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Compressor and receiver Sizing Question

Compressor and receiver Sizing Question

(OP)
Hello All,

I am quite new to this sizing of air compressors and wondering if anyone could advice me on my understanding. Thank you!

1. Lets say the assumptions for the Duty factor and cycles are correct. Why am i getting a Air receiver volume of 30730L, while my Air used in idle time is 37910 Litres. It does not make sense as i will have exhausted my air receiver?

Air receiver volume(L) = Air used in idle time(L)/ (Pout-Pin)KPa X (101.325)Kpa
Its the same equation except i am using in L.

2. How to determine which duty factors are suitable? The air demand is used for continous flow for a Process + around 10 control valves..

Calculation below.

Air demand 566.9325 Nm3/hr
157.48125 l/s

Number of Duty compressor 1
Standby 1
Compressor Duty Factor 60%
Capacity req per compressor 262.46875
Capacity selected 263

Maximum compressor starts per hour 6
Cut- In pressure 700 kPa A
Cut- Out pressure 825 kPa A
Compressor Cycle time 10 mins
Air used in cycle time 94488.75 Litres
Compressor run time 359.2728137 sec
Compressor Idle time 240.7271863 sec
Air used in idle time 37910.01821 Litres
No of Air receivers duty 1 No.
Air Receiver Volume required 30729.86076 L

RE: Compressor and receiver Sizing Question

What are the units associated with the 263 capacity? I assume it's L/sec at normal conditions.

During the draw down time (240.7 sec), the air used is 157.4 L/sec * 240.7 sec = 37886 L

This is supplied by the decrease in pressure in the receiver from 825 kPa to 700 kPa or a dP = 125 kPa. That volume in the receiver provides about 1.25x volume of air at Normal conditions (to be exact, you need to correct for normal temperature and operating temperature but I'm going to ignore than, you can work it through in more detail).

37886 L of air used / 1.25 = 30,300 L, pretty close to your number.

Think about what is happening here. The air consumed by your process comes from the decrease in pressure in the receiver.

So in your case, the compressor runs for 360 seconds and compresses the air in the receiver from 700 kPa to 825 kPa. During this time, the 263 L/sec compressor capacity is meeting your process requirements (157 L/sec) and compressing the air in the received. When the compressor shuts off, the air used for the next 240 sec is supplied by the air in the receiver dropping from 825 kPa to 700 kPa when the compressor starts up again.

Essentially, a change in 101.325 kPa = volume of air in the receiver consumed as air (neglecting as said before the temperature effect).

Rounding off the numbers. As the pressure drops from 825 kPa to 725 kPa, 30300 L of air is available to be used. If the pressure dropped from 725 kPa to 625 kPa, another 30300 kPa would be released. This would continue until you reach the minimum pressure or atmospheric pressure, whichever is lower.

RE: Compressor and receiver Sizing Question

(OP)
Hello TD2K thanks alot for your detailed explanation!

Regarding Compressor Duty Factor, what is the basis of choosing such value? example.. 60% = "60% of time running and 40% of the time idling"

RE: Compressor and receiver Sizing Question

Not to speak for TD2K, I use service factors in that range as well. You have to ask yourself the question "what is the longest that a failed air compressor could be out of service (i.e. how long to fix/replace, how often would it be expected to happen?)" If you ever get into a situation where a broken air compressor shuts your plant down you will have a very unpleasant discussion with your plant manager. To avoid that, it is common to design air systems with receivers big enough to last long enough to fix a small problem, install 100% redundancy, and/or keep max load under 60% of max hp. These three things are all kind of expensive on first blush, but I looked at one facility that a shut down cost $10,000/hour, and saving a few hundred dollars on air compression equipment a few years ago doesn't carry much weight when a 24 hour/day plant is down for 2 weeks waiting for a replacement air compressor.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Compressor and receiver Sizing Question

”Why am i getting a Air receiver volume of 30730L?”
It sounds like whomever is sizing the tank is doing so to give you a cycle time of ten minutes, or 6 starts per hour. This is a common target to run the compressor in on/off mode, saving unloaded run time (= saved energy = saved money). But not knowing the specifics, I’m guessing at that. I’ll discuss some ways you could get away with a smaller tank and still achieve (what I’m guessing is) your targets.

”How to determine which duty factors are suitable?
Your duty factor (60%) is determined by your demand (157 L/s) divide by your capacity (262.5…L/s?) (157/262.5=0.6).

Whether or not a duty factor of 60% is “suitable” depends on the type of compressor. Most reciprocating do not want to operate continuously and so duty factors of under 80% or so are needed. Most load/unload screw and centrifugal compressors can operate continuously and, due to reducing unloaded run time (in the case of screw compressors) or by-pass flow (in the case of centrifugal compressors), are more best suited to run as close to 100% duty factor as much as possible. However, given the difference between average and peak loads in most plants, this is not really feasible which is why multiple compressors or a VSD or, as in your case, running on/off can provide a lot of benefits. If you have a VSD, which is unlikely if your concerned with cycle times, then you want the load and therefore duty factor to vary somewhere between 50% and 80%. Any higher and you lose the advantage of the VSD (and due to inverter losses can actually be less efficient than a fixed speed). Any lower and you begin running near min-speed, where the VSD is much less efficient (and you can run into issues of the compressor not getting hot enough to evaporate water in the oil). With any compressor, leaving some ability to handle future changes in load is desirable.

As discussed above, the system appears to be sized to give you 6 starts per hour. Again, my guess is this is because you have a screw compressor and would like to run the unit on/off rather than load/unload. In this case, there are trade-offs between duty factor and receiver size. The main relationship between duty factor and starts per hour is a negative parabola where the number of starts per hour are 0 at 0% (the compressor never needs to turn on) and 100% duty factor (the compressor never can turn off/unload) and max at 50% duty factor. The shape of the parabola is dependent on pressure band (dP between load and unload points), storage size, compressor capacity and demand. Assuming the demand is unchangeable, you can reduced the required size of the compressor by either:
  • Increasing the pressure band. Note that operating at higher pressure increasing the power consuming by 1% for every 2 psi, roughly. So expanding the pressure band by reducing the load pressure set point is obviously ideal, if practically possible.
  • By selecting a compressor such that your duty factor shifts away from 50%. >50% is preferable as any compressor that gives you a duty factor under 50% is likely oversized. Just be careful it can still handle your peak loads plus future growth.
  • Installing multiple, smaller compressors (one trim unit and at least one base load that always runs), instead of one large compressor. While two smaller compressors will usually cost more than one big one, the control of the system can be much better and energy costs less.
Circling back to the original question, is 60% “suitable”?, the answer is probably yes. A higher load factor (i.e. smaller unit) would allow you to reduce the size of your receiver but you would need to make sure it could handle your peak load (as zdas04 indicates).

Note that I’m discussing the receiver size as a means to reduce the number of starts per hour to 6. Zdas04 appears to have approached your question from a different angle. He is talking about the receiver as a means of riding out compressor failures (at least temporarily). This is certainly another service the receiver can provide and this may factor into the size above and beyond the 6 starts per hour requirement.

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