Abstract
There are several thermo-fluid process modelling tools available on the market which can be used to analyze the off-design performance of thermal plants. These tools all offer the user with a simple convective heat exchanger component that requires the design-base process conditions as inputs. The tools would then calculate an effective overall heat transfer factor (UA) and make use of gas flow mass ratios to scale the UA value for off-design conditions. The models employed in these tools assume that the contribution of gas radiation is insignificant, hence only applies convection scaling laws. This paper presents an improved model which considers the contribution of the gas and particle radiation, as is often encountered in the first few heaters in coal fired boilers and heat recovery steam generators. A more fundamental scaling law is applied for the convection scaling and incorporates a cleanliness factor which allows for the consideration of fouling of the heater surfaces. The model’s performance was validated against a discretized tube-level heater model that solves the fundamental convection and radiation terms. The model is accurate within 1% for the cases considered, as compared to more than 20% error if radiation contribution is not considered.
Highlights
The ability to accurately predict the heat transfer occurring in a thermal application is key to a successful design
It has been shown that many commercial software tools make use of only a mass flow ratio to scale the heat transfer coefficient for gas-to-liquid heat exchangers in the off-design performance calculations
A new approach was presented where the overall UA term is split into a convection and a radiation term and treated separately
Summary
The ability to accurately predict the heat transfer occurring in a thermal application is key to a successful design. It is not unusual to find a vast volume of literature dealing with heat exchanger performance modelling. The choice of method employed by the designer or analyst is a function of the expected outcome as well as the available information about the specific heat exchanger. The tools generally offer three approaches: a) determine the physical design requirements of a heat exchanger, given certain performance needs; b) determine the performance of a heat exchanger given detailed design information; c) determine the performance of a heat exchanger given a known operating point. This paper deals with the last approach where the analyst is usually interested in the off-design performance of an existing heater without detailed knowledge of the physical design or physical process. The focus is on gas-to-fluid heat exchangers such as superheaters found in typical steam generating plants
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