airisagas.com

Control Storage III

Managing Compressor Cycling

The most energy efficient way to operate a fixed speed compressor in part load can be “load-no load” (provided there is sufficient storage). Without properly sized storage rapid cycling can be a problem in this mode. Typical airend mean time to failure should be 6-7 years or 52560-61320 hours of service. In order to achieve this in “load-no load” operation, we should attempt to limit the load cycles to 2 million cycles for this period of time. There are 585600 minutes in a year. We would want our total cycle time to be not less than 1.5 minutes or more in the realm of 3.0 minute. With this type of load-unload time per cycle, we can take advantage of the effectiveness of this mode and extend the mean time to failure simultaneously. Using these values, let’s see what we need to do to get our desired cycles.

Remember that your controls management requires you to only support the difference between supply and demand. This will be the positive and negative rates of change. Let’s say that your base load requirements are covered and the balance of the requirement is 200 scfm of trim with a 60 hp compressor available with a displacement capacity of 300 scfm. 50% cycling would be the hardest cycling. In this case let’s assume that we have a 10 psid dead band with the load pressure set at 100 psig and the unload pressure at 110 psig. The existing control storage is 10 scf/psi at 14.5 psia atmospheric pressure or 10 scf/psi X 14.5 psia X 7.48 gal/scf = 1084.6 gallons. This would include the supply piping and the tank or tanks.

With this scenario, the cycling would look like this:

Supply = 300 scfm – Demand 200 scfm = 100 scfm

10 psid dead band X 10 scf/psig = 100 scf of useful storage

Load time = 1 minute at 200 scfm + 100 scfm excess capacity from 100-110 psig.

Unload time = .5 minutes @ 100 scf of control storage capacity from 110-100 psig.

Total cycle time = 1.5 minutes

Assuming the same situation with 20 scf/psi of control storage:

Supply = 300 scfm – Demand 200 scfm = 100 scfm

20 psid dead band X 10 scf/psig = 200 scf of useful storage

Load time = 2 minutes at 200 scfm + 100 scfm excess capacity from 100-110 psig.

Unload time = 1 minute @ 200 scf of control storage capacity from 110-100 psig.

Total cycle time = 3 minutes

Optimizing the system’s efficiency always requires evaluating all aspects of the operation including extending the mean time to failure. You don’t have to sacrifice the mechanical wellness to improve efficiency.

 Control Storage Applications II

Event management is more appropriately coped with at the point of use. If you are going to support it at the supply, you need to measure it. This can be done with a pressure logger. You would identify this transient event based on the rate of pressure decay, time of the event, and the existing storage capacitance. If the pressure decay is 8 psid and the capacitance is 30 scf/psi, the event is 240 scf. One of the objectives of control storage is to manage the largest event in the system without turning on an additional compressor. If you adjust the next compressor at a load pressure, which is 5 psid below the previous load pressure, then you will need the event to be managed within 4.5 psid assuming that the event starts at the load pressure of the previous compressor, which would be the worst case scenario. The control storage required would be:

event scf X (atmospheric psia / allowable delta p)  X 7.48 gal/scf = required control storage

240 scf X (14.3 psia /4.5 psid) X 7.48 gal/scf = 5704.7 gallons

You will then need to subtract the existing supply storage from the full requirement. If 15 scf/psi of the existing storage is on the supply system, calculate the gallons equal to 15 scf/psi. 15 scf / psi X 14.3 psia X 7.48 gal/scf = 1604.5 gallons. 5704.7 gallons total storage for control on the supply side – 1604.5 gallons of existing supply storage = 4100.2 gallons new or additional storage.

To determine the value of the reduction, multiply the event volume time 60 seconds divided by the actual time of the event in seconds. Assuming that the supply system is being supported by the backup compressor or compressors, you will need to determine the rate of flow of the event because the supply does not understand the volume of the event, only the rate of flow it sees relative to pressure decay. If the event takes 30 seconds, the supply will see this as:

240 scf X 60/30 = 480 scfm for 30 seconds

The energy required would be for 480 scfm or the part load energy of the next compressor(s) that serve the event in real time. Please keep in mind that you may be modulating the supply compressors, which will support the event with less pressure decay by using on board power.

