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Recirculation Units Design Recirculation Units (RCU) design
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Recirculation Units Design

1. Introduction


Recirculation units (RCU) design is becoming increasingly common in the design of HVAC systems in pharmaceutical facilities due to their advantages over traditional systems. It is particularly convenient for use in off-site cleanroom manufacturing strategies. In this article, we will compare this system with another traditional system and examine the advantages and disadvantages in a specific example.

2. A typical Air Handling Unit Design


Let’s imagine an HVAC system that serves a suite with a series of rooms for an upstream biological production process. A traditional design, according to the ISPE HVAC guide, (you can find it here) would consist of the following components:

  • Outdoor air treatment (make-up unit) to humidify, dehumidify, and ensure overpressure in the rooms.


  • Air handling unit to supply the necessary airflow to achieve the required air changes per hour and design classification. The Air Handling Units cool down to the set point that the room with the highest heat gain requires.


  • Reheaters to compensate for the supply air temperature required for the rest of the rooms (those with lower heat gains)


  • Recirculation of room air to the air handling unit, through a suction fan located in the same air handling unit.


The diagram for this system would be as follows:

3. Recirculation Units (RCU) Design


The proposal with Recirculation Units (RCU) design would be as follows:


  • Treatment of the outside air, in the same way as in the previous case. Nothing would change here.


  • One or more RCU units for each served room. To determine the number of units, we will take into account factors such as the maximum flow rate to establish the most compact size of the unit possible.


  • Common exhaust fan for all rooms.


This would be the sketch:

4. Advantages and disadvantages of RCU systems


RCU systems have a large number of advantages, let´s see a few of them:


  • Drastically reduction of the probability of cross-contamination. Each RCU serves one room (or a set of rooms, PAL, MAL). From a GMP point of view, this approach is ideal.


  • We will require less duct area. Since the RCU units will always be close to the spaces they serve, we will require less duct surface area. In some countries, regulations for duct sizing require a maximum velocity. For high flow rates, this is usually a problem that forces an increase in duct area. By using lower flow rate RCU units, we can eliminate this restriction.


  • Similarly, the required static pressure will also be lower, and we can optimize the exhaust fans.


  • As mentioned, the reduced flow rate allows us to use more compact units. Here is an example of a unit:
  • Reheaters are not required. Each unit will adopt the supply temperature with respect to the heat gain of the served room.


  • In parallel, we save on the additional pressure loss of these reheaters.


  • The air from the room can be returned with the same supply fan, only an exhaust fan will be necessary, which can be common for all rooms, for pressure control.


  • They can be installed on a walkable cleanroom void.




  • Use of tempered chilled water. If the heat gains require it, it cannot be supplied at a temperature lower than 16°C.


  • More maintenance. We have more fans, filters, and valves.


  • More instrumentation.


  • All of the above results in a higher cost.


  • If installed on a walkable ceiling, precautions must be taken to avoid possible liquid leaks into the rooms.

5. Let´s take a look at an example


In the following example, we will present the same problem using the two proposed solutions for summer conditions. We will classify three areas corresponding to an upstream biological process: a formulation room, a cell culture room, and a buffer preparation room. All three rooms will have an ISO 7 classification, and according to the design ACH, we will need to supply the following filtered air flows: 16,875, 12,000, and 6,750 m3/h. Different heat gains and latent loads have been considered for each of them. The temperature and humidity conditions inside the rooms are identical.

To achieve the required positive pressure and ventilation, we will need 3,913 m3/h of outdoor air in the Swiss city of Visp. The outdoor design conditions are the following:

5.1. Air handling unit with reheaters design


We will have a make-up unit that we will use for dehumidifying the outdoor air. To do this, we will cool it to 9 ºC and then reheat it to 16 ºC. This will bring the dew point from 20.3 ºC to 8.7 ºC. The energy exchange, including the fans, is as follows (click on the image to enlarge):

I usually use a rough calculation to estimate the ventilation motor power to determine the temperature increase due to it. However, in this case, since it is an important value in the comparison, I have decided to use the actual selection based on Ziehl-Abegg manufacturer’s curves, obtaining approximately 1.7 kW.

Next, we estimate some losses in the ducts from the makeup unit to the main unit. In the main unit, the pre-treated air will mix with the return air through a return fan. Then, the air will cool to overcome the heat gains of the rooms.


Thereafter the cooling will take into account the gains of the fan and the supply ducts. Since the air is sent to rooms with different heat gains, we will determine the supply temperature setpoint by the room that requires the lowest temperature. The rest of the rooms will need to reach the supply temperature using reheaters. All of this is explained in the post, Heat Gains. The air condition in the AHU is shown in the image below (click to enlarge):

We obtain that the return fan will be 12.5 kW and the supply fan will be 19.7 kW according to the following curves:

And finally, dimensioning the reheaters, we obtain (click on the image to enlarge):

The difference between supplied and recirculated air is explained in the post, Cleanroom Air Balance

And with this, we have already sized the first option. Next, we will see the sizing with RCU.

5.2. Recirculation Units design


In this example, the sizing of the makeup unit will be exactly the same, nothing changes since we need the same treated outdoor airflow to supply it at the same conditions, i.e. 16°C and a dew point of 8.7°C.

This air will supply to the three RCUs, which will mix with the return air from each of the rooms. We will have an exhaust fan to regulate the pressure of the rooms. Click on the image to enlarge it.

The power of each of the supply fans is 8.6, 6.3, and 3.5 kW as we can see in the following performance curves:

Note that using RCU we do not need return fans.

6. Comparison between the two systems


In the table below, we will compare the two systems in terms of installed fan power, cooling, and reheat power.

7. Conclusions


As we can conclude from the table above, there are substantial improvements in the selection of Recirculation Units (RCU) design.


There is no cross-contamination between rooms or suites. We don’t mix recirculated air from a room with air coming from other areas.


On the one hand, with this system, we can reduce the ventilation power since we will not need a return fan to collect all the recirculated air from the rooms. Likewise, we can bring the supply airflow as close as possible to the place of the terminal HEPA filters, thus reducing the pressure drop.


We reduce the cooling power since it is not necessary to over-cool the air in rooms that do not have as much heat gain as others.


As a consequence of the previous point, we determine that it is not necessary to reheat.


Finally, we will say that this case is significantly favorable for an RCU system, in terms of energy efficiency and compliance with GMP. We have not discussed in this post about the cost, but the initial investment will be higher for an RCU system. We should study each case individually based on the criticality of the process, the available space for air handling units, energy efficiency, and other factors.

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