... glass vacuum tubeQuantity of vacuum tubes16,18,24, 30,Absorption rate93%thermal radiation rate6%Life-spanAt least 15 years Features:1) The tubes ... is average and can assure the vacuum tubes to use for a long time2) Whole glass vacuum tubes adopt super hard borax and silica ... before using, in order to avoid scalding Working PrincipleSolar vacuum tube was evacuated into the tank; the coating inside ...