, and to provide the proper area zoning segregation as required by the process application, cleanliness classification, etc. The primary system should be redundant. If the primary system shuts down for any reason, and redundancy is not provided, then all of the secondary systems will be without make-up airflow, which means the clean rooms will not be properly pressurized.
Another item to consider when utilizing this design, is that airflow isolation should be provided for all secondary systems from the primary system. You want to make sure that all other secondary units can operate if one secondary system is down.
With that said, the primary system fan control requires a wider air quantity and pressure range than what is normally required to compensate for filter load-up. You now have to compensate for make-up airflow to say, only four secondary units versus the maximum of eight. The primary system must be closely evaluated to perform this function.
It is important to note here, that components that utilize constant air volume such as constant/variable air volume boxes (boxes with automatic air volume dampers or valves) from the central system to provide active room pressure control are not recommended in this article. Not only am I not a big fan of these for clean room applications since air pressurization needs to be maintained, but many times they don't work. Do you think your 20,000 CFM system will adjust for 100 CFM required to meet one clean room air pressure? I don't think so! Also, it is difficult to obtain stable room pressurization while air volume dampers on the system are constantly moving or hunting to find the proper setting.
Conventional airside system definition
A conventional airside system provides all airflow, cooling and dehumidification from one system which conditions the return and outside air, with a full size cooling coil. All of the return and outside airflow flows through the cooling coil, whether the space requires this large of a cooling coil or not. This is obviously an energy hog, since all of the airflow is cooled down to essentially the saturation point. Remember, we don't always need all of this airflow cooled down for cooling and dehumidification purposes, but that's how cooling coils work. Also, this airflow must be reheated back up to the respective temperature for various zones to meet the required room conditions. I like to compare this to a residential air conditioning unit operating at night, with the bed covers acting as the reheat coils.
Fixed return air bypass system definition
Notice the term "fixed". When designing airside systems for clean rooms, modulating dampers tend to be problematic for the clean room applications, as I mentioned earlier. This of course, does not include systems specifically designed for room pressurization control, which should be segregated from the central systems. I indicated one reason for this earlier in this article.
Anyway, the fixed face and bypass is similar to a conventional airside system. However, this system utilizes an air handling unit where the fan is sized for the total airflow, and the cooling coil is sized only for what is required for the cooling. Hence, some return airflow bypasses the cooling coil, going directly to the fan.
It is very important to note, that the Psychrometric calculations (thermodynamic air property heat transfer equipment calculations), in concert with the individual room load calculations, must be closely evaluated! I can't stress this point enough. The engineer must ensure that the cooling coil is properly sized, and that the worst case room delta T (temperature of room - required temperature of airflow entering room) is utilized. This type of system will also decrease the size of the reheat coils since the leaving airflow is slightly warmer than that of a conventional system. Hence, less operating costs.
What the hell is he talking about? In a nutshell, if too much air is calculated to bypass the cooling coil, then the room temperature and humidity conditions will not be maintained.
Which system should I use?
If you have the time, money, and resources, an evaluation of all systems should be performed. Otherwise, the following chart can be used as a rule of thumb. The numbers indicate choice of preference for each application.
About the author: Joseph DeLaurentis has more than 21 years of engineering and design experience, including 16 years in the biopharmaceutical and microelectronics industries. His responsibilities have encompassed all design phases and tasks from conceptual design through construction turnover to owner. Joe attended Drexel University.
For more information: Joseph N. DeLaurentis, Lead Building Mechanical Systems Engineer, Kinetics Biopharm Division, 7 Penn Center, 1635 Market St., Philadelphia, PA 19103. Tel: 215-656-2544. Fax: 215.561.4444. Email: jdelaurentis@kineticsgroup.com.