One man was killed, another worker injured and two firefighters suffered minor injuries after an explosion Friday at a furnace equipment facility in Ayer, officials said.The man who died, identified yesterday as a man in his 50s, and another worker were rushed to the hospital following the explosion at Advanced Vacuum Systems, Middlesex District Attorney Marian T. Ryan, state Fire Marshal Peter Ostroskey, Ayer police Chief William Murray and Ayer fire Chief Robert Pedrazzi said in a statement.