Describe the bug
adjust logic to:
$$ Q_{\text{total}} \ [\text{kW}] = \text{mass flow} \ [\text{kg/s}] \times (h_{P2} - h_{P1}) \ [\text{kJ/kg}] $$
$$ Q_{\text{sensible}} \ [\text{kW}] = \text{mass flow} \ [\text{kg/s}] \times cp \ [\text{kJ/kg}\cdot\text{°C}] \times (t_{P2} - t_{P1}) \ [\text{°C}] $$
$$ Q_{\text{latent}} \ [\text{kW}] = Q_{\text{total}} \ [\text{kW}] - Q_{\text{sensible}} \ [\text{kW}] $$
Steps to recreate the bug
Versions: Revit , Rhino
Screenshots
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Adjust in load calculation UI
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also in GH
Test file
explanation...
Where we have Cooling process - two points ie P1(35 degC, RH 55 %, x 19.59 g/kg, h 85.61 kJ/kg ) and P2(12 degC, 100%, x 8.73 g/kg, h 34.14 kJ/kg)
We have also mass flow m = 1 kg/s
we know that to calculate the total load for this Q total = Q sensible + Q latent
Q sensible = mass flow * cp * (T P2 - T P1)
Q sensible = 1 kg/s * 1.01 kJ/KgK * (12 degC - 35 degC)= -20.20 kW
Q latent = mass flow * r0 * (x P2 - x P1)
Q latent = 1 kg/s * 2.450 kJ/kgK * (8.73/1000 kg/kg - 19.59/1000 kg/kg)= -0.03 kW
we know that Q total = Q sensible + Q latent
Q total = -20.20 kW + (-0.03 kW) = -20.23 kW
However when I use the equation:
Q total = mass flow * (h P2 - h P1)
Q total = 1 kg/s * (34 kJ/kg - 85.61 kJ/kg) = -51.47 kW
so
The discrepancy you're observing in the total heat load (Q total) calculations for the air cooling process using the psychrometric properties arises due to a few key factors related to the nature of the calculations and the properties of air.
Specific Heat Capacity (cp) Variation: In your calculation of Q sensible, you've used a constant specific heat capacity (cp) value of 1.01 kJ/kgK. However, the specific heat of air is not constant and varies with temperature and moisture content. The value of cp used in your calculation might not accurately represent the specific heat of air at the conditions of P1 and P2.
Use of Approximate Formulas: The formula for Q latent assumes a linear relationship between the enthalpy and the humidity ratio, which might not be perfectly accurate over a wide range of conditions. This is because the enthalpy of moist air is a nonlinear function of both temperature and humidity ratio.
Accuracy of Psychrometric Data: The values for enthalpy (h) at points P1 and P2 are taken from a psychrometric chart or calculated using psychrometric equations. These values are subject to the accuracy of the chart or the equations used for their determination. Small errors in these values can lead to significant discrepancies in calculations, especially when dealing with large changes in temperature and humidity.
Difference in Calculation Methods: The method using sensible and latent heat separately (Q sensible + Q latent) and the method using enthalpy change (h P2 - h P1) are based on slightly different principles. The enthalpy method accounts for the total energy change, including both sensible and latent heat, in a more integrated manner. This method is often more accurate for processes involving both temperature and moisture content changes, which is typical in psychrometric applications.
To resolve this discrepancy, you may need to:
Use more accurate or variable-specific heat capacity values for air at different temperatures and humidity levels.
Employ more precise psychrometric equations or charts to determine the enthalpies at points P1 and P2.
Ensure that all psychrometric properties (temperature, relative humidity, specific humidity, enthalpy) are consistently and accurately determined.
In practice, the method using enthalpy differences is generally preferred for calculating the total heat load in psychrometric processes, especially when both temperature and humidity changes are involved.
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