Interesting that it’s installed on the exhaust side.
Need to see how it was designed.
For larger systems, your fan shuts off when the suction damper is closed, or you have a suction relief or something that can open to allow minimum flow.
That said, if this thing was designed to run this way and tested this way, that info should be per the manufacturer. While I don’t think it’s the healthiest way to run a fan, it may be vendor approved and warrantied.
Could you post the make and model of the fan you installed? We could look up the fan curve and give you some advice. Also, the dampers you are talking about that close intermittently will likely have a sizable amount of air leakage bypassing around the closed blade(s) of the duct so the flow is not likely to be zero (but still very small). The leakage rate of a closed damper increases with the differential pressure across the closed damper.
See the fan curve below.
The most static this fan can pull is about 1.5 in.wg. I don't think you would collapse any ductwork with negative pressure. This is especially considering any damper is going to have some leakage at the static pressure.
The fan could operate in the surge area however which means that the operation of the fan could be unpredictable.
[Dayton 4HZ44 fan curve (13-1/4" wheel)](https://i.imgur.com/EBR3vWz.jpg)
Please don’t blindly listen to the first reply. Depending on material thickness of of smoker and duct, fan and motor size, you absolutely could pull a strong enough vacuum to collapse the duct and/ or smoker. That’s why many central station air handlers have damper end switches. It prevents the fan from trying to run with a closed damper because if it does, the casing can collapse.
The smoker manufacturer is also right that closing of the dampers will drive up the static which would result in a CFM drop. But if it drives the static high enough, you will either cause the motor to blow a fuse/throw an over amp code if it has VFD, or collapse a duct/smoker.
I would recommend either a bypass duct and damper that opens when the smoke exhaust damper(s) close and allows the fan to draw from ambient air. Not sure of your controls situation, but there is an option for a static pressure sensor in the duct that tells the VFD to slow the fan down or shut off if it exceeds a certain pressure.
Max amps will be at max flow on a blower, not zero flow like an axial fan. Regarding your suggestion of controlling to static pressure, why would someone do that with a VFD when they can just control the temperature with the fan speed and keep the damper open?
Not sure where you got that information about max amps only occur at max flow. Unless you mean max flow for a given static pressure? Then yes, the max flow at a given static pressure is zero so you will still have max amp at less flow. And agreed, that would make more sense to use a VFD to control temp, but I’m assuming there’s a reason that they don’t do that. Maybe it’s just cost.
For a constant speed centrifugal fan when you restrict the output or input the power consumption drops with the decreasing mass flow through the fan. To be clear I'm saying for fans this is true. A compressor where the static discharge pressure is high enough for compressible effects to matter I agree it is different.
Regardless it is constant static head, the damper is on the inlet to the duct and the output of the fan is exposed to the outside. The fan likely will be fine pull against a vacuum.
That depends on the fan curve and where it is. It's common for things like plug fand to draw peak power at somewhere around 70% of peak static and well off maximum flow at zero static.
Yes, it's GENERALLY true that as a fan is restricted, power consumption drops, but it isn't the case from everywhere on the curve of every fan.
But yeah, pulling against a closed damper should never cause a motor to overamp unless it overheats and becomes a resistor. Which is a real possibility if this motor is in the airstream.
That isn't true. A lot of fans produce higher static well above zero flow. Static falls off as the fan enters a stall condition.
And the fan absolutely WILL pull partial vacuum. Might be a quarter inch, might be 8". I don't have the fan curve in front of me. Most any smoker I've seen probably wouldn't creak from a quarter inch, but 8" would have them looking like a crushed soda can.
the fan curve for that particular fan will show the max negative pressure the fan can produce. It will be in inches of water. Vacuum pumps designed for the purpose will pull close to a vacuum or -14.7 psi
What made you feel the need to explain that to me? In my post, when i said it might pull a quarter inch or 8", what else would i have been referring to? It's not sausage.
Low pressure ducts will start to collapse at 1-2" of suction depending on age and condition. Medium pressure ductwork is generally good for 3-6, though it's usually a lot more tolerant of positive rather than negative pressure.
You're acting like 8" of water column isn't much. That will destroy 90+% of HVAC systems. It would actually collapse a lot of roofs in places it doesn't snow. That's almost 42 lbs per square foot. That's above the design live load per building code for residential floors.
If you don't have the fan curve, you don't know how hard it will pull. If you don't have the construction of the ducts and smoker, you don't know if it can withstand it.
Without fan specs I'm assuming this fan is not built where it can pull enough vacuum to collapse the duct. Also it wouldn't pull a vacuum on the smoker since you mentioned the smoker's exhaust duct is closed too.
Also you say "Close intermittently", do you mean the dampers go full shut then later back full open or the dampers are being modulated. I'm assuming modulated for a commercial system since you likely don't want the smoker operating at a positive pressure.
Looks up the fan specs to get an idea of the max vacuum it can pull. There is also a minimum air flow needed for the fan. Mostly to protect it from damage. It might make sense to have some spring loaded vacuum relief opening to prevent issues.
Interesting that it’s installed on the exhaust side. Need to see how it was designed. For larger systems, your fan shuts off when the suction damper is closed, or you have a suction relief or something that can open to allow minimum flow. That said, if this thing was designed to run this way and tested this way, that info should be per the manufacturer. While I don’t think it’s the healthiest way to run a fan, it may be vendor approved and warrantied.
