Heat Generated by Pressure Drop 4

[traindriver]

Has anyone had experience with generating heat (to heat a fluid to 240 F) by using pressure drop through an orifice plate? I want to heat hydraulic oil from 70F to 240F without using electric heaters, just pressure drop through a nozzle or an orifice.
Any help with general formulas would be most appreciated.

 

[iainuts]

The discussion on isenthalpic pressure drop across an orifice is all well and good, but it isn't the pressure drop that increases the temperature from 70 to 240 F. If heat isn't added to the system, then the only way to increase the temperature (assuming a closed loop hydraulic system) is to add work.

 

[mauricestoker]

Iainuts,

You're right, in the temp and pressure range given, the effect is frictional, and JT is of no importance. I would agree that without addition of heat, no work is performed.
I disagree with the rest, and addition of heat does not imply work is performed.

If you think in terms of energy and not work, consider a closed pressure vessel. When heat is added, no work occurs, but internal energy changes. Similar, if a paddle is installed and run by a pump, no work is performed but internal energy changes.

The rate of change in temperature divided by the rate of change in pressure is the JT coefficient. At the state point where change in pressure does not impact change in temperature, the coefficient is zero, and no JT effect takes place. When plotted against against presure and temperature (which is not normally near STP), the graph will be dome shaped. For points within the dome, throttling will cause decrease in temperature. For any point not on the graph or under the dome, throttling will result in increase in temperature.

If you don't agree, I'd recommend going to any cryo plant, or review the mechanical engineering thermo books. Most of all, this does not involve work or enthalpy, it involves internal energy and state points. If you can identify an argon plant that does not recognize this, please put me in touch as currently being an energy engineer, I'll show them how they can possibly save money.

 

[iainuts]

Hi Maurice,
Why are you concerned with the JT affect? What you're saying about the JT affect is all perfectly valid. But if heat can't be added to the hydraulic fluid, the only way to increase internal energy is to have work performed on the fluid. That can be done very easily by pressurizing the fluid with a pump and dropping the pressure across an orifice. That's all the OP is asking about. The only other consideration is to determine how much heat might be lost in the system as it gets hot. And yes, I'm quite aware of cryo plants, I've worked in the cryogenic field for over 21 years now, and much of my work is process related.

 

[mauricestoker]

Maybe I missed the initial question: can you use throttling to increase temperature? I must have missed the part about deadheading the pump.

If you dead head the pump, like mentioned above, you change the internal energy. What does that have to do with the original question, can use of orifice plate or throttling device increase temperature? If you know of another condition at which throttling results in increased temperature, I would very much like the education. Judging by some of the responses above, I would gather that many people assume that you cannot increase temperature by throttling. At least one response asked for the thermo behind it, and there it is.

 

[iainuts]

I think we all agree that we can't significantly increase the temperature of a fluid by throttling. That's too narrow of a focus, we need to step back and look at the system and the process.

Also, I'm not suggesting the pump be dead headed. Typical hydraulic circuits have pumps, and if the hydraulic fluid is forced to flow around a circuit in which there is a pump and a restriction such as an orifice, and if there isn't any work being done by the fluid nor any heat being removed, then the hydraulic fluid will gain all that energy being put in by the pump and won't be rejecting any energy. End result is the fluid gets hot. The reason the fluid gets hot is because energy is added at the pump which isn't removed by the isenthalpic expansion. So if energy isn't removed, the fluid gradually warms up from all the work being done on it. The fluid only rises by a few degrees with each cycle, but if that heat isn't removed through either a heat exchanger or by having that pressurized fluid do work on something, then the energy remains in the fluid and it gradually gets hot. I've seen hydraulic circuits run like this for about 15 minutes and get up to 200 F during that time.

We can apply the first law to this and find dU = Win. The equation provided by Drexl above is the same equation as this. It appears to me that Drexl has the same interpretation as I do regarding the intent of the OP.

 

[davefitz]

You can do it indirectly, and the Linde reference provided the hint.

One means of cooling a gas is by expanding the high pressure gas thru an isentropic expansion, otherwise called a turbine. Linde had done this in the past, and if there was no useful load for the turbine to drive, it might spin a shaft that is immersed in oil with appropriate shear devices in the oil filled space. The oil would then heat up. As long as the oil is not then transferring heat to the environment, it would continuously heat up.

So, in this case , a source of high pressure gas would expand across a turbine, the tubine shaft would drive a high shear propellor immersed in the oil bath, and the oil would heat up.

 

[mauricestoker]

Thank you, davefitz, you obviously get the point.

 

[mjpetrag]

Say you have compressed liquid water at 5000 psi and 400 F.
h1 = 381.14 Btu/lbm

You drop this across a valve to 500 psi at constant enthalpy.

T2 = 405.3 (interpolation)

DeltaT = +5.3 across valve


Now if you tried it with steam.
5000 psi @ 1000F -> 1365.5 Btu/lbm
dropping across the valve to 500 psi @ constant enthalpy
T2 = 715.3 F

DeltaT = -284.7 F



-Mike

 

[25362]

Miller's J-T inversion curve for all fluids shows that for 1.56 reduced pressure [Pr] -the liquid water case in hand-a reduced temperature [Tr] below 0.84 would involve heating on isenthalpic expansion.

In general, for a Pr[&asymp;]0 this would happen for all fluids at Tr<0.78 or Tr>5. This curve shows a maximum at Pr>11.8 beyond which all fluids would heat up on J-T expansion.

Liquid water and steam appear to comply with Miller's findings.

 

[chicopee]

I admit I was incorrect MortenA. I examined several phase diagrams and there are chemicals that exhibit a temperature rise with pressure drop in the liquid region when holding enthaly constant.