2001 NCEES Practice #127 - Mechanical - Engineer Boards
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2001 NCEES Practice #127

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Can someone explain why the answer to Problem #127 is 100F

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Change in temperature across a throttling valve is assumed to be 0 so T(8) = T(7) = 100F.

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22 hours ago, Buckley said:

Can someone explain why the answer to Problem #127 is 100F

Can you post a screenshot of the question?

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Question 127 ask you to find the Temperature at point 8.

Edited by MikeGlass1969

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3 hours ago, MikeGlass1969 said:

Question 127 ask you to find the Temperature at point 8.

Thanks Mike.

@Buckley if you perform an energy balance around the throttle device you’ll get h7 = h8

Since (for ideal gases) the enthalpy changes only with temperature, then if there is no enthalpy change, there is no temperature change, hence T8 = T7.

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Thank you everyone!

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20 hours ago, Slay the P.E. said:

Thanks Mike.

@Buckley if you perform an energy balance around the throttle device you’ll get h7 = h8

Since (for ideal gases) the enthalpy changes only with temperature, then if there is no enthalpy change, there is no temperature change, hence T8 = T7.

@Slay the P.E. I would have been trapped on this and picked 94F, the next temperature in the choices that is slightly lower lower than 100F, simply because the proportionate pressure drops slightly as well.

Edited by MecEng

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13 hours ago, MecEng said:

@Slay the P.E. I would have been trapped on this and picked 94F, the next temperature in the choices that is slightly lower lower than 100F, simply because the proportionate pressure drops slightly as well.

Yes, be careful.

Throttle devices are isenthalpic. It just happens that for ideal gases, isenthalpic = isothermal (regardless of the magnitude of pressure drop). That is not generally the case. For example, look at the throttle device in the vapor compression refrigeration cycle. It is constant enthalpy across the throttle valve, but the inlet is saturated (or sometimes compressed) liquid and the discharge is a liquid-vapor mixture at a substantially lower temperature.

Edited by Slay the P.E.

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On 2/19/2020 at 8:15 PM, Slay the P.E. said:

Yes, be careful.

Throttle devices are isenthalpic. It just happens that for ideal gases, isenthalpic = isothermal (regardless of the magnitude of pressure drop). That is not generally the case. For example, look at the throttle device in the vapor compression refrigeration cycle. It is constant enthalpy across the throttle valve, but the inlet is saturated (or sometimes compressed) liquid and the discharge is a liquid-vapor mixture at a substantially lower temperature.

I came to this thread yesterday to make this same comment but got sidelined at work before I could make it. Well done!  It should be noted that nearly every refrigerant system includes this element for the express purpose of cooling and thus the problem trap is to think of this sort of system, when the key element of this problem is that an ideal gas is used.

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