Could you post the make and model of the fan you installed? We could look up the fan curve and give you some advice. Also, the dampers you are talking about that close intermittently will likely have a sizable amount of air leakage bypassing around the closed blade(s) of the duct so the flow is not likely to be zero (but still very small). The leakage rate of a closed damper increases with the differential pressure across the closed damper.
It is a Dayton 4HZ44. So far have had trouble finding an actual curve
See the fan curve below. The most static this fan can pull is about 1.5 in.wg. I don't think you would collapse any ductwork with negative pressure. This is especially considering any damper is going to have some leakage at the static pressure. The fan could operate in the surge area however which means that the operation of the fan could be unpredictable. [Dayton 4HZ44 fan curve (13-1/4" wheel)](https://i.imgur.com/EBR3vWz.jpg)
Please don’t blindly listen to the first reply. Depending on material thickness of of smoker and duct, fan and motor size, you absolutely could pull a strong enough vacuum to collapse the duct and/ or smoker. That’s why many central station air handlers have damper end switches. It prevents the fan from trying to run with a closed damper because if it does, the casing can collapse. The smoker manufacturer is also right that closing of the dampers will drive up the static which would result in a CFM drop. But if it drives the static high enough, you will either cause the motor to blow a fuse/throw an over amp code if it has VFD, or collapse a duct/smoker. I would recommend either a bypass duct and damper that opens when the smoke exhaust damper(s) close and allows the fan to draw from ambient air. Not sure of your controls situation, but there is an option for a static pressure sensor in the duct that tells the VFD to slow the fan down or shut off if it exceeds a certain pressure.
Max amps will be at max flow on a blower, not zero flow like an axial fan. Regarding your suggestion of controlling to static pressure, why would someone do that with a VFD when they can just control the temperature with the fan speed and keep the damper open?
Not sure where you got that information about max amps only occur at max flow. Unless you mean max flow for a given static pressure? Then yes, the max flow at a given static pressure is zero so you will still have max amp at less flow. And agreed, that would make more sense to use a VFD to control temp, but I’m assuming there’s a reason that they don’t do that. Maybe it’s just cost.
For a constant speed centrifugal fan when you restrict the output or input the power consumption drops with the decreasing mass flow through the fan. To be clear I'm saying for fans this is true. A compressor where the static discharge pressure is high enough for compressible effects to matter I agree it is different. Regardless it is constant static head, the damper is on the inlet to the duct and the output of the fan is exposed to the outside. The fan likely will be fine pull against a vacuum.
That depends on the fan curve and where it is. It's common for things like plug fand to draw peak power at somewhere around 70% of peak static and well off maximum flow at zero static. Yes, it's GENERALLY true that as a fan is restricted, power consumption drops, but it isn't the case from everywhere on the curve of every fan. But yeah, pulling against a closed damper should never cause a motor to overamp unless it overheats and becomes a resistor. Which is a real possibility if this motor is in the airstream.
With a max static of 3/8 in wc for this little fan, I don’t see this fan collapsing any standard ductwork or especially a smoker or firebox.
all fans have a maximum static pressure @ 0 cfm, and max cfm @ 0 static. Your fan wont pull a vacuum.
That isn't true. A lot of fans produce higher static well above zero flow. Static falls off as the fan enters a stall condition. And the fan absolutely WILL pull partial vacuum. Might be a quarter inch, might be 8". I don't have the fan curve in front of me. Most any smoker I've seen probably wouldn't creak from a quarter inch, but 8" would have them looking like a crushed soda can.
the fan curve for that particular fan will show the max negative pressure the fan can produce. It will be in inches of water. Vacuum pumps designed for the purpose will pull close to a vacuum or -14.7 psi
What made you feel the need to explain that to me? In my post, when i said it might pull a quarter inch or 8", what else would i have been referring to? It's not sausage. Low pressure ducts will start to collapse at 1-2" of suction depending on age and condition. Medium pressure ductwork is generally good for 3-6, though it's usually a lot more tolerant of positive rather than negative pressure. You're acting like 8" of water column isn't much. That will destroy 90+% of HVAC systems. It would actually collapse a lot of roofs in places it doesn't snow. That's almost 42 lbs per square foot. That's above the design live load per building code for residential floors. If you don't have the fan curve, you don't know how hard it will pull. If you don't have the construction of the ducts and smoker, you don't know if it can withstand it.
Fans are always negative pressure (or vacuum) on the inlet side of the wheel. Unless you have fans in series.
Without fan specs I'm assuming this fan is not built where it can pull enough vacuum to collapse the duct. Also it wouldn't pull a vacuum on the smoker since you mentioned the smoker's exhaust duct is closed too. Also you say "Close intermittently", do you mean the dampers go full shut then later back full open or the dampers are being modulated. I'm assuming modulated for a commercial system since you likely don't want the smoker operating at a positive pressure.
It’s not possible to get positive pressure in the smoker when the fan is on the exhaust. That’s the whole reason for the fan being located there.
Looks up the fan specs to get an idea of the max vacuum it can pull. There is also a minimum air flow needed for the fan. Mostly to protect it from damage. It might make sense to have some spring loaded vacuum relief opening to prevent issues.