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Energy calculator verification

In my previous posts we looked at determining and calculating humidification levels using the psychrometric chart. Today we will look at the accuracy of the Gibbons humidifier calculators compared to a similar calculations carried out with a psychrometric chart.  

This calculator shows the energy savings available from replacing steam humidifiers with Gibbons humidifiers in a mixed-air system.

To verify that the Gibbons energy calculators give a fair reflection of the conditions that would occur in the real world, we have picked a set of conditions and compared the results of the calculator with the long-hand calculations and a psychrometric chart.

The conditions for calculation are as follows:

Outside air

Outside air temperature db = 5.2°C
Outside air temperature wb = 4.0°C
% of month = 16.1
Hours per month = 119.8 h
Outside air enthalpy = 16.46 kJ/kg
Outside air moisture content = 0.0045 kg/kg


Required room temperature (°C db) = 21.0°C
Required room humidity (%RH) = 50%
Return air temperature (°C db) = 23.0°C
Supply air temperature (°C db) = 18.0°C
Supply air volume (m3/s) = 10.0 m3/s
Minimum % fresh air = 10.0%
AHU run time as percentage of year (%) = 100%
Cost of electricity (£/kWh) = £0.10
Cost of heating energy (£/kWh) = £0.10
Cost of cooling energy (£/kWh) = £0.10
Coefficient of performance (COP) of cooling system = 3.0
CO2 emissions (tonne/MWh) = 0.544
Temperature rise across fan = 1.0°C



Calculated results                                                         Manual                   Energy

                                                                                        calculation             calculator

Return air enthalpy (kJ/kg)                                              43.0                        42.8
Mixed air enthalpy (kJ/kg)                                                36.9                        36.7
Damper position (% fresh air)                                       23.33%                  23.19%
Mixed air moisture content (kg/kg)                                0.0070                   0.0070
Room moisture content (kg/kg)                                     0.00780                0.00773
Required moisture content increase (kg/kg)              0.00080                0.00073
Specific volume of air (m3/kg)                                       0.834                     0.828
Mass of air (kg/s)                                                             11.99                      12.08
Humidification rate (kg/s)                                               0.0096                   0.00904
Humidification rate (kg/h)                                               34.56                      32.54 


Gibbons Humidification

Humidification power (kW)                                             0.17                        0.17
Heating power (kW)                                                         0.0                          0.0
Cooling power (kW)                                                         0.0                          0.0
Power cooling system inc COP (kW)                            0.0                          0.0

Total Power (kW)                                                            0.17                        0.17


Steam humidification

Humidification power (kW)                                            25.7                        24.4
Heating power (kW)                                                        0.0                          0.0
Cooling power (kW)                                                        23.98                      22.6
Power cooling system inc COP (kW)                          7.99                        7.53

Total power (kW)                                                            33.7                        31.9


Energy saving

Saving in total power (kW)                                            33.5                         31.7

Total energy saved (kWh)                                             4013.3                    3797.7

Financial saving                                                             £401.33                  £379.77

CO2 reduction (tonnes)                                                2.18                         2.07


With the chosen specific conditions in this example, the Gibbons energy calculator underestimates the savings by around 5.3%.

For more information, please don't hesitate to contact us on 01621 868138 or email

To download our humidification calculator please visit


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