Double pipe heat exchanger temperatures estimation using fractional observersM. Alegria-Zamudio, R. F. Escobar-Jiménez, J. F. Gómez-Aguilar, J. García-Morales and J. A. Hernández-PérezEur. Phys. J. Plus, 134 10 (2019) 496DOI: https://doi.org/10.1140/epjp/i2019-12939-8