Metric Conversion Question

[cobrajetken]

Air flowing in a duct has dry and wet bulb temperatures of 78F (25.6C) and 65F (18.3C), respectively. Use Psychrometric charts in English units (chart 1a) and SI unirs (1b) attached to find enthalpy,specific volume, humidity ratio and relative humidity in English and SI units. I found these points on the attached charts. I found these points on the two charts and the corresponding values asked for. The enthalpy for 78Fdb and 65F wb is found to be 30 BTU/lb and on chart (b) enthalpy for 25.6Cdb and 18.3Cwb as 52 kj/kg. When I convert specific volume from English to SI I find 13.56 cubic feet per pound equals o.86 cubic meters per kilogram. However, when I convert enthalpy in English units (30 Btu/lb) to SI I get 68 kj/kg which is way off from what I got on the SI chart 52 kj/kg). I racked by brain but I'm stumped. I'm missing something here and I'm not seeing it. Could you help me with this dilemma? Thank's. Ken

 

[MintJulep]

The starting point of zero enthalpy is arbitrary.

Your English and Metric charts likely have different zeros.

 

[DRWeig]

Relative enthalpies convert easily. Absolute enthalpies, as currently defined, have different zero points as Mint said.

I posted the answer to this one last year sometime. I will attampt to find it.

Best to you,

Goober Dave

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[DRWeig]

Found it. Read here:

http://www.eng-tips.com/viewthread.cfm?qid=326696

Best to you,

Goober Dave

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[katmar]

MintJulep is correct, but it is even more arbitrary than you might think. For the SI Units chart it is assumed that everything starts at 0[°]C. This seems logical enough. We assume the water is vaporized at 0[°]C and then the mixed air and water vapor is heated from 0[°]C to the saturation temperature.

But the Customary Units chart is a bit more complicated. It is assumed (again) that the water is vaporized at 0[°]C and the water vapor is heated from 0[°]C to the saturation temperature. But just to add a bit of interest, the air is assumed to start at 0[°]F and heated to the saturation temperature.

Unit conversion is such fun.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"