The purpose of a cleanroom recovery test is to determine the contamination decay rate for a cleanroom. The cleanroom is artificially challenged with generated particulate, and the ongoing particle concentration is continuously monitored with a light-scattering airborne particle counter. It is not recommended for unidirectional airflow cleanrooms, however, because the recovery rate is too brief to measure effectively. It is well-suited for all other classes of cleanrooms, including non-unidirectional ISO Class 5 rooms up to and including ISO Class 9.
The concept behind the room recovery test is to determine how rapidly the cleanroom can purge itself in the event of a high contamination incident. Room recovery, not surprisingly, is an exponential decay phenomenon. The theoretical recovery time can be calculated based on two factors: the clean air change rate and the uniformity of contaminant mixing.
An exponential decay equation has a particular form. Equation 1 describes an exponential decay equation:
Equation 1: Y = Ae^(-kt)
In this example:
- Y = the decaying room particle concentration at any time, t, during the decay
- A = the initial concentration at time = 0, the beginning of the decay, C(0)
- -k = the constant value component of the exponent, (-k*t)
- e = the base of the natural logarithm ~ 2.718
- t = the expired time at which the instantaneous value of Y is desired
To normalize empirical data and to be able to compare recovery trials for different rooms, the recovery time (t) is formally defined as the time required for the initial concentration to decay precisely two log scales, that is, 100:1.
Regardless of the initial artificial concentration, the room recovery time is the time required for the clean air supply to remove enough particulate to reduce the concentration to one percent of the starting concentration. The recovery test has nothing to do with the normal steady-state room particle concentration ISO class, as is often misunderstood.
Recovery times, t0.01, are a function of the room air exchange rate and, with a perfectly mixed initial concentration, should approach 4.6*τ, where τ is the reciprocal of the air exchange rate (3600/ACH) with recovery time measured in seconds. The reason for this is that if the base of the natural logarithm is raised (2.718)^4.6, you will get 100 – or two log scales.
Industry standards commonly invoke a number of parameters affecting cleanroom recovery rate. Some of the popular variables include air recirculation ratio, inlet-outlet airflow geometry, thermal conditions, equipment load, and air distribution characteristics. Presumptively, the most important factor contributing to the room recovery rate is the air change rate. However, in this study, inlet-outlet airflow geometry plays a very significant role.
Equation 2 gives the theoretical relationship between air change rate and recovery time as the solution to a mass balance equation. As an idealized equation, the assumption of perfect mixing is implicit.
Equation 2: V δC = F(t) – Q(t)C(t) δt
Translated into English, Equation 2 simply means: The volume-rate of change of concentration in the room is equal to the rate of contaminants coming in the room minus the rate of contaminants leaving the room. This mass balance equation could also describe your checking account balance or the number of your children under your roof.
After artificially contaminating the room for a few seconds, and with the artificial particle generation rate, F(t), equal to zero after shutting off the aerosol generator, and the HEPA-filtered airflow, Q, constant with time, t, the equation solution for particle concentration at any point in time during the test, C(t), becomes Equation 3.
Equation 3: C(t) = C0e^(-Qt/V)
Airflow, Q, divided by room volume, V, can be recognized as air change rate, ACH, as seen in Equation 4.
Equation 4: ACH = Q/V
Therefore, the time decay equation for room concentration becomes Equation 5.
Equation 5: C(t) = C0e^(-ACH*t)
This is now in the standard exponential decay form of Equation 1, where C0 is the initial concentration at the time, t = 0, after shutting off the artificial particle generator. If the desired C(t) = 0.01*C0, as defined by the recovery rate, the solution for t0.01, after correcting for time units becomes:
Equation 6: t0.01 = 4.6τ
Where the time constant, τ is seconds/air change, which can be found as 3,600/ACH. This means there is a direct, predictable relationship between room air change rate and room recovery rate. The recovery rate obviously decreases with increasing air change rate, so the reciprocal of air change rate, τ, is used. Longer times than 4.6τ indicate relatively inefficient cleanroom ventilation, while recovery times less than 4.6τ indicate better than expected performance.
Case Study
A cleanroom facility containing a combination of two distinct types of HEPA filter diffusers had extensive room recovery testing performed. In the first type, the HEPA filter screen had adjustable lamellae, which are designed to disrupt the unidirectional HEPA filter face discharge velocity profile, creating a sideways-directed turbulent flow. Rooms containing this type of air diffuser were equipped with typical low-wall return systems.
The second type of HEPA filter diffuser analyzed in this study was a combination supply/return diffuser whereby air is supplied to the cleanroom with continuous perimeter sideways-lamellae and center air return, self-contained in the same fan filter unit. These rooms had no additional air return provisions. This second type of diffuser finds its utility, according to the manufacturer, in ISO 14644-1:2015 class 8 rooms, whereby the design intent is turbulent dilution rather than unidirectional airflow.
In each room, the room recovery location was taken at the location in that room that had the highest airborne particle count from previous ISO 14644-1:2015 classification testing. At least three recovery trials were conducted in each room, and the precise 100:1 recovery time was calculated algebraically. In practice, most empirical reduction ratios will be between 105:1 and 150:1. This necessitates the need for algebraic solutions to determine the precise 100:1 recovery time. The 95 percent confidence recovery time was calculated for each room and is used as that room’s recorded recovery time, t0.01.
The particular rooms chosen for this study had bubble-style pressurization with respect to all surrounding rooms so that the precise air exchange rate could be calculated without additional complications from clean air infiltration.
The range of air exchange rates in the present study was fairly narrow, with a minimum of 25.3 and a maximum of 30.6 air changes per hour. The actual 95 percent upper confidence limit (UCL) recovery times ranged from 569 to 1,171 seconds. What is most apparent from these results in Table 1 is that all of the supply-return (Type 2) rooms had worse recovery ratios than all of the twist diffuser (Type 1) rooms.
In conclusion, it seems that a significantly better contamination reduction yield, ceteris paribus, comes from the type of room design having twist diffusers and conventional low-wall returns rather than the self-contained supply/return ceiling inlet/outlet fan filter. This study is limited to the two types of HEPA air distribution devices represented and does not evaluate the most common type of design, i.e., conventional terminal HEPA filters with low wall return. A direct comparison of that nature would isolate the beneficial effect of twist versus conventional supply diffusers.
However, the twist diffuser, in combination with conventional low-wall returns, was very effective in approximating ideal mixing conditions, as shown by the clustering around the 1.0 ratio. The twist diffuser and low wall return combination optimized the contamination reduction efficiency.
Looking to get NEBB Certified? Request your application today.