HPAC Magazine

Current technologies are available to modulate system or compressor capacity

August 19, 2016   By Dave Demma

It is morning again. You get ready for the day and make your way out to the garage where your car is waiting for the morning’s commute to work. You start the engine, make sure your radio is tuned to your favorite morning show and proceed to carefully back out of the driveway. Then you put the pedal to the metal and take off at full speed to the first stop sign. At the last moment you slam on the breaks, coming to a screeching halt just before the pedestrian walk. After making sure it is safe to proceed, you take off once again at full speed. While this style of driving might get one to work, it is hardly an efficient way to drive. The accelerator was designed for a specific purpose–to match the engine’s speed with the driving requirements at any given moment.
Why has it taken our industry so long to see the logic in this approach?

Compressors are selected for an application’s design condition, typically the worst case scenario in the middle of the summer, thereby ensuring that there is sufficient capacity to provide adequate comfort cooling (or product temperature in refrigeration applications) under all circumstances. One of the factors influencing compressor capacity is the condensing temperature, and this is directly affected by ambient temperature. In fact, as the condensing temperature increases, the compressor capacity decreases. For example, consider a compressor with a capacity of 50500 Btu at a -20F SST and a 105SCT. During the cooler months where the condensing temperature can be maintained at 70F, the compressor capacity increases to 66500 Btu.
Now think about that for a moment. The system load is typically at its highest during the peak ambient temperatures in the summer. This is the condition at which the equipment is selected to ensure that there is sufficient equipment capacity during the most miserable of conditions. As the load decreases with a drop in ambient temperature, the compressor capacity increases. Going back to the driving analogy, even though the speed limit has decreased to 25 m.p.h, the engine is still operating at full speed. It is time to implement an accelerator to limit compressor capacity.
There have been several methods used in an attempt to match compressor capacity with the actual load at any given point in time. The following is a review of the technologies currently being employed.

INVERTER DRIVEN COMPRESSORS
Mini-split/multi-split/VRF equipment manufacturers have been providing inverter driven compressors with their equipment for some time now. In simple terms, think of this as a compressor (rotary or scroll) with a built in VFD. So, functionally it is not too much different than a compressor applied with an aftermarket VFD but there is an added benefit. The manufacturer spends the necessary time in its research and development lab to design the inverter components that will vary the frequency to the absolute best specification for optimal performance of its compressor.

COMPRESSOR UNLOADING
Compressor unloading allows one or more cylinders in a compressor to temporarily become inactive for a portion of time to reduce compressor capacity. For example, a four-cylinder compressor could operate as a two- or three-cylinder compressor during times of reduced load. The issue with this technology is that it does not offer the ability to precisely match the compressor capacity with the load. If the actual load on a 10-ton system drops to 6.5 tons, unloading one of the four cylinders results in too much capacity, while unloading two of the cylinders results in too little capacity. To put it more plainly, if the speed limit is reduced to 40 m.p.h., and your car only offers the ability to drive at 33 m.p.h. or 47 m.p.h., how does this really serve your needs?

DISCHARGE BYPASS
While discharge bypass is not a compressor technology, it is a means of matching the system capacity with the load demand. A very stable option in maintaining a constant minimum suction pressure (minimum discharge air temperature or chiller fluid temperature) comes from bypassing discharge vapour to the low-pressure side of the system, falsely loading the system. The extra loading on the compressor will raise the evaporator pressure along with the evaporator refrigerant saturation temperature.
When used properly in air-conditioning applications this will prevent the saturation temperature from dropping to the point where frost buildup on the evaporator can occur. It can also be applied to prevent the compressor from operating below its design suction pressure, ensuring a reasonable compression ratio. If the discharge gas is bypassed to the evaporator inlet at the design evaporator temperature, proper refrigerant velocities for good oil return will be maintained as well.
Look at this method of matching system capacity with the load condition as you would driving the car with the engine at maximum RPM, while simultaneously applying the brakes to remain within the required speed limit.
So, this method does not offer reduced energy consumption for the lower load periods, but it does very nicely match the system capacity with the actual load. Efficient operation aside, using an electric step motor discharge bypass valve that responds to discharge air temperature will provide control within 1/2 degree F.

