Performance characteristics of artificial coatings applied to steam surface condensers

• Coating thermal performance is quantified in three ways.
• Details pertaining to the apparatus and experimental uncertainty are provided.
• Methods of predicting the overall coated condenser performance are considered.
• Insight into these coatings allows better management of condenser refurbishment.

Recent developments in artificial protective coatings (APCs) and their application techniques have made it possible to consistently apply uniformly thin coatings. However, documented testing procedures are limited, which specifically deal with the thermal performance of these coatings. Moreover, recently developed coatings and application techniques are unique in many aspects and there exists the need to test their combined performance. There are also insufficient industrial guidelines to regulate the selection and application of these coatings. This paper investigates the measurement of the thermal performance of these coatings applied to new brass tubes. The double-pipe counter-flow heat exchanger, that was designed, manufactured and commissioned, is described. Heated water is used to simulate condensing steam, thus enforcing repeatable convection coefficients that are similar in magnitude to condensing steam. The measured annular convection coefficients (tested on new uncoated tubes) indicate the heat exchanger achieves fully-developed hydrodynamic conditions, and the measured Nusselt numbers agree within up to ±5% of literature correlations. Three different coatings are tested, with thicknesses ranging from 40 μm to 130 μm. Thermal performance is measured in terms of the coating thermal conductivity, the effective coated-tube conductivity, and the coating factor. Additionally, the pressure drop measurements agree within ±5% of smooth tube predictions. The Heat Exchange Institute (HEI) method for determining the heat transfer rate of steam surface condensers (Heat Exchange Institute, Standards for Steam Surface Condensers, Heat Exchange Institute (HEI), Cleveland, Ohio, 2012, 11th edition) makes no provision for APCs. However, this paper shows how to modify the design cleanliness factor or the material correction factor to account for APCs. Single tube tests are related to the overall condenser performance, and hence are used to show how coating guidelines may be determined in terms of the coating conductivity and thickness.