Model formula

Computational fluid dynamics (CFD) simulation has proven its validity and value in predicting air movement in cleanrooms. However, the user's engineering judgment is still crucial to a successful CFD simulation. This article will explain how to apply CFD software, and how to ensure that all the relevant physics are included in the model. For example, in a cleanroom application, all diffusers and major flow obstacles must be represented. Significant heat sources such as human occupants and equipment must also be included.

The “building envelope” can be thought of as everything that separates the interior of a building from the outdoor environment, including the windows, walls, foundation, basement slab, ceiling, roof, and insulation. In the context of the CFD model, consideration of the envelope and in particular, which aspects of the cleanroom envelope are to be included in the model, is an important first step. The 3D volume for which air velocity, temperature, and so forth are to be calculated is called the solution domain. The extent of the domain is determined by the objectives the person is trying to achieve with the CFD results, as well as the practical consideration of the available data.

Solid-surface aspects of the building envelope are normally used to construct the edges of the solution domain. The reason for this is that the setting of CFD boundary conditions such as ambient temperatures are more easily applied to solid surfaces than to fluid planes or open surfaces where there can be wide variation of conditions. At solid surfaces, the variation of conditions is usually much smaller, because cleanrooms are typically located inside the overall building structure and are therefore not sensitive to variations in external conditions such as solar loading.

Process equipment and filters

With the overall solution domain of the CFD model set, the focus can then be turned to the objects that populate the cleanroom. One form of model to be considered is one in which nothing populates the cleanroom or clean corridor, and the system is balanced as an empty unit. However, the reality is that there are usually significant amounts of equipment and operators in these clean spaces, thereby ensuring that the conditions in the room are significantly different from the empty space scenario.

The issue of equipment and operators must, therefore, be addressed in most cases. To include every object in detail is not feasible from the viewpoint of computational tractability, nor is it necessary. In reality, the only objects that need to be included are those that present a significant blockage to airflow or those that dissipate heat, as these are the items which will affect the desired laminar flow conditions within the cleanroom.

Representation of HEPA filters in the simulation is necessary when the details of the ceiling plenum in the cleanroom are to be included in the model. For example, representation is required if non-uniformities in the flow distribution over the ceiling into the cleanroom need to be evaluated.

The representation relies on the conversion of the pressure drop versus airflow rate curve for the HEPA filter into appropriate CFD boundary conditions. A standard means of representing a resistance to airflow is to equate the pressure drop to the velocity through the HEPA filter, as shown in equation 1.

The main issue is then the representation of the loss coefficient, ƒ. This is handled in different ways by the various CFD codes. The curve for most HEPA-filter types can be essentially thought of as providing a linear pressure drop with increasing flow rate - for instance, ?p ? v. There should be enough flexibility in the CFD software to include such a relationship. Care should be taken in evaluating the manufacturer's data-specifically, to check whether the performance data refers to the media making up the filter, or the whole unit. In the former case, additional detail may be required to ensure an accurate representation of the filter.

A representation of a perforated plate will also have to be added to the model immediately underneath the HEPA filter. This will add an extra pressure drop to the system represented by the ceiling - in this case, the pressure drop will follow a more traditional ?p ? v2 relationship.

Diffusers, leakage and pressurisation

The representation of diffusers is similar to that of HEPA filters: it should be developed with available manufacturer's data for accuracy. Typically, the manufacturer's data is displayed to give the distance from a reference point to where an isovel (line of constant speed) is reached. This “throw” data, for example, provides the distances from the face of the diffuser to the 50, 100, and 150 fpm isovels.

The usual diffuser type defined in cleanrooms, namely the laminar diffuser, is readily defined as a single-fixed flow device. However, laminar diffusers typically have low free-area ratio faces, on the order of 10 to 20%. Therefore, it is important that the entrainment characteristics (i.e. the way that the flow at the diffuser face ingests the air from the rest of the room) are handled correctly. This means that the CFD code should account for the local acceleration of the airflow through the face of the diffuser.

The issue of leakage and pressurization also needs to be carefully considered in the definition of the cleanroom model, since even relatively small flows through the fabric of the cleanroom can impact flow conditions within the room. One obvious path for leakage or pressurisation in the cleanroom is the door crack. The relationship between the pressure drop and flow rate through the door crack is determined by a variation of Equation 1; see equation 2.

This equation can also be applied to process holes in the fabric of the cleanroom, with the flow coefficient dependent on the shape of the hole. The user can, therefore, set the flow coefficient for the crack and the pressure beyond the room, which, in turn, calculates the flow rate, or set the flow rate at that location.

If no pressure boundary condition is set in the cleanroom, the values of pressure that are calculated by the CFD model are variations in pressure about an arbitrary base value. In order to determine actual pressure values in the cleanroom, there must be at least one pressure boundary condition in the cleanroom, which acts as a datum point.

building the grid

The following should be considered in terms of building the computational grid for the cleanroom analysis:

The grid should be refined in regions where high gradients are expected. In the cleanroom, for example, there are several areas where the gradients of particular physical values are high:

• Regions close to flow devices or cracks in the cleanroom fabric; for example, diffusers, door cracks, etc.
• Regions close to physical objects or objects that dissipate heat; for example, pieces of equipment, technicians, etc.
• Sources of contamination; for example, spillages, technicians.

This article has discussed some of the key issues and engineering judgements required to ensure accurate, reliable and useful CFD simulations for cleanroom design. When properly applied, CFD simulation can generate substantial improvements in cleanroom design by enabling engineers to identify problems and evaluate potential solutions long before equipment is purchased and construction begins. CFD simulation may also be very useful after the commissioning stage, in order to fine tune and optimise air handling and ventilation equipment.