  Macroscopic entanglement witnesses have been put forward recently to reveal
nonlocal quantum correlations between individual constituents of the solid at
nonzero temperatures. Here we apply a recently proposed universal entanglement
witness, the magnetic susceptibility [New J. Phys. {\bf 7}, 258 (2005)] for the
estimation of the critical temperature $T_c$ in the nanotubular system ${\rm
Na_2V_3O_7}$ below which thermal entanglement is present. As a result of an
analysis based on the experimental data for dc-magnetic susceptibility, we show
that $T_c \approx 365$ K, which is approximately three times higher than the
critical temperature corresponding to the bipartite entanglement.
