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Rated heat flow of heat exchangers in the Modelica Buildings Library

asked 2018-10-31 09:27:07 -0600

akey gravatar image

I am using the Modelica Buildings Library for a project and would like to know how to calculate the input parameter, 'nominal Heat Flow Rate,' that is required for several of the heat exchanger components i.e. the 'StratifiedEnhancedInternalHex' and the RadiatorEN442_2 models.

Is this 'rated value' something that is usually specified by the manufacture? I've searched several technical specifications and cannot find it listed. Or is this something you determine experimentally?

Or does this 'rated value' have to be modeled separately, using knowledge of the flow regime? As the model already requires input parameters such as pipe diameter, mass flow rate, pipe conductivity etc; why is this value not being calculated automatically in Modelica?

Sorry if I am missing something here, but if you could point me to a reference, I'd much appreciate it.

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answered 2018-10-31 12:57:18 -0600

updated 2019-05-17 09:31:39 -0600

Hey akey,

I believe the nominal heat flow rate is the nominal capacity of the radiator at nominal conditions, i.e. Power output under nominal mass flow rate and nominal deltaT conditions for a fixed ambient air temperature.

For instance, the rated radiator capacity is provided by manufactures generally for an inlet water temperature of 75ºC, outlet temperature of 65ºC, for an ambient room temperature of 20ºC (but also for 70ºC/50ºC/20ºC and 55ºC/45ºC/20ºC) and for a given water mass flow rate. Those are rated conditions. However, during dynamic simulation, those conditions are not fixed and the "real" instantaneous radiator power output will vary as those conditions also change, following the radiator characteristic equation.

You can find an example of this data on pages 3,4 and 5 of a radiator catalog data here:

The nominal heat flow rate is the rated capacity of the radiator as provided by the manufacturer under the test conditions defined by EN442. If you are not working with specific equipment, It can also be the peak heating demand of a room which would be supplied by the radiator. The same applies to heat exchanger models.

You can also find documentation on this here:

I hope that I was clear enough. Good luck for your project.

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@Redux Thanks very much for taking the time to answer and your English was perfect ;). Yes for radiators it seems the output is well defined in technical datasheets. The problem I am having is with the internal coils found inside a heat storage tank. Heat transferred will be a function of convection over inner walls, conduction through the pipe material and then convection from the outer walls into the storage medium. I am yet to find any storage tank technical specification that provides nominal performance of the coil. Perhaps I should use the rated output of the cylinders if available?

akey gravatar image akey  ( 2018-11-02 03:38:12 -0600 )edit

@akey: The heat exchanger configuration in the tank model is such that you do not need to know the geometry of the coil. Rather, it requires to specify, for the design conditions, the fluid temperature in the tank, the fluid temperature inside the heat exchanger, and the required rate of heat transfer. The model then computes the UA value which it will use during the simulations.

Michael Wetter gravatar image Michael Wetter  ( 2018-11-02 10:02:19 -0600 )edit

@Michael_Wetter Thanks for your reply. It seems that in order to calculate the UA of the coil, the user must supply the 'nominal heat flow' of the coil (Q_flow_nominal). I have not seen this information supplied by the manufacturer of any hot water storage tanks; however this information seems readily available for radiators. Apologies if this information is available, I just haven't come across it. It seems like quite a critical value so any assumptions would have a large impact on the validity of the model.

akey gravatar image akey  ( 2018-11-03 09:32:48 -0600 )edit

@akey I think the nominal heat flow data you are looking for can be found on (as examples):

tabes from pages 4 and 6 for an DHW tank example:

table on page 3 for steam-water heat exhanger example:

Fábio Batista gravatar image Fábio Batista  ( 2018-11-09 11:28:05 -0600 )edit

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Asked: 2018-10-31 09:27:07 -0600

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Last updated: May 17 '19