Heat Exchange


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Heat Exchanger Shell-Side Inlet: 200tonnes/hr NH3 @ -33degC Heat Exchanger Shell-Side Outlet: 200tonnes/hr NH3 @ approximately +3degC Heat Exchanger Tube-Side Inlet: 3.5 bar steam I am analysing the instrumentation and controls on an heat exchanger. I initially want to evaluate the thermodynamics of the heat transfer before I analyse the suitability of the current instrumentation and control strategy. Please educate me! Any process books suitable for an Instrument guy?
Repost your question in few days. In the mean time, try to reach the web site of ChemEng.com They have a forum and all sorts of links.
Hi William, You could probably get a rough indication of the steam mass flowrate using a heat balance equation:- Ms * Cs = M-NH3 * C-NH3 * T Ms * 650kcal/kg = 200,000kg/h*1.29kcal/kgC*36degC Ms = ~14,290 kg/h steam at 3.5 barg Ms- Steam flowrate in kg/h Cs- Enthalpy of saturated steam at 3.5 barg. M-NH3- Mass flowrate of NH3 C-NH3- Specific heat capacity of NH3 (taking its average value betweeen -33 to 3 degC) T-NH3- Temperature difference of inlet & outlet of NH3 regards, Andrew Ng Plant Werx

Bruce Durdle

William, 1) What is the pressure of the ammonia? From suitable tables, and pressure and temperature info, you can find out the specific enthalpy of the ammonia at inlet and outlet. The difference is the amount of energy you have to add. 2) What is the temperature of the steam at the exchanger inlet, and what is its conditon at the outlet? From these data, find the change in energy per kg of steam. 3) From the energy needed to heat the ammonia, and the change in energy available per kg of steam, find the steam flow required. If you are seriously interested in a suitable book, I have one I have put together for a technician training course that has got the essentials as far as I am concerned. I can send you a copy for GBP10 + postage. It covers thermodynamics, heat transfer, fluid mechanics, rotating machines, electric motors. Cheers, Bruce

Heavner, Lou [FRS/AUS]

2 comments: -If there is a phase change in the ammonia, don't forget it. Latent Heat of Vaporization, if it is being vaporized. If there is a phase change, it will probably swamp the sensible heat contribution. I don't have NH3 property info handy. Also,your heat capacity will be different for liquid vs vapor NH3. -Is there enough surface area to achieve the heat transfer? Assuming you are just doing a sanity check on the process engineer's work, I guess you will have to trust he got that part right. If not, your steam flow will be limited by how fast it gets condensed, the steam control valve would run wide open, and your ammonia won't reach target temperature at the target flowrate no matter how good your controls are. Regards, Lou Heavner Emerson Performance Solutions