The collection of the NCG from the digester blow requires the efficient condensing of the blow vapors without relief to the atmosphere or the intake of ambient air into the system. The batch blow releases a large amount of heat in a short time resulting in a high condensing water demand, thus most systems are direct contact condensing.

Multiple blow tanks may be connected to a single condensing system. The system is designed to take a batch process and turn it into a continuous heat source. A system will include pressure and vacuum relief devices, a cyclone separator for removal of entrained fiber, a direct contact primary co-current condenser, an accumulator tank, an indirect secondary condenser, heat recovery and a condensing water supply pump. Heat recovery may be done using a number of different methods including heating process water, boiler feed water heating or pre-evaporation of black liquor.

The mechanical pressure/vacuum relief valves are located both on the blow tank and accumulator tank. Vapors from the cyclone separator enter the primary condenser mounted on top of the accumulator tank. Condensing water from the bottom of the accumulator tank is used to condense the bulk of the blow steam. The condensing water supply pump is a low head axial flow type pump. A small fraction of steam is passed through to the secondary condenser for final cooling of the NCG. The hot condensate from the primary condenser is accumulated in the top of the accumulator tank and is pumped to heat recovery for utilization on a continuous basis. The secondary condensate is sent to turpentine decanting or treatment.

Many mills have extended periods between blows. To prevent ambient vapor space condensing, padding steam is added to the blow tank to maintain a slightly positive pressure within the system. A backpressure is maintained in the accumulator tank by an automatic isolation valve between the secondary condenser and the accumulator tank to minimize padding steam condensing in the secondary condenser.

Even with the installation of a cyclone separator the possibility exists that fiber will be carried into the accumulator tank. The Lundberg Associates equipment is designed with large openings in all of the equipment to minimize the risk of fiber pluggage.