This paper presents a systematic analysis of the thermodynamic performance of spiral turns in spiral plate heat exchangers (SPHEs), including non-adiabatic sources such as effects of heat leakage to the environment and fluid friction. These sources can reduce the thermal performance and increase the irreversibility of SPHEs. First, the critical factors of the heat loss rate to the environment, internal heat transfer rate (HTR), and channel temperature distributions are specified based on modeling the SPHE with hypothetical heat exchanger networks. Also, this modeling is validated with the results of channel temperature distributions by computational fluid dynamics simulation. Second, besides examining the spiral turns by entropy generation methods, entransy-based parameters are developed to analyze the SPHEs based on generated heat due to fluid viscosity in their channels for the first time. Finally, to show the method applicability proposed, an optimal designed single-phase counter-current SPHE is explored as a case. Three scenarios are introduced to evaluate the performance and irreversibility, namely heat leakage and no heat leakage to the environment and transferring the net heat between the streams. Results highlight the effects of non-adiabatic conditions, such as reductions of around 5.46%, 2.25%, and 2.42%, respectively, in the heat transfer area, total HTR, and overall heat transfer coefficient. Furthermore, findings confirm the performance reductions and irreversibility increments in non-adiabatic conditions and assert the importance of covering the outermost channels appropriately.