Stack Overflow

WE'VE ALL HEARD stories about roastery owners who have had to deal with complaints from their neighbors about coffee smells and smoke. Trouble is, aggravated neighbors don't always come to the roastery door to discuss their concerns in person. Many times, they file a formal complaint, which may lead to downtime at your business as you work with air quality control officials to rectify the problem.

To help avoid these issues from the start, roasters should take a look at their exhaust stacks and--if they don't already have one--install a no-loss stack.

Let's back up a moment to start with the correct terminology, as it helps us define what we are talking about. The no-loss stack is actually just a stackhead - a part that ends, or caps off, your stack. So any stack can theoretically be made to be "no loss" simply by adding a no-loss stackhead. No-loss stackheads are designed to eliminate back pressure on positive pressure-exhausting equipment, while protecting the equipment from the rain.

FIGURE A. Types of Stack Terminations

None of these should be recommended for use with coffee roasters, except for the no-loss stackhead at bottom right. (The no-loss stackhead shown is only one type.)

China Top

Goose Neck

Gas Vent Rain Cap

T-Top

No-Loss Stackhead

Keeping your neighbors happy is one reason to adopt a no-loss stack--but it's not the only reason. Fundamentally, the more efficient your machinery is, the better your air quality is likely to be. As a roaster, your core motivation for installing a no-loss stackhead should be making your roasting operation as efficient as possible.

Operational Efficiency

The primary reason that no-loss stackheads are important to coffee roasters is that they are, simply, the correct stack termination for equipment with positive pressure exhausts like coffee roasters. Both drum and air roasters have positive pressure exhausts, so this is the correct termination for both types of equipment. If you are using any type of stackhead other than one with no loss, then you are reducing the efficiency of your roaster.

Possible problems caused by back pressure on coffee roasting systems include:

Jet-Cap

1. Reduced batch capacity
2. More frequent cleaning of blowers (drum) and ducting (air and drum)
3. More variability of product profile
4. More wear and tear on equipment
5. Less energy-efficient operation
6. Smoke in roasting space
7. Chaff not being separated from the coffee

The higher the velocity and/or volume of your exhaust at stack termination, the more problems you will experience without a no-loss stackhead on your installation. This is due to the increased back pressure caused by higher-velocity and higher-volume exhausts.

CAVEATS to the use of no-loss stacks

There are always exceptions to every rule, so here are a few for the use of no-loss stackheads.

1. They are designed for stacks that end vertically.
2. There can be environmental factors that may reduce the effectiveness of the rain protection of a no-loss stackhead. These factors can include trees, walls or other buildings in close proximity that may cause rain to be forced straight down when equipment is not in use.
3. Some inspectors do not understand, or may not allow, no-loss stacks--generally because they are unfamiliar with their use. (For these situations, it is possible to modify a China cap to reduce back pressure and the negative effects of pushing smoke down.)

Figure A (page 48) shows multiple types of stack terminations, none of which should be recommended for use with coffee roasters, except for the no-loss-type stackhead. (The no-loss stackhead shown is only one type.)

No other stack termination will give you the positive effects for pollution control that the no-loss stackhead will.

POLLUTION CONTROL

The secondary reason for using a no-loss stackhead has to do with pollution control--more specifically, with reducing your chances of getting nuisance complaints caused by smoke and smell emanating from your stack. With a no-loss stackhead, the pressure of the exhaust helps to push the smoke and smell higher into the atmosphere. Better winds and a wider effluent plume allow for greater dispersion and dilution of both smoke and smell.

With better dispersal and dilution rates, your effluent becomes less offensive, and you will be less likely to receive a nuisance complaint. It is always better to try to reduce your chances of getting a nuisance complaint than it is to deal with air-quality officials after a complaint has been filed, regardless of the final outcome of the complaint.

No other stack termination will give you the positive effects for pollution control that the no-loss stackhead will. Some, like China caps and goose necks, actually force the smoke down (where it can get sucked into HVAC or fresh-air makeup systems), while others, like T-tops, drastically reduce exhaust velocity.

But what about the rain? How does a no-loss stackhead that is open on the top keep rain from coming down the stack?

No-loss stackheads are designed to keep rain out of a stack by using the principle that rain does not fall straight down. Don't ask me how it works exactly. But, believe me, it does work. (Being located in Florida, we get plenty of rain to put this to the test.)

A no-loss stackhead can be adapted for use with any ducting material. This allows for an easy and inexpensive upgrade to all existing roasting operations.

Whether you are installing a new roaster, or just need to fix an old or troubling installation, a no-loss stackhead will help your equipment run better, your coffee taste better and your neighbors stay happier. At around $100 per stack, a no-loss stackhead is one of the best returns on investment you can make.