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Ask the Expert: HVAC Cooling O&Ms and FIMs

Posted by Danielle Marquis

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September 24, 2015 at 6:15 AM

Question: How can I improve the energy efficiency of my facility's air conditioning system?

 

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Answer: HVAC cooling system issues, which include problems with component maintenance and temperature control, can take several forms. An energy audit will typically identify energy inefficiencies in one or several of the following areas:

 

  • Multiple air conditioning compressors start simultaneously
  • Chiller evaporating and condensing temperatures are not optimized
  • Chiller is operating during cold weather to provide air conditioning
  • Reheat coils are used to maintain zone temperatures
  • Building utilizes a dual duct or multizone system
  • Air conditioning load trips circuit breaker on extremely warm days
  • Air of inadequate volume or temperature is being discharged through grills
  • Refrigeration condensers or coils are dirty, clogged and/or not functioning efficiently
  • Chilled water piping, valves and fittings are leaking
  • Chiller operation is not optimized (short-cycling)
  • Refrigeration compressor short-cycles
  • Multiple parallel chillers have no isolation schedule for extended light-load operation
  • Steam, hot or chilled water leaks are evident
  • Steam, hot or chilled water valves do not shut off tight
  • Conditioned air or heated water is discarded

 

The suggested operations and maintenance ("O&M") improvements and facility improvement measures ("FIM") will vary based on your facility's unique mix of inefficiencies, but what follows are some general guidelines.

 

Suggested HVAC Cooling O&Ms:

 

  • Adjust controls to stage compressors.
  • Increase chiller evaporator temperature following manufacturer's recommendations.
  • Decrease chiller condensing temperature following manufacturer's recommendations.
  • Tighten wire lugs if loose.
  • Replace defective circuit breakers.
  • Clean condenser on air cooled systems.
  • Clean scale build-up in condenser or water cooled systems.
  • Defrost evaporator coil if iced. Determine cause of icing and correct.
  • Clean evaporator coil, fins and tubes.
  • Clean or replace air filters.
  • Fire damper may be closed. Open and replace fusible link if necessary.
  • Balancing damper may have slipped and closed. Open to correct position and tighten wing nut.
  • If fan is rotating backwards, reverse rotation by reversing electrical contacts.
  • Clean condenser coil and/or water tower nozzles.
  • Determine if normal operating temperatures and pressures have been identified and if all gauges are checked frequently to ensure design conditions are being met.
  • Increased system pressure may be due to dirty condensers which will decrease system efficiency. High discharge temperatures often are caused by defective or broken compressor valves. Repair or adjust as required.
  • Inspect the liquid line leaving the strainer. If it feels cooler than the liquid line entering the strainer, it is clogged. It is very badly clogged if frost or sweat is visible at the strainer outlet. Clean as required.
  • Clean coils and/or other elements as needed on a scheduled basis. Include dehumidification coils.
  • Repair joint or piping leaks.
  • Repair or replace valves.
  • Raise chilled water supply temperature (Note: This is especially important if system was designed for a 75 degree Fahrenheit space temperature and the space setting has been raised to 78 degrees Fahrenheit for energy conservation purposes).
  • Remove scale deposits from condensers.
  • Check refrigerant charge. If low or leaking, then find and repair leak. Recharge system.
  • Repair electrical control circuit if required.
  • Reset high/low pressure control differential settings if needed.
  • Evaporation coil may be iced up or dirty. Defrost and clean.
  • Liquid line solenoid valve may be leaking. Repair or replace.
  • If frost is detected on the liquid line strainer, it is clogged. Clean strainer.
  • Clean condenser coil.
  • If condenser is a cooling tower, ascertain if spray nozzles are plugged. Make sure water flow is unobstructed. Clean towers of leaves and debris. 
  • Remove scale deposits from shell/tubes on water condensers.
  • Repair suction valves in compressor, if needed.
  • Develop load vs. capacity matrix.
  • Isolate unneeded chillers.
  • Repair leaks and/or repair/replace valves for steam, hot or chilled water lines.

 

Suggested HVAC Cooling FIMs:

 

  • If you don't have automatic controls, purchase and install. This will allow any additional compressors to cut in only when the main compressor can no longer satisfy space conditioning load.
  • Provide a water temperature system injecting cooling tower condenser water directly into the system's chilled water circuits. Except for pumping and cooling tower fan horsepower, this provides free cooling. Special care must be taken in treating and filtering condenser water.
  • If system is forced air, using DX coils and air cooled condenser, install economizer cycle to obtain free cooling.
  • Convert to variable air volume system if the reheat coils are not necessary to supply heat during the heating season.
  • Convert dual duct or multizone systems to variable air volume, if building has a separate heating season.
  • Install controls to automatically reset hot and cold deck temperatures.
  • