Determining Water Flow Requirements based on Air Flow and Design Assumptions for Revit 2012

Heat Transfer Principles

The liquid flow required through a coil is dependent on the amount of heat to be added or removed, also known as the load on the coil. 

The amount of heat transferred to or from a mass can be determined from the following formula.

Q_s = m x c x DT

where 

When computing flow, the heat required is determined from a heating / cooling load analysis.  The specific heat of the fluid is typically an assumed constant, and a temperature change of the fluid through the coil is usually determined based on project requirements; thus, one can determine the required flow.  Typically, airflows are specified by the designer in round numbers.  So, once the flow for the load is known, the flow is adjusted.  For example, if the flow based on the exact load works out to 542, the designer may specify the flow as 550.   Assuming the same DT, this will result in slightly different Q_s, which needs to be found to determine the water flow.

Usually, flow of air is expressed volume rate per time, instead of mass per time, so another factor is necessary to take into account density.

Q_s = q x ρ x c x DT

where

Inspect all the units for the right side of the equation, you will see that everything cancels, except the volume and time..  Revit is able to handle the conversion of units (in this case minutes to hours), so we don’t need to take that into account in the equation. 

Revit Family Implementation

Boxed text like that below indicate parameters to be created in the family definition.  To be able to schedule or tag any of these values, define them as Shared Parameters. 

unitTEMPERATURE  = 1 °F (not actually needed, explained below)

c = Air Specific Heat x unitHEAT / (unitDENSITY x unitVOLUME x unitTEMPERATURE)

Q_s = q x ρ x (Air Specific Heat x unitHEAT / (unitDENSITY x unitVOLUME x unitTEMPERATURE) x DT

Since density is assumed constant, this can also be defined as a type parameter:

Since the flow required at each unit is specified by the user, this should be defined as an instance parameter:

The default value is arbitrary, and as with all instance parameters, should be set to a value as required for the instance within the project.

Since the temperature change is usually derived based on entering and leaving temperatures, these will also be defined as parameters:

The temperature parameters may be instance or type, depending on the flexibility you need in your project.

Revit does something ‘interesting’ when doing math on temperatures. 

Heating Air Delta T = Heating Leaving Air Temp - Heating Entering AirTemp = -434.67°F ; -258.15°C

In this case, it is off by 459.67 °F or -273.15 °C, which happens to be related to absolute zero.  To account for this, add 0 in the formula:

Adding 0 to the temperature to get it to work out is easy enough… however, in the formula to determine the volumetric flow, there is multiplication and division of temperatures, which introduces some other complications.  To simplify this, cancel the temperature units in the temperature change result to avoid having temperature units in the equation. 

Heating Air Delta T / 0°F

Remember, this is not division by 0, but actually division by 459.67 of 273.15, thus, there is no division by zero error.  However, the result now must be multiplied by 459.67 or 273.15.  Using a constant parameter makes this a bit easier to maintain:

 Then, as before, we need to adjust:

In the Calculated Heating Load formula, we will substitute ‘Heating Air Delta T as Number’ for DT, and get rid of the unitTEMPERATURE factor:

Now that the Calculated Heating Load based on the specified airflow is known, the water flow can be determined.