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I'm designing a boiler and would like to know if the efficiency-ntu methodology can still be applied when there is a phase change in one of the streams.
Also, any hints on to where I can read up about refractory bricks as applied to shell & tube heat exchangers (my boiler)? I'll be using a AES design.
Thank you,
Chrispap
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Boiler
Started by Guest_Guest_chrispap_*, Mar 15 2005 05:26 PM
3 replies to this topic
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#2
mbeychok
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Posted 24 March 2005 - 07:37 PM
Chrispap:
I don't know about how other readers have taken your question, but it doesn't make sense to me.
NTU (Number of Transfer Units) methodology is used in designing absorbers, and perhaps distillation columns in some cases ... in other words, for mass transfer designs. I have never heard of it being used for boiler design or heat transfer design.
Would you please spell out in detail what you are trying to do?
I don't know about how other readers have taken your question, but it doesn't make sense to me.
NTU (Number of Transfer Units) methodology is used in designing absorbers, and perhaps distillation columns in some cases ... in other words, for mass transfer designs. I have never heard of it being used for boiler design or heat transfer design.
Would you please spell out in detail what you are trying to do?
#3
siretb
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Posted 25 March 2005 - 04:04 AM
NTU can be applied to heat transfer. the definition is similar to what is used for mass transfer, except that enthalpy is the variable
We have then NTU =Integral[from H=initial enthalpy ; to H=Final Enthalpy ; dH/(H-H*)] where H* is the "equilibrium enthalpy". Refer to Perry for more details.
Thgisd approach can be used when there is a phase change; one example is its use for calculation of cooling towers, where water is evaporated or condensed
We have then NTU =Integral[from H=initial enthalpy ; to H=Final Enthalpy ; dH/(H-H*)] where H* is the "equilibrium enthalpy". Refer to Perry for more details.
Thgisd approach can be used when there is a phase change; one example is its use for calculation of cooling towers, where water is evaporated or condensed
#4
Guest_chrisp_*
Guest_chrisp_*
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Posted 25 March 2005 - 10:40 AM
Yes, sorry if my question was poorly posed. The effectiveness-NTU methodology can be used when designing a heat exchanger, we were told it is prefered over the LMTD approach as (if I remember correctly) avoids itteration when one of the stream outlet temperatures is not known.
In my set -up so far, I have one stream coming in at 1143K needing to be cooled to 723K and coolant water comming in at 293K and exiting at approximately 435K.
Flow rate for hot stream: 26.36 kg/s
Cp for hot stream: 2.37 kJ/kg K
By a heat balance (addressed in another post of mine) :
2.37*26.37*(1143 - 293) = m (Dhv + 4.18(435 - 293))
I find the flow rate of cooling water to be: 9.7478 kg/s
However as this stream (of water) is entering at 293K and exiting at 435K well above water's boiling point, there is a phase change, and I suspect that the overall heat transfer coeffcient will not be steady, and can thus not be used in the effectiveness-NTU approach.
My question therefore is what can be done in such a situation when one stream is undergoing a phase change. How do I calculate my overall heat transfer coefficient in such cases?
Your help would be most appreciated, I am very stuck
Thank you!
Chrispap
In my set -up so far, I have one stream coming in at 1143K needing to be cooled to 723K and coolant water comming in at 293K and exiting at approximately 435K.
Flow rate for hot stream: 26.36 kg/s
Cp for hot stream: 2.37 kJ/kg K
By a heat balance (addressed in another post of mine) :
2.37*26.37*(1143 - 293) = m (Dhv + 4.18(435 - 293))
I find the flow rate of cooling water to be: 9.7478 kg/s
However as this stream (of water) is entering at 293K and exiting at 435K well above water's boiling point, there is a phase change, and I suspect that the overall heat transfer coeffcient will not be steady, and can thus not be used in the effectiveness-NTU approach.
My question therefore is what can be done in such a situation when one stream is undergoing a phase change. How do I calculate my overall heat transfer coefficient in such cases?
Your help would be most appreciated, I am very stuck
Thank you!
Chrispap
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