heat summary

[Thomas Bohdal]

Hi, I have some questions on the heat summary in Ventsim Design:

 

1) What is the difference in heat summary between TOTAL HEAT SUMMARY, and HEAT ADDED TO NETWORK? (These appear to be the same in a model I am running)

 

2) What does exhaust heat difference mean exactly (ie: difference between what and what?)

 

3) I am exploring these outputs as I would like to know which of 2 different scenarios makes the mine hotter or cooler in a specific deep mine:

                a) adding more flow, which may absorb more strata heat, but also may remove more heat in the exhaust

                b) closing some airways, which should reduce the strata heat (as there is less strata exposed)

There are too many airways to simply compare temperatures so I am looking at only the summary for airways with >0 flow.

What is the best value in the heat summary to indicate that the mine is relatively hotter or cooler?

 

regards,

[Craig Stewart]

Hi Thomas

 

1. Total Heat Summary is the positive Heat Inputs into a model.  It excludes negative heat sinks such as refrigeration.  Heat Added to Network is the net balance of positive and negative inputs.  Perhaps the wording can be improved in Ventsim ......

2. "Exhaust Heat Difference" is part of the auditing tool that ensures no significant heat is being lost in the model calculations.  Models are simulated to converge to a solution with an acceptable error (for example with 0.1m3/s airflow imbalance, or with 0.1C temperature imbalance).  If these limits are too high, or the model has lots of bias towards to extreme of this limit, some heat energy can be lost between junctions due the imbalance.  Imbalances of more than a few percent or 0.1C warrant further checks to ensure limits are not too high - any imbalance can be reduced by decreased acceptable error limits (in the simulation airflow (flow) and simulation heat (zero mass flow, temperature) settings, at the expense of longer simulation times.

3.  The figure you are probably after is the total sigma heat (similar to enthalpy) change.  The summary doesn't directly provide this, but you can simply divide the HEAT BALANCE TOTAL by the TOTAL MASS FLOW and look at the difference between the figures in the different simulations.  You'll find increasing airflow will increase the total heat absorbed from the rock, but not at the same rate as the airflow increase - therefore you'll likely end up with lower sigma heat (and therefore wetbulb) temperatures.  In most cases, with warm rock and limited airflow and cooling, the best design option is to limit or stop airflow to active work areas, unless there is a need to frequently re-enter non-worked areas (which may require some time for the ventilation to cool again)

[Thomas Bohdal]

14 hours ago, Craig Stewart said:

3.  The figure you are probably after is the total sigma heat (similar to enthalpy) change.  The summary doesn't directly provide this, but you can simply divide the HEAT BALANCE TOTAL by the TOTAL MASS FLOW and look at the difference between the figures in the different simulations.  

Thank you this was helpful. I did this, and also compared with a hand-calculation comparing only strata heat and air free cooling.

The scenarios are a) leave as is, b) completely open partial blockages to let more air through c) completely block partial blockages.

The two methods show improvement in scenarios (b) and (c) but differ in which is the preferred of the two. This may be because my hand calculation ignores moisture.

Providing results below for interest:

 

Sigma Heat (caculated as per instruction above); option (b) is preferred.

Case	Sigma heat, kJ/kg
a) Initial state	24.4
b) Partial blockages cleared entirely	23.3
c) Blockages hermetically sealed	24.3

 

Simplistic calculation of change in air and strata heat flow; option (c) is preferred.

Case	Change in Airflow m3/s	Delta cooling power of incoming air, kW[1]	Delta rate of heat flow from strata heat, kW	Delta rate of heat flow, net of air and strata, kW
a) Initial state	-	-	-	-
b) Partial blockages cleared entirely	+17	-340	+210	-130
c) Blockages hermetically sealed	-11	+225	-508	-283

 

---

[1] Assuming air at 20°C, threshold at 26°C DB),

 

[Craig Stewart]

Hi Thomas

Nice summary.  Remember that any kW calculation is an energy flow rate which depends on both the contained Sigma Heat and the mass flow of air.  If you have a solution that decreases airflow, this will also show decreased energy flow kW, but this does not necessarily decrease contained energy or the air per kg (Sigma Heat) which is the most important consideration for the cooling power of air on human physiology