Manage energy in your HVAC systems: Your questions answered

Webcast presenter Charlie Martin answered questions about topics such as 3-phase motors, AHU maintenance, efficient systems, and evaporative cooling units.
By Charlie Martin, Advanced Energy August 27, 2018

Charlie Martin, senior energy engineer, Advanced Energy. Courtesy: Advanced EnergyThe "Manage energy in your HVAC systems" webcast was presented live on Aug. 15, 2018, by Charlie Martin from Advanced Energy. The webcast can be found here. He supplied written answers to some of those questions that weren’t addressed from the webcast attendees:

Question: Are 3-phase systems more efficient?

Answer: As a general rule, this is true. In an industrial setting, 3-phase power is readily available and HVAC equipment tends to be relatively large had have larger motors. Three-phase motors are more efficient than similar equipment with single-phase motors. Single-phase motors are not normally available in large HVAC equipment. However, single-phase motors are economical for smaller HVAC equipment typically found in residential settings. So a small single-phase HVAC unit may have a higher EER rating than a similar size 3-phase unit. Also, HVAC manufactures who serve the residential market for small HVAC units will have a larger selection of single-phase air conditioners and heat pumps including high efficiency options, and may only offer a standard efficiency 3-phase unit.

Q: What manufacturer makes the most efficient systems?

A: All major manufactures offer high efficiency options. DOE efficiency standards mandate minimum HVAC efficiency standards. Regarding efficiency, it is important to focus on the manufacturer’s published EER, IEER, and SEER for smaller DX equipment. For chillers the focus should be on the full-load and IPLV kW/ton values based on AHRI standards, and not the manufacturer name.

Q: What if any procedures do you recommend for AHU maintenance such as checking delta P and filter changes?

A: If you have a building management system (BMS), it is good to continually monitor air system static pressure (delta P) and have the BMS trigger an alarm when delta P gets high. As filters get dirty delta P increases. With VAV systems you could also monitor fan power as delta P increases, the fan speed will need to increase to compensate for the increased pressure drop across the filter and more power will be required. The amount of pressure increase varies.So, it is hard to say at what pressure you should change the filter. Initially, a visual inspection is in order along with monitoring pressure. When the filters are sufficiently dirty, note the associated delta P at that condition. Then you can set the alarm when delta P approaches that pressure. If you do not have a BMS or ability to monitor delta P, you are pretty much left with visual inspection and setting up scheduled filter changes.

Q: What is a typical fouled approach to trigger cleaning of condenser and evaporator tubes?

A: Each chiller manufacturer should provide design specifications regarding approach temperatures across the evaporator and condenser heat exchangers. At full load cooling approach temperatures will typically be higher than at lower cooling loads. So you will need to note the loading condition along with the associated approach temperature. A good approach temperature for 40% load may be 1 to 2° F, while a good approach temperature at foul load may be between 2 and 3° F. So, if the approach temperature at 40% load rises to 4-5°, and/or the full load approach temperature rises to 5-6° it probably is time to clean the heat exchanger. If a constant speed chiller design lift is 55° F at full load and both the evaporator and condenser’s approach temperatures have increased by 2°, the lift has now increased to 59° F. This results in an increase of 7% lift and about a 10% increase in power.

Q: Any recommendations for small-scale evaporative cooling units?

A: Referring to direct evaporative coolers, often called swamp coolers, they are relatively low energy consumers as the fan motor is typically the only device using electricity. So, the fan motor efficiency is the most important consideration and should be relatively high. Also, the wetted media needs to be cleaned periodically and replaced as necessary. It acts as an air filter as well as a platform for cooling the air. Dirt build-up increases static pressure and will cause the fan to slow down and reduce performance. Being a wet filter, as it gets dirty and if the fan cycles off, the media can be become a good environment for biological growth. This can lead to health hazards if the media is not maintained properly.

Q: Can you post the efficiency numbers quoted?

A: With new or replacement RTUs and other unitary DX equipment, we should focus on both the Full-load efficiency—EER rating and seasonal or part-load efficiency—SEER or IEER rating. The higher the more efficient the unit. Examples include:

  • Typically, a 12 EER or higher is preferred for equipment less than 20 tons along with SEERs greater than 16 for equipment and IEERs greater than 13-14
  • High-efficiency air-cooled chillers are available with full-load efficiency > 10.3 EER and part-load efficiency > 13.4 IPLV.
  • High-efficiency water-cooled screw and scroll chillers are available with full-load efficiencies < 0.6 kW/ton and part-load efficiencies < 0.5 kW/ton.
  • High-efficiency water cooled centrifugal chillers are available with full-load efficiencies < 0.55 kW/ton and part-load efficiencies < 0.4 kW/ton.

Q: On a high level, how much energy we could save by increasing 1 degree in the temperature set point?

A: Of course savings from increasing cooling set point temperature varies greatly based on many factors, including the base temperature, the number of hours system operates, the efficiency of the HVAC equipment, the nature of the cooling load, and how the cooling load compares with the capacity of the HVAC equipment. However, on a high level we may expect to achieve a 3-4% savings in energy for each degree of temperature increase in set point.

Q: How do you enhance the vapor recompression cycle performance?

A: Referring to the vapor recompression cycle, there are several ways to enhance the performance of DX air conditioners and chillers.

  1. Reduce lift where possible. That is keeping the condenser temperature as low as practical and the evaporator temperature as high as practical.
  2. Use the highest efficiency compressor practical and employ good maintenance practices to maintain its efficiency.
  3. Maintain proper refrigerant charge and fix refrigerant leaks soon after they occur.
  4. On low-pressure equipment, ensure purge units are in good condition to remove non condensable gases from the system.
  5. Use multistage chillers where practice, which contain intercoolers and multiple stage compressors, reducing compression power requirements and increasing cooling capacity.