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Select face velocity for Air Handling Units
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How to select face velocity for Air Handling Units in Pharmaceutical applications

1. Introduction


The face velocity passing through cooling coils is one of the most important design factors that directly affect the effectiveness of heat exchange and the size of air handling units (AHUs).


Traditionally, the recommended face velocity passing through the cooling coils should be between 2.0 and 2.5 m/s. See ASHRAE 90.1. This is to avoid water droplet carryover and to maintain reasonable pressure drops.


We have mentioned this sometimes in this blog. See the post, Energy Savings in Pharmaceutical HVAC Design, for instance. Pharmaceutical HVAC systems consume a lot of energy. In this post, we will attempt to establish a criterion for how to select face velocity for Air Handling Units in Pharmaceutical applications, based on a rational decision.

2. What does face velocity affect?


We observe that different face velocities affect the following:

2.1. Unit size


Firstly, as mentioned in the introduction, the unit size will be larger when the face velocity is lower. This has implications for the equipment footprint and costs as the main disadvantages. However, the pressure drop of the filters will be lower and therefore energy consumption will decrease.

2.2. Number of coil rows


As air velocity decreases, residence time increases, so the number of coil rows and fins per mm of the coil will be lower. The result will be a lower pressure drop.

2.3. Condensate tray size


Typically, in cases where dehumidification exists, the condensate tray should be longer the higher the velocity through the cooling coil.

2.4. Filter pressure drop


Undoubtedly, the pressure drop in filters is one of the factors that contribute most to operational costs in pharmaceutical HVAC systems. The face velocity does not affect the pressure drop of the terminal HEPA filters. However, the filters installed in the AHU do play a relevant role: the higher the face velocity, the higher the pressure drop.

3. Let’s take a look at an example


As I always like to do, let’s see an example of a real case for the selection of an AHU with a capacity of 30,000 m3/h and an available static pressure of 850 Pa. We will study its behaviour in terms of pressure drop and ventilation power for two face velocities: 2.54 m/s and 2.0 m/s.


The results are compiled in the following tables:

3.1. Size


The first thing we can see is that for the same flow rate and available pressure conditions, the AHU is larger with the lower face velocity. This implies that the initial cost of the unit will be higher, and the equipment footprint will be larger as well.

3.2. Pre-filter


Next, for the same type of filter, ePM1 70%, the pressure drop will be lower for the lower air velocity. However, as mentioned before, having a larger unit will also require more filter surface area. Therefore, the investment in filters will also be affected.

3.3. Fan


Next, we can see the behaviour of the fan. We can observe that the fan operates more smoothly with a lower air velocity. This is also reflected in the efficiency and absorbed power. Working at maximum pressure drop, the absorbed power is approximately 5% higher for an air velocity of 2.54 m/s.

3.4. Coils


Now let’s take a look at the sections for cooling and heating coils. We can see that working at lower air velocities requires us to use coils with fewer rows, as it increases the residence time of the fluids in them. The pressure drop is reduced by approximately 20% for cooling and 47% for heating.

3.5. Filtration stage


Finally, we will see the behaviour of an 85% ePM1 filtration stage, common in pharmaceutical HVAC systems for the protection of absolute filters. As in the case of pre-filters, we can see that efficiency increases.

4. Conclusions


So, after analyzing all the data we’ve seen so far, we can say that the static pressure of the AHU at a speed of 2.0 m/s is 1202 Pa. The specific fan power is 1.79 kW/m3/s. For the unit at 2.54 m/s, the static pressure is 1251 Pa (4% higher). The specific fan power is 1.87 kW/m3/s (4.5% higher).


But, is this really cost-effective?


As we’ve seen, the unit at the lower speed is bigger. So do the energy savings compensate for the larger investment?


Let’s do some rough calculations assuming that both units work for the same amount of time, 8000 hours per year and that the price of electricity is 0.2 €/kWh.

However, this is an approximate calculation. To compare it properly, we should carry out a detailed analysis such as Net Present Value. This analysis should include maintenance costs and the variation of pressure drop over time. We have not done this to avoid making this post too long. In any case, we see that the savings are significant and the return on investment is attractive. So we can determine that a velocity of 2.0 m/s is convenient.


But, what if we lower the face velocity even more?


The answer is that it is not worth it. Because it is not a proportional decrease. If we lower the velocity below 2.0 m/s, we do not obtain significant energy savings. In the graph below, we can see how the SFP (Specific Fan Power) varies with different velocities in cooling coils.

Here’s the graph showing the variation of Specific Fan Power (SFP) with different air velocities in the cooling coils:

Therefore, we can see that air velocities below 2.0 m/s do not result in significant reductions in SFP. We could consider this value reasonable. Hopefully, this article helps with how to select face velocity for Air Handling Units.

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