  We have compared the absolute temperature data of the CMB spectrum with
models for CMB spectra distorted by a single or two heating processes at
different cosmic times. The constraints on the fractional energy injected in
the radiation field, DE/E, are mainly provided by the FIRAS instrument aboard
the COBE satellite. Under the hypothesis that two heating processes have
occurred at different epochs, the limits on DE/E are relaxed by a factor 2 both
for the earlier and the later process with respect to the case in which a
single energy injection in the thermal history of the universe is considered.
In general, the constraints on DE/E are weaker for early processes than for
relatively late processes, because of the wavelength coverage of FIRAS data.
  We considered also the FIRAS calibration as revised by Battistelli et al.
2000, that, in the case of the favourite calibrator emissivity law proposed by
the authors, implies significant deviations from a planckian spectrum. An
astrophysical explanation of this, although intriguing, seems difficult, both
in terms of CMB spectral distortions and in terms of a relevant millimetric
foreground.
  Future precise measurements at longer wavelengths as well as current and
future CMB anisotropy space missions will provide independent, direct or
indirect, cross checks.
