Mixed and stratified ventilation in manufacturing halls – what you need to know.

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The energy-efficient operation of hall ventilation is insured according to the current view by regulations in the Energy Saving Ordinance (EnEV) and the Energy-related Products Directive (ErP). They prescribe high requirements on the efficiency of room ventilation equipment. In fact, the actual energy saving starts much earlier, namely with the selection of the calculation method that is most suitable for the particular requirements on ventilation of a hall. The methods are different and if the application is not adapted there can be significant fluctuations in the required incoming air flows. That makes it clear how important it is to select the calculation method adapted to the application.

In manufacturing halls with a high load level, beneficial heat loads and specified harmful substance limits can only be maintained in the long term if the ventilation measures are adapted to the processes. The task of hall ventilation is to distribute the incoming air flow by means of a monitored and thus calculable room air flow within the hall so that heat and harmful substances that are released will be optimally picked up and removed with the extract air. Three principles are available for the room air flow, namely:

-       Displacing

-       Mixing

-       Stratifying

The individual room air flows are considered below with regard to their possible application in manufacturing halls:

1. Displacement flow

In the displacement flow, the supply air flows through the hall in a piston shape from one enclosing surface (wall, floor or ceiling) to the opposite surface without backflow. To avoid disruptions caused by transverse flow, air speeds between 0.2 and 0.5 m/s must be selected for this flow type. That leads to area-related supply air flows of 720 to 1800 m³/(h∙m²), i.e. values that are significantly in excess of those which are normally encountered in manufacturing halls. As a result, this guidance is restricted to special cases such as aircraft painting systems.

The principle of displacement flow_Hoval

Figure 1: The principle of displacement flow

2. Mixed ventilation

In mixed ventilation, warm room air contaminated with harmful substances is mixed with cool, less polluted supply air in order to achieve specified, low heat and harmful substance loads in the working area. Assuming complete mixing, the same thermal and substance conditions are achieved throughout the entire volume of the hall. Mixed ventilation systems can be implemented in various ways: individual air jets vectored downwards from the hall ceiling (see Fig. 2) or tangential ventilation with horizontal air supply under the ceiling or above the working area.

The principle of mixed ventilation_Hoval

Figure 2: The principle of mixed ventilation

3. Stratified ventilation

The principle of stratified ventilation is based on using the thermal upcurrents at the load sources in the working area in order to remove the burden to the upper, unused hall area (see Fig. 3). This involves creating layer of supply air at the load sources in the working area by means of low-impulse inflow, with the height of the layer depending on the power of the thermal upcurrents which are established and the magnitude of the incoming supply air flow. Such stratified ventilation systems can be achieved with three types of air guidance:

-       Zonal displacement flow

-       Zonal mixed ventilation

-       Stratification flow

These have the following properties:

3.1. Zonal displacement flow

In this air guidance, supply air vents with a large area are generally used at individual workplaces to keep a limited protected area free from harmful substances. When used over a large area, this type of flow would once again revert to displacement ventilation with the restrictions as already described.

The principle of zonal displacement flow_Hoval

Figure 3: The principle of zonal displacement flow

3.2. Zonal mixed ventilation

In this guidance, the cooler supply air is input into the hall through supply air vents in the floor. The supply air flow is largely vectored in the same direction as the thermal upcurrents, meaning there is little disruption. This type of air guidance requires either false floors or air ducts in or on the floor, i.e. conditions which are scarcely possible to achieve in production halls. In practice, this air guidance, which offers very high performance levels, cannot be used for manufacturing halls.

The principle of zonal mixed ventilation_Hoval

Figure 4: The principle of zonal mixed ventilation

3.3. Stratification flow

Stratification flow involves disrupting the thermal upcurrents in the hall to the lowest possible extent by the supply air flow. Ideally, the supply air passes through large air events so that air speeds of 0.5 m/s at the vent surface and 0.2 m/s in the working area are not exceeded. There are also three variants in this case:

-       Supply air entering above the working area:

The cool supply air flow accelerates as it moves towards the floor due to its higher density, and spreads out there although only leading to minor disruptions to the thermal upcurrents at heat sources. This effect can be reduced all the more by cutting the fall height of the supply air.

