Residential furnace's fuel nozzle flow rate....how accurate?

[ben686]

I need to determine the oil used for two furnaces (and one water heater) connected to a 2000 gallon tank, shared with 4 other furnaces. Although commercial off-the-shelf oil flow meters (eg, mechanical/electro-mechanical) could be used, I was looking for a cheaper solution (especially considering the cost of installation for two in-line fuel flow meters with an accuracy of 1%). The measurement term would be one heating system of roughly 5-6 months, with a continuous 3rd measurement of a hot water heater.

I've been using an hour meter / runtime counter so far to count the number of minutes and hours, etc for a particular furnace and multiplying the count by the nozzle etched 'gallon per hour' rating.

My concern is how accurate the system would be, and what the cause/solution would be to those inaccuracies. I would belive that an accuracy of within 10% should be attainable due to defecencies within the design. Would any in-accuracies be able to be 'averaged' or designed out of the system.

Not sure what extent of info is necessary, but I will try to come up a someone accurate model off the top of my head (might have some oversights, though!):

The entire system consists of a 2000g oil tank on the eastern side of the building, enclosed within a vinyl sided wooden framed building. Copper piping from the tank gradually decends ~5' to a two-stage oil circulating pump and oil filter assembly within 20' of the tank. Pressure is (roughly) 4psi within this loop, with unused oil returning to the tank (and I would assume air or a reduced flow depending on oil demand from the system). Piping extends vertically (~4') and laterally before decending to each of the furnaces. OSV's (oil safety valves) are installed in the vertical section of pipe within 3' of nozzle. The size of the lateral system setup is fairly small, entire system (including furnaces) is roughly 300SF. Design uses 5 oil hot air furnaces and 1 oil hot water heater.

Two of the furnaces are faily old (guessing 1960-1970's) with the following nozzle sizes:
1.75, .85, .65, 1.75, 1.75. From what I recall, the larger nozzles may be of a slightly different type, with a 80deg spray...not sure of all of the specifics.

-Thanks

 

[quark]

An accuracy of 10%???(sigh...). Then what is the need of measuring flowrate?

The figures given on the nozzle are maximum flowrates through the nozzles at the specified pressure and they increase with increased pressure and viceversa. Further, the orifice becomes wider due to aging(mainly erosion)and flowrate varies and it may be significant.

How are you accounting for returned oil incase of modulated burners?

PD Meters(oval and gear etc.) are not too costly and can work well within 2% error. Have one in supply and one in return line.

Regards,

 

[jmw]

Look at the FuelCom solution from FTI of Phoenix for flow and return metering.

JMW

http://www.ViscoAnalyser.com

 

[ben686]

I'm looking to meter each furnace so I can bill the tenant for the appropriate amount of oil usage. Therefore, I can't simply do a bulk measurement of usage from the tank...I have to put a meter on the six diffent units.

Previously, the only type of measuring that I was able to do was pro-rata based on the leased SF of the building, with which the tenants tenants are agreeable. My motivation is to bill the parties responsible, and gather as much information as possible about the system. Needless to say, the current implementation has been effective, I can evaluate a years worth of usage to the second of all of the burners simultaneously to track performance and possible maintence issues; I can give the tenant a hard copy of the bill showing his exact usage including start and stop times, duration, frequency, etc, and not simply a Qty*price invoice.

Although the installed system may not be 100%, I find it hard to believe that it's "probably substantially inaccurate and should not be used." Achieving 100% accuracy would probably require turning the furnace off!

You mention pressure at the nozzle...I would assume that would be the "head" pressure off the furnace's oil pump, rather than the supplying line's pressure. Line pressure of the circulating loop is fixed at 4psi. What factors would cause the pressure to fluctuate significantly?

Finally, how much of an aging/erosion issue would it be if the nozzle is changed every year (600-1200 gallons) or six months (300-600gallons)?

 

[Timelord]

ben686,

Are all the furnaces the same? If they are the simplest solution may be to meter the water heater usage, meter the tank and monitor the on time of the burners in the furnaces. The usage could be allocated based on time after the water heater usage is subtracted from the tank delta. One flow meter for the water heater, four hourmeters on the burners and a reasonably accurate level indicator on the tank is all it would require.
Assuming the furnaces are identical.

Timelord

 

[ben686]

I checked around about some of the flow meters...the tech's reply so far has been that the circulating line pressure should be as high as 20psi or more to be able to operate the components within most of the the mechanical metering devices on the market.

 

[quark]

Though I can't quantify the changes in flowrate with respect to time(and due to erosion), you should better manually check the flowrates by filling a measuring flask. This may be difficult with compact burners but you should find a way out. Though we installed meters for our boilers, I do check the flow manually once in a week.

Regards,