By Bob Gray
From the January/February 2015 issue
For reducing energy consumption of the HVAC systems in the typical building, can we say that it is a “no brainer” to apply variable speed drives (VSDs) on everything from pumps to fans? Traditionally, these have been two of the major targets for implementing Energy Conservation Measures (ECM) within facilities. VSDs provide a means to operate a fan or pump much more energy efficiently. It is commonly stated that the only way to achieve additional energy savings over a VSD, is to turn the pump or fan off. But there is more facility professionals can do in this regard.
Over the past decade, the HVAC market focus for energy conservation has been on quick and easy applications of VSDs applied to the HVAC pumps and fans within a building. And substantial energy has been saved through the application of VSDs.
Moving forward, there is a need to expand on what has been learned over the years—and take control to the next level. This can be done by looking deep within the control strategy and algorithms of the cooling equipment itself. By implementing ways to enhance the internal operational characteristics of this equipment—and then operating the VSDs applied to the pump and fan motors to complement that operation, the control strategy can be taken to the next level.
Example 1: Air-cooled chiller supplying chilled water to a commercial facility. In more recent years, HVAC OEMs have applied a VSD to the condenser fans in an effort to help save energy and reduce short cycling of the compressors during lower outside air temperature conditions. Using the VSD in this case, users can isolate a single circuit or stage of the compressor and better maintain the fixed head pressure to avoid short cycling. This operation keeps the compressor online to provide mechanical cooling to appease the load.
Why not take the application of the VSD to the next level by controlling all of the condenser fans of both, or all three, of the refrigerant circuits—and incorporate floating high (head) pressure control as well? Floating high pressure control is the allowance of the high side refrigerant pressure to vary up and down along with outdoor air temperature in air-cooled or water-cooled chiller systems.
By taking advantage of less than design outside air temperatures, an operator can reduce the condensing temperature within the condenser, which reduces the lift on the compressors, reducing the energy consumption. The application of the VSDs is taken to the next level by creating a better operating environment for the compressors. This move of adding the floating high pressure control to the mix saves up to 30% of the total energy consumption of the chiller. Users can also apply VSDs to their chilled water pump motor, using the same microprocessor/controller used for the condenser fan control to monitor the delta-T of the chilled water loop. The control algorithm resets the chilled water flow, based on the delta-T of supply and return temperatures of the chilled water loop. Note: Users need to ensure they have met the OEM’s minimum chilled water flow requirements through the evaporator of the chiller.
To apply effective floating high pressure control, operators would monitor via the microprocessor/controller the outside air temperature, plus the high and low pressure values of each refrigerant circuit of the air-cooled chiller.
Example 2: Water-cooled chiller supplying chilled water to a commercial facility. As in Example 1, operators can apply the same principle for controlling the chilled water pumps, adding to that, the condenser water pumps as well. In this application, the same controller can be used to control the speed of the cooling tower fan(s) and reset the condenser water temperature. Reducing the condenser water temperature reduces the lift on the compressor, which in turn substantially reduces energy consumption.
The same monitoring points stated in the air-cooled example must be used with the water-cooled chiller, However, operators would want to monitor outside air wet-bulb temperature instead of dry-bulb as in the case with the air-cooled chiller example.
An understanding of OEM minimum and maximum pressures and temperatures requirements must be applied to the software algorithm in order to protect equipment and prevent the chiller from surging.
The Common Theme
In both examples, users will have taken systems with individual loops of control and provided synergy by bringing them together into a single control operation that sees the complete system as opposed to a single control loop.
In order to implement effective floating high pressure control, operators must vary the speed of the condenser fans with a variable speed drive, using a microprocessor controller with the FHP control algorithm, and provide analog inputs for outside air temperature, high side refrigerant pressure, and low side refrigerant pressure.
The interesting thing about this control approach is that it can be added as a retrofit to existing chiller plants, without replacing air conditioning equipment. If the air conditioning equipment that is mechanically sound, applying this as a retrofit brings older equipment up to the efficiency standards of newer equipment. For most retrofit applications, the typical return on investment is less than three years.
For both air-cooled and water-cooled chillers, in applying floating high pressure control the life of air conditioning equipment is extended. Meanwhile, there is an improved environment for the compressors to operate within. Reducing lift requirements reduces the mechanical strain on the compressor(s), and helps to alleviate short cycling during cooler outside air conditions.
In addition, using the new control technology makes it possible for the equipment to communicate with a building management system (if one is in place) through protocols such as typically already in place through one of the popular protocols such as BACnet, LonWorks, and Modbus.
Gray is the U.S. HVAC/R marketing segment manager for Schneider Electric. With more than 25 years of experience in the HVAC/R industry, his expertise ranges from commercial and industrial automated controls to large mechanical HVAC/R systems. He holds a bachelor’s degree in mechanical engineering and marketing.