Parallel hoods – Two ways to get “greener”

By Frank Sorrels

Saving money (greenbacks to some people) on drying energy is of paramount importance in today’s economy. Combine that with the opportunity to be an environmental topnotch citizen by helping reduce greenhouse gases, and you have two winners.

In my article in the October/November 2006 issue of Tissue World, I pointed out that connecting hoods in series does not save air side drying energy. If a thermodynamic boundary is constructed around a tissue machine drying system, applying the conservation of energy and mass law of physics clearly verifies this fact.

When the blowing air is in a normal humidity ratio range, the air side energy requirement will be the same regardless of the method of connection. Increasing the blowing air humidity ratio decreases the amount of make up air required and this in turn can greatly decrease the cost of the drying energy.

In the series connection, the entire amount of make-up air enters the dry end section where it is heated. That’s about twice the amount of air the dry end would heat if the sections were connected in parallel. So there is no energy savings.

With hoods connected in parallel the make-up air to both hood sections will be fresh air with a humidity ratio around 0.013 kg of water vapor per kg of dry air. But when the sections are connected in series, the WE makeup air, which is the DE exhaust air, will have a much higher humidity ratio; perhaps 0.15 or more.

Thus, as the series system exhaust air humidity ratio is increased, a limiting value will be reached. For hoods connected in parallel, this limit does not exist and both hood sections, WE and DE, can operate at maximum humidity ratio, saving a significant amount of burner fuel.

With hoods connected in parallel, the system is easier to operate, easier to balance, and it can dry the product, at a reduced speed, if one burner or supply fan is inoperable. Most importantly it offers the opportunity to fully optimize the hood performance from an energy consumption standpoint.

Select your new machine with the hoods connected in parallel. If your existing machine has hoods connected in series, consider changing the ductwork to that of a parallel arrangement and then fully optimize.

Generally speaking, there are two major things that will contribute to lower air side energy consumption. One is operating the system at the highest humidity ratio and the other is the use of an air to air high efficiency heat exchanger to preheat the make up air.

As an example, consider a 5.5 m width tissue machine running at 1460 m/min and producing a 14.5 g/m2 bone dry sheet and with natural gas as the burner fuel for the parallel connected hoods. If the machine was operating with a bowing air humidity ratio of 0.23 kg of water vapor per kg of dry air and did not have a heat exchanger, the performance curves would be similar to those shown in Graph 1.

Increasing the humidity ratio of the blowing air to 0.48 kg of water vapor per kg of dry air would reduce the air side energy requirement about 25.7 %, a very significant savings. Using an air to air heat exchanger of high efficiency installed in the exhaust duct to preheat the make up air to the machine, the air side energy requirement would be further reduced about 15.4 %. The performance curves would be similar to those shown in Graph 2.