29 May Run around coil optimization in pharmaceutical applications
In this post, Run around coil optimization in pharmaceutical applications, we’ll explore how we could save a few bucks on energy consumption.
I’ve been working in the past on a construction project for a bio-pharmaceutical facility that required 100% outdoor air HVAC systems due to the uniqueness of the processes involved (multiproduct biological products). As we discussed in our previous post, Energy Savings in Pharmaceutical HVAC Design, this requires energy recovery systems to make it viable. Specifically, with run-around coil systems, as is a common approach in pharmaceutical HVAC. If you are not familiar with this system, you can figure out more here.
However, a traditional run-around coil system with a simple exchange between the extraction air and the outdoor air supply can be optimized in cases where dehumidification is also necessary. Let´s compare a traditional system with an improved one.
2. A simple system
The system proposed for energy recovery consists of an air handling unit with the following coil sections:
- Run around coil from the extraction unit. In this coil, we recover heat from the extraction process, let’s say at 22°C with outside air at 32°C. The efficiency of this exchange is usually around 60-70%. Glycol water is usually used as the exchange fluid.
- Sensible heat cooling coil (optional). If the outside temperature is high, we sometimes need to install a pre-cooling coil to remove sensible heat. We can use either tower cooling water or chilled water for this coil.
- Cooling coil. This coil is responsible for removing the latent heat required to achieve the desired dew point in the supply air. You can see its behaviour in the post Cooling and dehumidification in Excel. Chilled water is usually used for normal dew points. Glycol water is usually used for lower humidity.
- Reheating coil. The outlet temperature of the dehumidification coil is very low, typically between 9-11°C if we use chilled water. It can be even lower if we work with glycol water for lower humidity. Therefore, we need to reheat to supply air at the required temperature to overcome the loads. It is common to connect to the hot water distribution system for this purpose.
The sketch for this system would be as follows. The coils needed for heating operation in winter are omitted.
3. An improved design
As we already know and have seen, the process requires both cooling and heating. We can use the heat recovery circuit to take advantage of the different types of required heat exchange. Let’s see how.
The idea is to install a double 3-way valve system, directing the water from the extraction recovery to two different batteries depending on whether dehumidification is required or not. Therefore, we will have the following configuration:
- Run around coil 1. If dehumidification is not required, heat is exchanged with the extraction battery. The first valve coming from the exhaust unit is open and the second one is closed. The operation is the same as in the previous case.
We will have the same sequence of coils as in the previous case to achieve the same results:
- Sensible heat coil (optional)
- Cooling coil for dehumidification.
And we will add the following coil:
- Run around coil 2. This is the coil we will use for energy saving during dehumidification. When required, the control system will send the glycol water from the extraction coil here, instead of from run around coil 1. This will cool the glycol, and then we will send it to run around coil 1 while reheating the supply air.
Through this system, the objective is to have a smaller cooling and reheating coil, and therefore lower energy consumption than in the simple system.
The diagram would be as follows:
4. Let´s see an example
Next, we will look at both systems using the same example taken from real life. We will try to recover heat from a set of rooms at 25ºC and 58.9% relative humidity. The incoming air will be in the worst conditions at 32ºC and 49.2%. The dew point in these conditions is 20ºC, and the goal is to dehumidify it to a dew point of 10ºC. The airflow, both in and out, will be 14,400 m3/h.
4.1. Simplified design
The treatment of the incoming air will consist of the following stages, as defined in the previous section:
- runaround coil
- pre-cooling coil, using tower water at 12-18ºC
- dehumidification coil using chilled water at 6-12ºC
- reheating coil
The operation is independent of the dehumidification needs. The pump will run continuously, exchanging heat between the incoming and exhaust air.
Thus, after sizing the coils, we will obtain the following values. We have used software from a manufacturer for the calculation of the recovery coils (click on the image to enlarge).
As we can see, we achieved a recovery efficiency of 71.4%, with a total heat exchange of 23.3 kW.
4.2. Run around coil optimization in pharmaceutical applications
Next, let’s take a look at how to size the system to recover reheating heat using a second coil. The system requires two 3-way valves to divert the glycol water to one coil or the other. If dehumidification is required, it will be sent to the second coil and then to the first. If dehumidification is not necessary, it will only be sent to the first coil. The operation in this case would be identical to the previous one.
Thus, for reheating we have a recovery efficiency of 45.4% for the second stage and 73.2% for the first stage.
We can even see in this case that a reheating coil wouldn’t be necessary. The second stage provides us with enough heat exchange to reheat.
In the table below we summarize the results obtained from both systems:
As we can observe, the numerical comparison of both systems confirms what we were expecting: the energy requirements decrease systematically with two stages of heat recovery.
I hope this article helps improve the energy efficiency of pharmaceutical HVAC systems, especially those that use 100% fresh air.