Supply air entering above the working area_Hoval

Figure 5: Supply air entering above the working area

-       Supply air entering close to the floor in the working area:

The lowest destruction is achieved by arranging the air vents in the working area directly on the floor, although there is still an acceleration of the cooler supply air above the height of the air vent.

Supply air entering close to the floor in the working area_Hoval

Figure 6: Supply air entering close to the floor in the working area

-       Supply air entering close to the floor in the working area with impulse stabilisation:

This downward flow can be decreased by reducing the downward movement with small, high-impulse, horizontally vectored supply air jets from the air vent, with the effect that the thermal upcurrents are hardly disrupted at all. This procedure is referred to as impulse stabilisation:

Supply air with impulse stabilisation_Hoval

Figure 7: Supply air with impulse stabilisation

4. Which air guidance methods remain available for practical use?

These considerations show that the requirements on ventilation of a manufacturing hall can only be achieved in practice using two types of air guidance: mixed ventilation and stratified ventilation.

But which hall ventilation variant is the better solution depending on the procedures in the hall?

5. Application limit of mixed ventilation

In mixed ventilation, and air status is established filling the space in the entire hall which is actually only required within the working area. This leads to the conclusion that high loadings will quickly bring this air guidance principle to the limits of its application possibilities. The basic equation on removing a heat load from a hall gives the proportion showing that the required supply air flow increases in direct proportion to the increasing heat load. With very high heat loads, this results in such high air flow rates that air speeds of unreasonable velocity would occur in the hall. Although the inverse proportionality to the temperature differential does have a reducing effect, it only provides limited assistance because in practice only values from 10 K to 15 K can usually be achieved.

This indicates that the principle of mixed ventilation is more suitable to lower heat loads. Higher loads lead to larger thermal upcurrents, meaning it can be expected that stratified ventilation will be more suitable when heat loads are larger.

6. Concept of stratified ventilation

For a better understanding of its function, we will consider a heat source in an enclosed hall. This gives rise to a thermal upcurrent which inducts ambient air and transports it upwards. For reasons of continuity, an equally large air flow is returned from the upper to the lower hall area, where it is once again inducted into the thermal upcurrent. In practice, this return flow of polluted air into the working area is not desirable. The conditions can be improved by mechanical ventilation involving removing part of the polluted air from the hall in the upper area of the hall and supplying the same amount of cooler supply air in the lower area. The return flow will be reliably, completely avoided if the extract air flow removed from the hall is as large as the thermal upcurrent which is followed by an equally large supply air flow. The following conclusion can be reached: the supply flow required for stratified ventilation is calculated directly based on the magnitudes of the thermal upcurrents.

Concept of stratified ventilation_Hoval

Figure 8: Concept of stratified ventilation

7. Heat loads with mixed and stratified ventilation

The heat load to be removed with mixed ventilation is made up of the total value of the loads of the production systems, solar radiation, lighting, people as well as transmission and heat flows absorbed or given off by parts of the building. These values must be calculated or measured, something that is occasionally a difficult and complicated procedure, especially when production systems are involved. However, with regard to ventilation technology, only the proportion of the heat loads to be dissipated by convection (which must be calculated in addition) need to be considered.

In stratified ventilation, the magnitudes of the thermal upcurrents above production systems significantly depend on the heat quantities which are output from their surfaces to the hall air by free convection. These are surfaces and bodies which can be imagined as combining to create production machines in a simplified view, i.e. vertical and horizontal surfaces as well as cylinders (machining centres, injection moulding machines). In the case of horizontal surfaces and horizontal cylinders, the output heat flow is introduced directly into the calculation. At vertical surfaces, it does not appear explicitly, meaning that it does not have to be calculated either. The same applies to vertical cylinders, which can be treated using the equation for vertical surfaces to a good level of approximation. The calculation of the heat output from horizontal surfaces and cylinders according to  uses the particular convective heat transfer coefficients ; calculation methods are specified for these in the literature and in codes of practice. As a result, the convective heat loads to be removed can be calculated.

This calculation offers a significant advantage compared to mixed ventilation. The proportions of the heat load to be removed convectively are calculated directly as a component of the process. This means they do not have to be calculated from the total load or estimated.

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Author
Tobias Brugger
 
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