In the previous post called Auto Dual Controls I, we discussed protecting against a base load compressor failure. If the storage for this is larger than the event requires, defer to the larger storage that will handle both separately.

Auto Dual Controls I

“Backing Up a Base Failure”

Most equipment manufactures the compressors with auto dual controls not only to save their clients money, but also to be able to handle the failure of a base machine or a transient demand event. Unfortunately these good intentions will not get auto dual to function correctly on their own. You have to plan the management of these events, which requires more information regarding machine adjustments, properly sized control storage, and knowledge of the event.

We will only consider a base compressor failure. The idea is simple. We don’t want to run an extra compressor 8760 hours a year, so we don’t lose the plant on a unit failure. In order to make the back up compressor available for a cold start, we must start the unit with auto dual engaged and let it time out. This will engage the auto dual controls. If an operator shuts off the compressor manually, the auto dual function will not be available. In many compressor controls, the auto dual timing is adjustable from a minimum (usually 10 minutes) up to 30 minutes. Sometimes this is fixed. The purpose of this control is to allow the unit to operate unloaded long enough that it can immediately start after it shuts off without violating the minimum starts per hour. On older compressors, this is accomplished with anti-recycle timers. The auto dual timer doesn’t usually engage until the compressor unloads. Some compressor controls also provide a prevent/start or cool down timer. This is used for cool down after the motor is shut off. This function engages sometimes after auto dual and sometimes after a manual shut off. It can also occur when either function is engaged. This is typically engaged from 2-30 minutes. If you attempt a manual or automatic restart before the timer has timed out, the unit will trip out and require a fault reset. Often the timer will reset on the fault stop. You must have more than one back up compressors or a very predictable flat load to use this function. Next you need to know the cold start permissive time required on the back up compressors to bring it from off in auto dual to full load. This can either be a default fixed time or an adjustable time delay value typically measured in seconds. Depending on the manufacturer, the size, and the type of compressor, there may be several timer functions involved to acknowledge or adjust. The time could be as fast as 6 seconds for a smaller, older compressor with electro-mechanical controls. More recent compressors with microprocessor controls are more likely to have more complex protective functions. This involves time. Watercooled compressors and non-lubricated compressors typically have more complex and time consuming cold start permissives. In some cases there may be as many as 4-12 adjustable or fixed time functions requiring between 15-135 seconds required before a full start. With larger two stage compressors, the lugging forces will likely require more time for a start than a single stage compressor. If a timer is completed before the sump pressure reaches a preset minimum value, the unit may trip out the start attempt. You must also consider the starter wind time. This could be from 2-3 seconds for an across the line full voltage start or up to 30 seconds for an adjustable start reduced voltage or 3 step starter. Synchronous motors with exciters will require a great deal more winding time. You must determine in advanced what the permissive starts, auto dual timing will be, and if you intent to have a prevent start timer engaged before you determine the next action.

If the largest base load compressor is a 1500 scfm unit, this represents 25 scf/second. You must multiply the total time required in seconds to start the back up compressor from either a cold or hot start to full load to determine the volume removed from the system while you wait. If this is 18 seconds, the air lost will be 18 seconds times 25 scf or 450 scf. You must now determine the amount of control storage required to limit how much the pressure will drop before the back up compressor will match the lost capacity.

Remember that the start permissive begins from the load pressure of the back up compressor.

The following formula will illustrate how this is done to determine the control storage capacity in gallons:

Lost Volume in Scf X (Atmospheric Psia /Allowable Psid) X Gallons per Cubic Feet = Gallons Required

Example:

450 scf X (14.3 psia/ 7 psid) X 7.48 gal/cf = 6876.3 gallon

If you already have existing control storage, you must subtract this from the total to determine the amount required to handle an auto dual start on the largest base compressor failure without production seeing the event. Don’t forget the timer adjustments and functions you wish to use.

Write them down and provide them to the start up service personnel so they are also working in this same direction. The right control storage without the right controls adjustment will not provide this precise result.