Double pipe heat exchanger temperatures estimation using fractional observers M. Alegria-Zamudio, R. F. Escobar-Jiménez, J. F. Gómez-Aguilar, J. García-Morales and J. A. Hernández-Pérez Eur. Phys. J. Plus, 134 10 (2019) 496 Published online: 09 October 2019 DOI: 10.1140/epjp/i2019-12939-8