Heater vs. Goal Seek for target outlet temperature, combustion fuels

[Fred_K]

Hello!

A question on how to inerpret the Heater functionality vs. the Goal Seek function.

If I want to have a target outlet temperature (dry bulb) for my airway, and I would like to know how much energy input is required to achieve this. I can run a Goal Seek with the airway to be adjusted and the target airway to be the same. A solution for the target dry bulb will be sought for a sensible heat source.

Because that's taking a rather long time with my model, I was thinking of using the heater functionality. Set the outlet temperature to my target value, select a fuel (even though I may not use it, as I said I'm more interested in the energy) and back calculate from the amount of fuel required.

The differences between the values are quite significant. Why is that so, how does Ventsim calculate in each instance (point source?)

I understand that the Goal Seek function may not have been designed with this kind of usage in mind and the calculation is different - is that the reason? Is there a latent heat factor involved somewhere (liquid fuels such as diesel also produce water during combustion)? 

Should I be trusting the Heater option rather than the Goal Seek function?

Cheers!

Fred

[Martin Griffith]

Hi, 

This is most likely down to the fact that the Goal Seek is only adjusting the sensible heat source, while the Heater is adding a mixture of heat and moisture. Though both solutions return the same dry bulb temperature, check the wet bulb temperatures, they are likely very different. With the Heater, it will take the fuel yield of H2O and inject that into the air as well; furthermore, there is a difficulty with these features where we are only setting either dry or wet bulb and not the other; we need to make assumptions about the other. In the context of Heaters, these assumptions are based around their use in colder environments

So it depends on what you are trying to do. One difficultly you have is that to adjust for a drybulb temperature, you can add sensible heat, or you can simply change the amount of moisture in the air. The dry bulb temperature can depend strongly on this; wet bulb temperatures are more often the focus in underground environments because they have a more direct relationship to the energy inputs, as well as having limits that are more easily calibrated to the limits of human exposure.

In short, what are you trying to model with this heat input?

[Fred_K]

Thank you for your answer, Martin!

I have actually removed all the combustion products from the fuel that I have set up: CO2, CO, H2O, etc. all set to "0". My assumption is that this way, only the heat would be considered in a kind of complete combustion scenario. For completeness' sake and confirmation, the Ventsim heater would not add the H2O to the intake air if the indirect option was chosen, correct? Have not checked this. Results are very different regardless.

What I'm trying to do is simply raise the dry bulb temperature from -X to +Y and find out how much energy input I would need for this.

I thought the heater option would be a nice and easy way of doing it (indirect, as I'm considering heat exchangers, with combustion products set to 0 as stated above), but the difference in results left me wondering how it works.

Cheers,

Fred

[Martin Griffith]

Remember that there are a variety of ways to get the drybulb temperature up from one temperature to the next. Consider your psychrometric chart (below), you can increase the drybulb temperature by going to the right on the x-axis. Therefore this could involve a de-humidification process at constant wetbulb temperature. Or it could involve pure sensible heating, or a combination of sensible heating + latent heating or cooling. 

Also bear in mind that in an underground environment, there is rarely ever a purely sensible heat transfer process. Underground environments are often humid and there is always some degree of wetness on the hot rock walls. Any heat transfer process will involve surrounding ambient moisture.

For the Heater tool we are trying to model an actual Heater underground. We are setting a dry bulb temperature, but not the wet bulb temperature. We make an assumption that the resulting air state will have the same relative humidity + any extra moisture from the burnt fuel. This is ok for underground Heating where we are generally heating from sub-zero temperatutes to slightly greater than zero to avoid surface entries icing up, so, along with the moisture from the burning, we're generally at higher humidity. But I think for other Heating scenarios (such as say increasing to higher temperatures of ~30 degrees), this might not be a valid assumption, and I've seen in my own models investigating this that indeed that might be a problem. 

Your other option is to not use the Heater function and instead set the drybulb AND the wetbulb temperature, thus removing any assumptions in the simulation. But then you would have to come up with a target wetbulb temperature. But if you are modelling for human comfort and safety then you're probably more interested in the wet bulb temperature, but I don't know what temperatures you are modelling.