A loop-seal having a horizontal immersed tube bundle in the recycle chamber with aeration at the bottom of the supply and return chambers or a conventional loop seal with heat exchanger (LSHE) was used for this study. Heat transfer behavior of the immersed tubes with aerations at the side walls of the recycle chamber and without side aerations was investigated. An experiment was carried out in a circulating fluidized bed system. The riser has a cross sectional area of 100mm x 100mm while the supply and return chambers of the loop seal have the same cross-sectional areas. The tube has a diameter of 12.7mm. Sand having an average diameter of 310pm was used as bed particles. In all cases, heat transfer tends to be highest and lowest at the tube bottom and the tube crest. respectively. The side aeration can improve particle mobility in the stagnant zone at the tube crest, while bottom aeration cannot, even at a high velocity. Without the side aeration, there is a large difference in the local heat transfer profiles between the lower half and upper half of the tube. With the side aeration, the profiles tend to be comparable. As a result, the average heat transfer coefficient around the tube in this case is higher, and thermal stress due to temperature difference in the tube is lower than for the case without side aeration. In addition, the LSHE with side aeration has higher responsivities to thermal load and solid circulation rate variation than that without side h aeration. (C) 2020 Published by Elsevier B.V.