Fume hood air handler cost

Don't get myopic. There is a reason for the old velocity numbers. You can slow the RPM of the fan to offset the velocity numbers, but at what real cost? the real cost is NOT the conversion, but the changeup of the air velocity, and how it effects the removal of the fumes over a predetermined period of time. Striving for efficiency at the cost of safety is downright stupid.

We actually already determined that a flow of 0.75 m/s is unnecessarily high and actually creates eddies and vortices that can cause fumes to leave the hood. It is probably safer to run it at 0.5 m/s.

(Probably -- a word meaning I don't know what I'm talking about) causing eddies and vortices? I would investigate the configuration of the hood prior to changing the rate of extraction.

Un-necessarily high is SUPER if safety is involved.

I agree with the previous post that changing the airflow for economic reasons while increasing health risks is unwise and even criminal. If the airflow is improved and becomes laminar with the change then it would be a good thing.

The fume hoods in our lab are all belt driven due to corrosion and explosion concerns. In order to change the airspeed, all you have to do is change the sheaves on the motor and/or blower unit.

Your response to your instructor should include the information he/she requested to satisfy the requirements of the course you are taking.

You will need to figure that out for yourself.

(as others have said) If you are going to jeapordize your welder. painter, fabricator etc. etc. you should go for the maximum possible safety configuration. COST IS NOT AN ISSUE when health and safety is concerned.

I would answer the question with the calculations he/she requires with a caveat "SAFETY RULES AT ANY COST"

NUMBERS ARE NOT THE ISSUE

If the fans are driven by belts and pullies, a change in their ratio could reduce fan speed. Or it could be as simple as installing a sheet metal damper to restricy airflow. Also some Air handler motors have multiple speed windings that can be changed just by hooking up one or two different wires.

Just install a damper before or atter the fan, and adjust the system by partually closing the damper.

That's all.

Do not install a damper. It will only create a resistance to the system. Determine your reduction in airflow and install a variable speed drive on the air handler. You can estimate your motor savings by applying the fans laws. If you have multiple flow hoods you may want to insert a static pressure sensor in your duct and control your VFD to maintain a static pressure setpoint. This will provide you with the ability to maintain a system airflow with a diversity in operation of the flow hoods. Use a T&B tech to determine your setpoint. Fan law will provide you with an estimate of fan savings. What about heating and cooling? Is the air tempered? What is the temp setpoint during heating and cooling? What is the climate? What are the hours of operation? You can calculate sensible heating and cooling using the following equation: 1.08*CFM*(ENTERING TEMP - LEAVING TEMP). CFM is your reduction in airflowm, ENTERING TEMP would be you average outside air temp for heating and cooling, LEAVING TEMP would be your AHU discharge temp for heating or cooling. I am assuming your system is 100% outside air. The equation is in units of BTU/hr. For heating energy multiply by the hours of heating, convert to Therms and multiply by the cost for natrual gas ($/therm). I am assuming the heating source is NG. To convert your BTU/hr of cooling divide by 12,000 to convert to tons and then multiply by the efficiency of you cooling source (kW/ton) and then multiple by your hours of cooling to obtain you kWh. You can then multiply you kW savings (fan and cooling) by your monthly demand rate times your months of cooling. Also, multiply your kW reductions by your hours of operations to obtain your kWh savings. Multiply your kWh savings by your cost of electrical energy ($/kWh). That should do it.

The damper increases resistance, lowers air amount - BUT decreases motor power consumption.

No needs to replace fan and motor - no expenses. In addition, you save money on electricity.

The desirable effect is achieved

Academicaly could be interesting the point to reduce the air consuption by costs reason , but really what matters is the eficiency of exctracion of fumes .

Said that I think that at first what matters is to analize the type of profile velocity at wood because one thing is the mean averadge velocity at wood and other if the velocity at the suction point of fumes are enough for a correct extraction .

