Mollier part 3: how to use a Mollier diagram

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Mollier part 3: how to use a Mollier diagram
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Mollier part 3: how to use a Mollier diagram

– by Jeroen Fijan, 23/04/20

This is a quick, no-nonsense guide to using the Mollier diagram. While we’ve done our best to make this as easy as possible to understand, it does require some knowledge about what the lines stand for – so if you’re unfamiliar with the diagram, please read the first and second parts of this blog.

Getting started

The diagram is used to engineer an air handling unit. In this example, we will show you how to calculate the following aspects of a unit using the Mollier diagram:

  • Cooling coil
  • Heater capacity
  • Steam humidifier

We start by establishing the outside and inside conditions. This is our field of operation, which sets the boundaries within which we work.

A common mistake is to set the conditions extremely high. To be ‘on the safe side’. We often get requests for outside conditions like 50°C – 80%RH. Not only are these impossible ambient conditions, it will make the installation unnecessary heavy and more difficult to operate.

For this example, we will assume the following conditions:

Outside:          35 °C – 70% RH
Inside:             21 °C – 60% RH

Energy efficiency

Every system reduces energy loss by using the heat or cold that are already present and transferring part into the outside air. There are a range of ways to reduce energy, each of which has its pros and cons. In this example, we will use one of the most common methods: a recirculation rate of 50%.

image 1 recirculation air.png
Image 1: Recirculation rate

Calculating cooling coil

The cooling coil we are using is chilled water-fed, with a temperature of 12 °C in and 6 °C out.

1: Marking points in the diagram

We start with the easy part, that is putting the fixed points into the diagram. We begin by locating the outside and inside condition and drawing a line between them.

2 - outside inside conditions - xs.png
Image 2: Outside and inside conditions

2: Determining the mixing temperature

Remember, we are recirculating 50% of the air for energy efficiency purposes. This means the temperature in front of the cooler will be a mix between outside and inside conditions. To calculate the mixing temperature, we use a simple equation:

equation 1.png

Let us assume a total air amount of 20,000 m³/h. With a recirculation rate of 50%, this gives us 10,000 m³/h for both outside and inside air streams. Now we can calculate the mixing temperature:

equation 2.png

This gives us a mixing temperature of 28 °C.

3 - mixing temperature - xs.png
Image 3: Mixing temperature

3: Cooling the air

The cooler dew point (CDP) is 9 °C and set on the saturation line. You’ll get the CDP by taking the average temperature of the cooling coil, which is 12 °C in and 6 °C out.

To determine the cooler capacity, we follow the lines of constant enthalpy between the mixing temperature and CDP.

4 - cooler capacity - xs.png
Image 4: Cooler capacity

This gives us 72 kJ/kg – 27 kJ/kg = 45 kJ/kg

Heater capacity

The temperature behind the cooler will be around 9 °C. We can round up a degree because of the residual heat of the ventilator, resulting in a temperature of 10°C. Room temperature is 21 °C, giving a temperature differential of 11 °C.

5 - heating the air - xs.png
Image 5: Heating the air

Now we have enough information to calculate the heater capacity using the Q=m*c*ΔT equation. For this example, resulting in a heater capacity of 76kW.

Humidifying the air

In this example, we use a steam humidifier to bring the air up to a relative humidity of 60%.

Cooling the air extracts a lot of moisture from it.

6 - extracting moisture by cooling - xs.png
Image 6: Extracting moisture by cooling

Now, heating the air brought it to a temperature of 21 °C without changing the absolute humidity. Looking at this line, we see a gap of 2.2 g/kg between our current point and the desired value.

7 - humidifying the air - xs.png
Image 7: Humidifying the air

Now we just need to convert the air amount of 20,000 m³/h to kg/s to find out how large the steam humidifier needs to be. For this example, resulting in a humidifier capacity of 15.2 kg.

Wrapping things up

So, does this cover every detail? Of course not.

For one thing, we didn’t integrate the bypass factor, which accounts for the fact that not every air particle running through the cooling coil is affected by the decrease in temperature.

And what about all the different methods of energy recovery, like the heat recovery wheel, twin coils or cross heat exchangers? Or adiabatic humidifying?

Like I mentioned above, this is a quick and dirty description of how the Mollier diagram can be used to give a fairly accurate estimation of an air handling unit’s capacities.

To get familiar with the diagram, you can try using different conditions. This example only included summer conditions: see what happens with the capacity of the heater and steam humidifier when you set the outside conditions to -5 °C and 20% RH. To get you started, download our free mollier diagram here.

And, as always, get in touch with one of our professionals for more specific or more advanced information.

Jeroen Fijan | R&D Manager

Jeroen Fijan has been working at Heinen & Hopman since 2001. He started as a draughtsman and, over the years, worked his way up to the top of the R&D division. Sustainability is a top priority in the quest to improve H&H’s products and processes.