Thermal calculation issues for flue gas condensers in gas boilers
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Dear experts, for a natural gas boiler with an exhaust gas temperature of 100°C, I would like to design a heat pipe flue gas condenser to reduce the gas temperature to 40°C in order to recover heat and use it to warm the exhaust gases. The vapor pressure of water in the exhaust gases is 14 kPa, corresponding to a dew point temperature of 53°C. The cooled flue gas is heated using the sensible heat of part of the flue gas and the latent heat of vaporization of water vapor; the heat release on the hot side is equal to the sensible heat of the flue gas plus part of the latent heat of vaporization associated with the condensation of water vapor. The temperature of the flue gas on the cold side is 40°C; after absorbing heat, it raises the temperature of the exhaust gas. However, the actual flue gas temperature at the cold-side outlet reaches 210°C. Question: 1. Is this approach feasible? 2. Why is the flue gas temperature at the cold-side outlet so high? Attached is a thermal calculation table; please feel free to provide feedback from everyone in the forum.
Hello, I was thinking of using the flue gas at the hot side outlet as the cold-side medium, with a gas temperature of 40°C. Is that not feasible?
It cannot be calculated simply based on the specific heat of the flue gas; are you currently calculating that the flue gas at the hot end is at 40°C (in co-current flow) and at 210°C at the cold end? In practice, the flue gas at the hot end cannot be reduced to below ~90°C, while heating the flue gas at the cold end to around ~70°C represents the limit; this also ensures heat balance.
Can the temperature be lowered unless a cold-side medium with a lower temperature and higher flow rate is used?
That’s roughly it; the heat pipe is merely a means of transfer. Removing this element means, in simple terms, that it’s not possible to use smoke at 100°C to heat another stream of smoke to 210°C℃”
:Victory: Got it. I’ve done relatively little work on heat exchanger calculations. Thank you for your guidance; have a happy New Year
I really don’t understand why heat pipes are used – what is the purpose of the heat absorbed by those heat pipes? What are the advantages? Innovation is fine, but it must be valuable!
Heat pipes are different from ordinary medium circulation pipes such as water pipes. In general, with regular medium circulation pipes, if there is a problem with one of them, the entire system stops functioning; however, heat pipes operate independently, so if one heat pipe fails, it does not prevent the heat exchanger from continuing to be used.
May I ask how you determine the amount of water vapor that condenses? I’ve been learning about this topic recently; it’s urgent! !
There is information available online on this topic, regarding the return water of condensing boilers; the amount of vapor that condenses is related to the dew point temperature and partial pressures
The sources I found are very varied; I’m a beginner. Are you involved in the design of heat exchangers?