The rate equation for heat transfer is: Q = U × A × MTD where:…
Two-Phase Liquid/Gas Heat Transfer
This section describes sensible heat transfer in two-phase hydroprocessing feed effluent heat exchangers, where phase change (boiling or condensing) is not controlling.
Two-phase heat exchangers are usually designed to operate in the annular or churn flow regimes on both shell and tube sides. For these flow regimes, liquid coats the heat transfer surfaces and a continuous gas phase flows in the core. Liquid droplets are entrained in the core flow. Heat transfer resistance between the gas and the liquid is negligible and can be ignored. The governing resistance is convection and conduction in the liquid film covering the heat transfer surface. This type of flow can be modeled as a pseudo single-phase fluid using “no-slip” mixture density and velocity, mixture heat capacity (or enthalpy), liquid viscosity and liquid thermal conductivity. “No-slip” means that the liquid and gas are assumed to flow at the same velocity. These pseudo single-phase fluid properties can be input to the HTRI single-phase simulation program or used with the simpler methods described in the previous section. This approach has been validated against Company in-plant test data for well mixed flow only.
When horizontal two-phase heat exchangers operate in stratified flow regimes, liquid accumulates in the bottom of the exchanger. If the outlet nozzle is on top, a stagnant liquid level rises in the exchanger as needed to force the upper part of the exchanger into a mixed phase flow regime where net liquid transport is possible. For this situation, heat transfer varies as flow rate to the 1.6 power, and pressure drop is nearly independent of flow rate. This is simply the effect of varying level of stagnant liquid.
If the outlet nozzle is on the bottom, vapor flows in the upper part and liquid flows in the lower part. The fraction of the exchanger associated with each phase is more or less in proportion to their relative volume flow rates.
Stratified two-phase flow in exchangers results in very poor thermal performance and should be avoided.
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