Assessing Steam Consumption with an Alarm Clock Step 4 – Converting Cycles to Volume and a Look at Vacuum Condensate Pumps

Those of you who have been following this string of blog posts about assessing steam load profiles by monitoring condensate pump cycles probably can see where things are heading at this point. Since we’ve figured out a way to manipulate our logger data to a point where we can count pump cycles, and recognizing the fact that the condensate simply is condensed steam, it would seem that all we would need to do is multiply the number of cycles by the volume discharged per cycle and “bingo” we have our steam consumption.

The volume discharged per cycle seems pretty easy; just multiply the receiver length times it’s width times the level change when the pumps run, right? Well, like most things in HVAC, the answer is “it depends”.

Lets take a look at a typical condensate pump and receiver.

condensate-pump.jpg.

Just to keep the record straight, that is not a picture of the condensate pump that the data we have been looking at came from. There are two reasons for that. One is that I forgot to take a picture of the condensate pump that the data came from. The other is that the picture is of a vacuum type condensate pump.

Vacuum condensate pumps are a special breed of condensate pump designed to serve vacuum return condensate systems and variable vacuum steam systems. Vacuum return condensate systems are designed to operate the return below atmospheric pressure. This keeps air out of the steam and condensate systems, which has a number of advantages including the promotion of steam distribution.

Variable vacuum systems actually maintain sub-atmospheric pressures in both the supply and return systems as a method of temperature control.   You can find out more about these technologies in the ASHRAE Handbook of Equipment and Systems, Chapter 10 Steam Systems and on the ITT portion of the Bell and Gossett web site.

It should be noted that neither technology is intended to lift condensate on its way back to the receiver. This is a common misconception but the reality is that a well designed condensate system needs to flow by gravity back to the collection points with out a rise in the pipe. If the return pipe elevation does increase, then the return system will be flooded to the level of the highest portion of the return piping, which can cause a number of problems.  More on that some other time.

The reason I wanted to highlight vacuum condensate pumps in the context of our current topic is because the inside of the receiver is more complicated that its rectangular shape belies, as can be seen from this cut-away taken from the Bell and Gossett catalog data for the unit in the picture above.

cut-away.jpg

As you can see, the inside of a vacuum condensate pump tank can have multiple variable shaped compartments. (I won’t go into the details of why but if you’re curious, the explanation is contained in the B&G catalog data for the unit). That means that if you are applying this technique to such a pump, you need to do more than length, times height times width to come up with the volume of condensate discharged.

In addition, for the equipment in the illustration and most similar equipment, at least one pump must operate round the clock because the vacuum is created by recirculating condensate through a venturi. Thus monitoring pump amps alone will not be enough to tell you when condensate is discharged. You may have to look for a change in amperage associated with pumping condensate out vs. recirculating condensate or monitor the solenoid valve that trips the discharge cycle.

All of that said, the pump we have been looking at data from was in fact a standard condensate pump. Thus, the volume discharged per cycle is a simple calculation as illustrated below. Note that the calculation was based on the external dimensions of the receiver, which assumes that the wall thickness is insignificant relative to the volume contained. The level change was based on monitoring the pump operation for several cycles while observing the gauge glass. If there isn’t a gauge glass, you may need to insert a tape measure or stick into the tank through the vent to assess the level change.

Condensate receiver height = 38.000 inches

Condensate receiver width = 38.000 inches

Condensate receiver level change per cycle = 3.625 inches

Volume pumped per cycle = length x width x level change = 5,235 cubic inches

Volume pumped per cycle = 3.03 cubic feet

Gallons per cubic foot = 7.48

Gallons of condensate per cycle = cubic feet x gallons per cubic foot = 22.66 gallons per cycle

Pounds per cubic foot = 62.4

Pounds of condensate per cycle = gallons per cycle x pounds per cubic foot = 189 pounds per cycle

So, the bottom line at this point is that every time the pump cycles, it discharged 189 pounds of condensate. In turn that means that in the time between the previous pump cycle and the current one, 189 pounds of steam made their way back to the receiver after condensing somewhere out in the system.

If the leaks are minor, and there is little or no use of steam in a process that does not return condensate, then we can sum up the pump cycles in a given time frame, say a day, multiply them by 189 pounds of steam per cycle and “presto” we have our steam consumption for the day.

Next, I’ll look at how you perform those steps in the spreadsheet we have been using. I’ll also look at how you can draw a picture of the load profile in addition to assessing consumption.

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David Sellers
Senior Engineer – Facility Dynamics Engineering
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