When possible the best and economicaly is to use VACUUM systems where the low cases of suction could be smaller - so near the source - . With this we can make a better capture and use low consuption of air and low energy consuption of energy .

If great hoods are needed we can always - after a good selection of the good profile of hood - and using a INVERTER adapt the rotation of fan to the needs . With that we assure good capture and savings , the possible ones for the application, without compromise the solution and good capture .

As said each of the two possibilities indicated are better , depending on concret problem .

The velocity is a result of the volumetric flow rate, which is in turn based upon the designed turnover rate. Reducing the the velocity from 0.75 m/s to 0.5 m/s will either entail reducing the volumetric flow rate (& hence, the turnover rate) by 1/3, or increasing the cross-sectional area of the hood by 50%. Reducing the turnover rate will have an adverse effect on safety, so it would be better to increase the size of the hood.

Thanks for quantifying my answer. If the professor has thrown in eddies and other issues, trust me, the answer is not reducing the speed, but a review of the hood design is definitely in order. As far as using vacuum as one writer observed, by default, that is what they are doing. The lift achieved is measured in inches water column, as you already know, measured in very low numbers considering the conversion of 13.54 inches of water column is 1 inch of mercury. It never ceases to amaze me the bull sh** the professors throw out there for our young want to be Engineers. They want $70 grand or better to start, and then can't even get out of the blocks to do anything.

Cost savings from reduced air flow is a function of the climate in your area. Building HVAC systems typically cool and reheat make-up air to reduce RH in hot/warm months, and they just heat make-up air in cooler, less humid, months, so whatever the cost of conditioning air is will reduce energy costs by 1/3. There are several energy-saving technologies just now, or recently, coming into the market that can provide additional energy savings.

There's lots of discussion here about safety when reducing air flow in hoods. Having dealt with just this issue for several years, I will say that without testing each hood for proper performance at, say, face velocities of 60 feet per minute, there is no blanket operating parameter that can be applied. Each installation of fume hoods will be unique, even if the labs are "exactly" the same. The difference in "reality" and "exactly" is that each fume hood of the same model in similar labs is probably connected to the same exhaust duct with different suction pressures. Even the locations of make-up air registers in the labs will have an effect on fume hood performance -- you just can't add up air flow numbers and know for sure what you have. Even walking in front of a fume hood can cause enough disturbance to cause fume spillage in some installations.

So-called "high-performance" fume hoods have been designed to address the problem of paying for conditioned air that is blown out of the buildings by designing them to operate well at 50-60% of the time-worn specification of 100 feet per minute face velocity, and the ones I have tested, and had tested by outside agencies, work well when installed properly in laboratories with good air flow characteristics.

Bottom line? You can do an economic analysis, but you won't know if it's anywhere close to "reality" until and unless you test it.

As far as fan costs are concerned, most will not have to be replaced. The pulleys can be changed to reduce air flow, and the motor will not use as much energy, and 1/3 reduction in air flow should be within the efficient range of the fan curve (if it isn't, then the fan was probably mis-sized in the first place). New fans range in price from a few hundred to a few thousand dollars. We can't know what to tell you in that regard without knowing what size fans you now have.

You have to understand how the existing fan system works. If you are talking about the unit conditioning the space the fume hood is in, it may be a mixed air system and it would simply involve adjusting the percentage of outside air.

Modern fume hoods are made with doors. They are made so that varying amounts of air are drawn through the unit depending on door position. There are also some good retrofit kits for fume hoods to convert them to variable flow. Retrofitting fume hoods is a common energy saving retrofit strategy. Payback may be quick if the hood is used a lot.

Some fume hoods have an integral make-up air fan with perhaps some minimal conditioning of the make-up air. If that is the case, there will probably be a volume damper in the duct somewhere.

If a 100% outdoor air fan is supplying air to the space, it may already have a variable flow capability. Usually labs have more than one fume hood, so the make-up air system has to be able to adapt to the number of hoods that are running. The control may be monitoring lab static air pressure relative to the adjacent corridor to keep it slightly negative.