Reduce HVAC energy consumption with FHP and VFD operations

Air-cooled chillers can take up a lot of room while expending massive amounts of energy. Integrating FHP and VFD operations is one way to cap energy consumption.

By Robert W. Gray May 12, 2014
I’m going to make a bold statement right up front by saying: All air-cooled chillers are over-sized. Now let me explain why I take this position. Among other considerations, Air –cooled chillers are typically sized with three basic parameters in mind:

  • Building load
  • Maximum outside air temperature (design temp)
  • Structural characteristics

Regarding the building load, when an engineer calculates the tonnage that is required to cool the building, they take into consideration many building characteristics such as insulation, lighting, windows, internal equipment heat load, and the number of occupants, just to mention a few.  

Most chillers are sized for a “worst case” scenario where the building is 100% occupied and the chiller would cover all the “known” internal loads, plus the warmest outside air temperatures in that region (typically 95 to 100 degrees). The purpose of this article is not to go through the specific calculations to determine the required chiller tonnage, but just make the point regarding the contributing factors.  

For commercial buildings, the internal load is constant and calculated using a simple means of BTUs/hour as produced by equipment, lighting, and people. Of course when calculating the actual tonnage requirement for the entire building, all the other factors come into play regarding the building characteristics as well as weather. The load that remains unaffected by weather or external condition is referred to as the “base load” of the building. It’s safe to say, this base load occurs monthly.    

The graph in Figure 1 shows the base load and how the heating and air conditioning adds to the total energy consumption of that building:

HVAC can account for as much as 50% of the total energy consumption of commercial buildings. As we all know, the outside air temperature affects the level of comfort in the building and therefore the operation of the HVAC equipment. If your building has working outside air economizers on central air handling systems, you have the ability to supplement the mechanical cooling with “free” cooling when the outside air temperature is within an acceptable range.  

This doesn’t answer the question as to why I stated that all air cooled chillers are oversized.What we have learned over the years regarding cooling equipment is that only 5% of the time does the chiller actually have to run at 100% capacity. That occurs when the outside air “design” temperature is met or exceeded and the building is fully loaded internally. But again, this is only 5% of the total run-time for the air-cooled chiller. So, 95% of the time the air-cooled chiller is running at something less than full load. In other words, it is running in a part load condition. While the chiller is running partially loaded, the chiller’s compressors and condenser are physically oversized for the delivered tonnage required for the building.  

OEMs have produced more efficient air-cooled chillers, and they in turn have gotten more efficient, while making advancements in controlling the part load operation. Staging in a multiple compressor bank has helped to deliver better part load efficiency. Improved designs and materials used in condenser coils have improved the BTU transfer which helps to keep the physical size down for the condenser section. But, what can we do to better our part load performance of our existing chiller?

We can achieve up to 30% reduced energy consumption of our installed air-cooled chillers simply by applying floating high (head) pressure (FHP) control with variable frequency drive (VFD) condenser fan operation. FHP has been around for many years, but until recently, it has not been effectively applied to AC equipment. FHP takes advantage of the excessive capacity of the condenser section at part load and cooler outside air temperatures to dramatically decrease the delivered KW/ton of the chiller (See Figure 2).  

Some of the HVAC/R equipment OEMs are starting to provide a factory option for some form of high pressure control, however, very few are utilizing true FHP with VFD condenser fan control.

From the factory, most of the OEMs air-cooled chillers are designed to maintain a fixed head pressure, even if they use a VFD for condenser fan operation. They typically use the VFD to limit the speed of the condenser fans in an effort to reduce the BTUs ejected to the outside air through the condenser coil. This operation helps to keep the compressor on line during colder outside air temperatures by stabilizing the head pressure.  

These OEMs have missed the most beneficial piece of FHP control, which is compressor energy savings. FHP with VFD condenser fan operation reduces the head pressure by utilizing the excessive capacity of the condenser to float the condensing temperature in relation to the outside air dry bulb temperature. This operation reduces the mechanical requirements of the compressor, better known in the industry as the “lift”. The lift is the difference between the inlet low pressure or suction side of the compressor and the outlet or high pressure side of the compressor.     By reducing the lift, you reduce the amount of work the compressor has to perform, which substantially reduces the KWH consumption of the compressors up to 30%.

So before you decide to replace your 5- to 15-year-old chiller for the sake of installing a more efficient unit, consider applying proper control of the condenser fans through true floating high pressure control with VFDs. The retrofit is typically less than 10% of the cost of replacement and yields an ROI of fewer than 3 years.  

As a side benefit, FHP allows the compressors to operate within less stringent mechanical requirements by reducing the lift. The FHP also helps to avoid compressor short cycling during colder outside air conditions. This extends the useful life of your existing air-cooled chiller.  

So, bottom line; as a result of your air-cooled chiller being oversized 95% of the time, you can achieve 30% energy savings, extend the useful life of the chiller, avoid the high dollar capital expense of replacement and achieve a fast ROI. Now that’s living in excess!

Robert Gray is an HVACR business segment manager for Schneider Electric. Edited by Jessica DuBois-Maahs, associate content manager, CFE Media,