DIGITAL SCROLL TECHNOLOGY
Although scroll compressors have been in use for some time, it was not until 2000 that digital scroll technology was available for commercial HVAC applications.
The digital scroll differs in design from the standard scroll in that it has the ability to allow the top scroll plate to lift approximately one millimetre from its normal position. This is enough of a distance to completely eliminate the scroll plate’s ability to compress vapour. The result is 100 per cent unloading capability.
This is where the digital aspect comes in. Through the abilities of electronic controllers, the compressor can now operate in segments of cycle time, which is comprised of the sum of loaded state cycle time and unloaded state cycle time. Plainly speaking, the compressor operation can be broken down into cycles of 20 seconds, with some portion of the 20 seconds of operating completely loaded, and some portion of the 20 seconds operating completely unloaded.
The capacity of the digital scroll can vary between 10 per cent and 100 per cent. The compressor capacity can be calculated from a ratio of percentage of loaded state time to the total cycle time. For example, if the loaded state time were 10 seconds and the total cycle time were 20 seconds, the compressor capacity would be 10/20, or 50 per cent of total capacity.
Given the wide range of capacities the digital discus can operate under, it can offer a very precise solution for matching the compressor capacity with the system load. Not only can design parameters be maintained more closely but there is the added benefit of an opportunity for energy savings.

DIGITAL DISCUS
The same technology that allows a scroll compressor to operate loaded anywhere from two seconds to 20 seconds for each 20-second cycle time has been applied to some discus compressors. The unloading is accomplished by blocking the suction port of the given cylinder to be unloaded. This will allow the bank of digitally controlled cylinders to operate anywhere from 10 per cent to 100 per cent capacity. If it is a three-cylinder compressor, the three cylinders will constitute a single bank of cylinders, and will load or unload simultaneously. The four-cylinder and six-cylinder compressors will have one bank of cylinders that remain 100 per cent loaded at all times. As such, the four-cylinder will unload down to 50 per cent and the six-cylinder down to 33 per cent.
Unlike the scroll compressor, which is completely inaccessible in terms of removing/replacing parts, the semi-hermetic compressor heads and valve plates are removable. With a valve plate/head replacement, the addition of the solenoid coil, discharge temperature sensor and controller, a discus compressor can be converted to a digital discus. If capacity modulation is required, this allows one to upgrade a standard discus compressor to a digital discus compressor easier and less expensively.

VARIABLE FREQUENCY DRIVES
Another option for providing compressor capacity modulation is to apply a variable frequency drive (VFD) to a standard compressor. A VFD works by converting the input current to direct current, and, from this, generating a simulated AC signal at varying frequencies. Varying the frequency will vary the motor speed, thus varying the compressor capacity.
While VFDs can provide good compressor capacity modulation, there are several things that must be taken into consideration. Most VFDs are capable of generating frequencies from 2.5 Hz to over 300 Hz. This is well outside the normal range for the typical compressor motor, so the frequency upper/lower limits must be kept within the compressor manufacturer’s recommendations.
In addition, the frequency range, which ultimately determines the motor speed, will be dependent on the compressor’s ability to provide proper lubrication at the reduced speed. Again, the VFD must be setup based on the compressor manufacturer’s specifications.
An additional advantage of a compressor outfitted with a VFD is that the startup frequency results in a low motor speed/torque, which reduces the normal high startup current. Not only does this reduce electrical consumption, the low torque start also relieves stress on the motor/compressor.
The bottom line is there are many options available to better match equipment capacity with load to provide maximum comfort in air conditioning applications or product integrity in refrigeration applications, without having the pedal to the metal whenever the equipment is in operation.

Dave Demma holds a degree in refrigeration engineering and worked as a journeyman refrigeration technician before moving into the manufacturing sector where he regularly trains contractor and engineering groups. He can be reached at ddemma@uri.com.

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