20 Mar Desiccant Dehumidification in Excel
This is a new post in our psychrometry design series. Now is time to talk about how to plot a process including desiccant dehumidification in Excel. May you like to consult our post Cooling and dehumidification in Excel, since we will use a few formulas explained there.
Desiccant dehumidification is another way to reduce the moisture content from the air. So the first question is: When using this instead of cooling? The answer is not easy to respond to, it depends on many factors. But there are here a few guidelines:
- Normally, cooling dehumidification is most economical than using desiccant wheels. This is applicable when the air moisture and temperature are high.
- It is recommendable to use desiccant wheels when we want to achieve a low humidity. This is a very common requirement in some pharmaceutical processes such as lyophilization suites.
- Finally, cooling and desiccant dehumidification systems are more economical when used together. Both technologies complement each other. The example we will study in this post will cover this situation.
2. How does a desiccant wheel work?
A desiccant wheel consists of a rotary wheel containing a material attracting moisture from the process air (which we want to dehumidify). Retained humidity is released to a counter air stream (reactivation airflow). For best performance, the equipment heats the reactivation air to lower the relative humidity and increase the moisture recovery.
In the sketch below we can see the principle of operation:
3. Desiccant calculation in Excel example
Let’s see a typical example of desiccant calculation in Excel. The system consists of an Air Handling Unit providing dehumidified and cooled air to a suite with several rooms where there is lyophilization equipment in a typical vaccine facility. As the heat loads are slightly different from one room to another, we design a reheater system for each room. See below the basic schematic for the main AHU (reheaters are not shown):
We obtain the airflow from a separated calculation, as explained in Cleanroom Air Balance, so, let’s get started.
3.1 Input data
The first point is to input the design data:
- Location & Altitude. Near Cork (Ireland) at 31 meters over sea level.
- Outdoor & Indoor temperature and humidity conditions. In this example, we will try to achieve a room relative humidity of 25%.
- Above indicated air flows.
- Sensible and latent heat to remove. See Heat Gains for guidelines.
Moreover, we will use our well and useful psychrometric functions addon you can find here https://wcec.ucdavis.edu/resources/software-resource-applications/ and the explanation on how to use, here: Cooling and dehumidification in Excel
3.2 Pre-Cooling calculation
As stated in the introduction above, the most economical solution to achieve low humidity from hot and humid air is a combination of cooling and desiccant dehumidification. This is our case.
So, the first step is to remove humidity per condensation as much as we can with chilled water. As our available chilled water temperature range is 6-12° C, we decide to lower the temperature to 10° C by saturating the cooling coil (99.8% RH).
Therefore, we calculate the resulting humidity ratio (absolute humidity), and other physical characteristics as enthalpy, density, etc.
Afterwards, we may know the required cooling capacity:
And consequently, we can plot the cooling line in the psychrometric chart. We explain how to plot the psychrometric chart here: A Psychrometric Chart in Excel
3.3 Desiccant calculation
Now we arrive at the crucial point of this calculation, and we will need the help of simulation software to determine the desiccant wheel exit conditions of the air. By knowing this point we can continue simulating the remaining processes: air mix, heat gains from fans, and sensible cooling. In the end, we will know the supply air condition. And, finally, by plotting a line between the supply air condition and the room condition, we will calculate the room sensible heat factor (RSHF).
This is usually an iterative process. If the calculated RSHF is greater than the design RSHF (in our example is 0.87), that means the dehumidification is not enough, and consequently, we have to select another desiccant wheel in terms of sizing or to modify any operational parameter of the desiccant wheel.
Otherwise, if the calculated RSHF is lower than the design RSHF, the dehumidification is excessive and we will have to decrease the dehumidification capability.
So, we will input the wheel size and the characteristics of the fans (to calculate the heat delivered by them). We will start with a wheel size 1525 x 200 mm (diameter x width):
Thereafter we will input the working conditions of the desiccant wheel:
- Process In Airflow: airflow to be dehumidified (14,400 m³/h)
- Process In Temperature: the defined pre-cooling coil temperature (10°C)
- Humidity Process In: the humidity ratio of the exit pre-cooling coil (7.6 g/Kg corresponding to 0.0076 Kg/Kg)
- Regeneration In Airflow. This is the ratio of process airflow that we will use as regeneration air. In this case, 1/3 (0.333) or 4,795 m³/h
- Temperature & humidity of regeneration air. We will use as regeneration outdoor air, so 28°C and 10.7 g.Kg)
- Heating temperature. We will use saturated steam as heating fluid for the regeneration air, thereafter we will put 140°C
- Wheel speed. Set at 24 rpm
- Model selection. 1525×200
With these inputs, we obtain the following results:
- Process outlet temperature: 39.1ºC
- Process outlet humidity: 1.35 g/Kg (3.1 % RH)
- Regeneration outlet temperature: 35.2°C
- Regeneration outlet humidity: 29.47 g/Kg (29.47% RH)
- Required regeneration air heating capacity: 179.75 kW (steam power required)
- Air pressure drop through the wheel: 253.26 & 321.74 Pa, necessary data for fan sizing.
Thus we can plot the dehumidification line in the psychrometric:
3.4 Air Mix process
Now it is time to add the temperature increase due to the fans (it is sensible heat, so humidity ratio remains constant):
According to our sketch, the next process is the air mix:
3.5 Sensible Cooling
The final process is to cool down the dehumidified and hot supply air to the required temperature. Following our example, this temperature will be 12ºC. As the air is dry enough, no condensation will take place and all the heat exchanged will be sensible. We will consider the heat effect of the supply fan in this step.
3.6 Room effect
By the last, we have to double-check that the obtained supply condition satisfy the room humidity increase. We will plot a line from the supply to the room conditions. The slope will give us a relationship with the Room Sensible Heat Factor (RSHF), and with the data collected until now, we can get the calculated RSHF by dividing the sensible enthalpy by the total enthalpy (sensible plus latent).
The room sensible enthalpy can be calculated as:
And the latent as:
The resulting RSHF calculated is 0.81 Hs/(Hs+Hl). That means that the dehumidification is too high. We should try another desiccant exit condition that increases the humidity exiting the wheel in order to optimize and adjust the design.
We have a few options to increase the humidity ratio at the wheel exit:
- Reduce the wheel diameter. Nevertheless, the resultant air velocity through the wheel would be too high and consequently the pressure drop too. So we are going to discard this option.
- Reduce the wheel speed. We adjusted a velocity of 24 rpm. If we decreased let’s say to 12 rpm it should work.
- Reduce the reactivation air temperature. In this particular case is our preferred option, as it adds an energy-saving advantage.
Reducing the reactivation temperature from 140ºC to 110ºC, we obtain:
With these new values, the new calculated RSHF is 0.86, close enough to our target of 0.87:
Finally, our wonderful chart remains as follows: