19 Feb Heating and Humidification in Excel
This is a new post following our series of psychrometric Excel series calculations. Now, we are going to see how to implement calculations for heating and humidification in Excel.
We will follow the same schema we used in the post Cooling and Dehumidification post.
To do so, we are going to use the psychrometric add-in you can download from here:
You may be also interested in plotting the psychrometric chart as we discussed in our post:
In particular, this is the cooling and dehumidification process that we are going to study:
It consists of the mixing of fresh air (OA) which we will heat and then humidify. This fresh air will be mixed with a return airflow with the room condition of temperature and relative humidity.
In case we have heat losses, we will include this in the calculation, in order to know the heating temperature to compensate for these heat losses. Also, we will calculate the amount of steam required to humidity the air.
Let´s get started.
2.1. Site location
In cell C8 we put the STD_Press related to C7. Here we obtain the barometric pressure at the elevation site.
2.2. Outdoor and Indoor conditions
Other inputs we need are:
- Outdoor temperature and relative humidity
- Indoor temperature and relative humidity
In our last post https://www.pharmaceuticalhvac.com/determining-outdoor-design-conditions/, we discussed how to import climatic data into excel.
With this data, we can calculate the outdoor and indoor air density and absolute humidity, that we will need later on (Dry_Air_Density and hum_rat2 functions).
Here we can calculate the necessary airflow, as described in our posts, About Air Changes, and Cleanroom Air Balance.
In the example below, it is manually written, but you can link to the result of the calculation based on the air changes:
The recirculation ratio is the recirculated airflow divided by the total supply airflow.
2.4. Heat losses
The following input we need to know for our heating and humidification in Excel calculations are the heat losses. Normally, in clean rooms, the heat losses are not significant since they are located inside insulated buildings. Nevertheless, we can calculate separately by using software like HAP or Trace (or other Excel calculation sheets). In this example, we will consider a ground floor with a total heat loss through the slab of 5 kW. Then, if there are heat losses we will need to calculate the supply temperature to compensate for this loss.
Nevertheless, as mentioned, is quite common that this value is close to zero. In this case, the supply temperature will be the same as the room’s indoor set point.
In this example, we can also include a humidification allowance, let’s say 10%.
Arrived at this point we have all the necessary data to start with the calculations. These will be:
- Heating temperature
- Air mixing humidity
- Air mixing temperature
- Psychrometric characteristics (enthalpy, dew point, air density) for each point (outdoor, indoor, mixing, heating, humidification,…)
- Humification capacity
- Outdoor conditions, temperature and relative humidity (or wet bulb temperature)
- Indoor conditions, temperature and relative humidity (or wet bulb temperature)
3.1. Heating Temperature
Let’s get started by preparing the table with the known outdoor conditions of temperature and humidity, and proceed to calculate the remaining air properties at this condition. For example the enthalpy:
The calculation of supply temperature is an iterative process that satisfies that the heat of the air mixed with the return compensates for the heat losses, in our example, 5 kW. To solve this, we will use an iteration module, that you can download from here.
In our previous post Cooling and dehumidification in Excel we explained how to insert the visual basic code into our Excel book.
Since we are not humidifying yet, the humidity ratio remains constant:
Then we can know all the psychrometric characteristics of the heated air, like relative humidity, and others:
After this, we can calculate now the humidification target, which is the room setpoint plus the 10% allowance:
And the remaining properties, in the example below, the dew point:
Now, we can present the indoor conditions, since we have all the data to complete:
3.3. Air Mixing Humidity
Now it’s the turn of air mixing humidity calculation. We will use the following formula:
3.4. Air Mixing Temperature
Finally, it is time to figure out the air mixing temperature. We will use the well known formula:
And obtaining the following results:
3.5. Heating Power
Subsequently, we have all the necessary to know the heating power in kW, based on the fresh air flow and the difference of enthalpies:
3.6. Steam capacity
At last, we can get the required amount of steam for humidification:
As always we say, it is a good idea t to end with a visualization of the process, to detect possible inconsistencies or errors. In our example, everything looks good: