LMAO trying to calculate mixing of air streams...

Dr. Barber said:

So, when mixing two air streams (say, the return and the outside air) you use either:

1) The equations (which the new handbook doesn't have), or

2) A psychrometric chart to use the lever rule technique (which requires a ruler, which you're not allowed to bring to the test)

This would be laughable if it weren't so serious.

Click to expand...

Dr. Barber said:

So, when mixing two air streams (say, the return and the outside air) you use either:

1) The equations (which the new handbook doesn't have), or

2) A psychrometric chart to use the lever rule technique (which requires a ruler, which you're not allowed to bring to the test)

This would be laughable if it weren't so serious.

Click to expand...

Dr. Barber said:

So, when mixing two air streams (say, the return and the outside air) you use either:

1) The equations (which the new handbook doesn't have), or

2) A psychrometric chart to use the lever rule technique (which requires a ruler, which you're not allowed to bring to the test)

This would be laughable if it weren't so serious.

Click to expand...

OldSquaw said:

Are you talking about something like calculating the temperature of two air streams mixing? Can't you just use the mass flows? For example, 100lbm/hr at 50F and 50lbm/hr at 70F so the mixture would be 57F. Or are you talking about something more involved where this technique doesn't work? 

I'm not working in HVAC and none of the practice problems I have done have used the lever rule, presumably because it cannot be used in the new test format.

Click to expand...

Dr. Barber said:

Yeah, temperature, humidity ratio, and enthalpy of the mixture would be weighted averages of the incoming streams, and the mass flow rates of those streams are the weighing factors. You're right.

I was thinking of for example if you have the two input conditions in the psych chart and they ask for resulting relative humidity or dew point -- typically I would use the graphical approach (lever rule) to locate the resulting point in the chart and then read the r.h. or the dew point.  Now you have to find dry-bulb and humidity ratio as you described and then locate the resulting point in the chart. Meh, not a big deal, I guess.  

Click to expand...