In cold weather, it’s all about inlet adjustment

With the very cold temperatures in the upper Midwest, ventilation systems in many production facilities are operating at their minimum levels. While these levels were supposedly sized and designed based on moisture removal rates, many producers struggle with wet pens this time of year. Another common complaint is higher than expected ammonia levels at specific locations within a facility.

The solution many producers use to correct these complaints it to increase the amount of minimum ventilation, generally by speeding up the variable fans on stage 1 in their controller. While this may resolve the problem, the increased ventilation rate always increases the heat removal from the facility and the resulting propane usage.

In a large number of cases the problem with minimum ventilation isn’t the rate (or minimum speed setting in the controller). Rather, it is the inlet settings.

As an example, for each 24” fan operating at a 50% setting (assuming all other controller settings such as motor curves etc. are set correctly for that fan), to get air to enter the facility thru the inlets at 800 fpm you need a total of 3.75 square feet of total opening into the room assuming the fan is rated at 6000 cfm at full speed. With 2-24” fans operating at 50% on stage 1 the total room inlet requirement becomes 7.5 square feet.

This is the total of all air leaks into the room, including pit lids not setting tight on the pump port, curtain pockets, mice holes in curtains, missing door sweeps, missing door knobs on old doors, shutters on idle fans and the designed ceiling inlets. The problem isn’t fan sizing – the problem is too many air inlets and air not entering the room at 800 fpm thru the ceiling inlets!

Lets start with pit lids. If you have a 2” gap along 4 4-ft pit lids this is a total opening of 2.7 square feet that is leaking air into the pit/room.

Shutters on unused fans are another major leak into rooms at minimum ventilation. I’ve seen data suggesting shutters on 36” fans leak at about 300 cfm at normal operating conditions with leakage much worse when wind impacts the shutters. A shutter on a 36” fan typically measures about 45” x 45” or 14 square feet. This means shutters leak at approximately 21 cfm per square foot of surface area (when they are clean and not missing shutter blades, etc). Shutters on 54” tunnel fans typically are about 25 square feet (5’ x 5’) meaning they leak about 500-550 cfm/shutter.

In my experience problems with bad air quality (high ammonia levels) and wet pens are due to the designed inlets not having the correct velocity. Air ‘dumps’ into the pig space and drops to the pit before it has a chance to pick up moisture, etc.

With the correct inlet velocity (800 fpm) with a 7.5-8 ft ceiling most growers should ‘feel’ the inlet draft in their face at about 12-15 ft back from the inlet. Anything less than this and there are too many ‘inlet’ holes into the room. When you get inlet velocity correct you have good air mixing throughout the room and pens generally become drier and complaints about air quality are minimal.

Before you increase the minimum ventilation setting in the controller and spend more money on propane, it pays to check you inlet adjustments! You need to make the sum of all holes into the room is small enough that air from the inlets enters at 800 fpm.

6 thoughts on “In cold weather, it’s all about inlet adjustment

  1. Dear Dr. Brumm, I think there is a little mistake. ” you need a total of 3.75 square feet of total opening into the room assuming the fan is rated at 6000 cfm at full speed. With 2-24” fans operating at 50% on stage 1 the total room inlet requirement becomes 7.5 square feet. ” shoulde be ” you need a total of 7.5 square feet of total opening into the room assuming the fan is rated at 6000 cfm at full speed. With 2-24” fans operating at 50% on stage 1 the total room inlet requirement becomes 3.75 square feet. “

  2. No, my math is correct. If a 24″ fan at 50% cfm is 3000 cfm, to get 800 fpm inlet velocity, the total inlet area must be 3.75 square ft (3000 cfm / 800 fpm). Two fans at 50% becomes 7.5 sqft (6000 cfm / 800 fpm). These numbers double with fans at 100% of rated speed, capacity. However, the problem is most often inadequate inlet velocity at minimum ventilation, most often due to much inlet opening into the room from such things as curtain pockets, curtain holes, door sills, etc.

  3. Great article as always, is there a relationship between the cubic area of the room and the fan capacity (CFM) to achieve needed static pressure static.

  4. No relationship between room size and size of fan needed to maintain static pressure. Static pressure is all about the relationship of the inlets versus how much air is removed. That’s why you need to do the math and aggressively restrict all possible inlets at very low ventilation rates so that pressure can be maintained.

  5. Mike on a different subject, do you know the relationship between fan capacity and the area of cool cell needed. If I have 50,000 cfm max capacity, what area of cool cells do I need to keep air speed low enough through the cool cell to be effective.

  6. 6 inch thick pads are sized based on 350-375 cfm per square foot of pad area. In general 6 inch thick pads should not be operated at over 400 cfm per square foot of surface area as the static pressure rises above 0.05 inches w.g. and the cooling efficiency (temperature drop) declines too much.

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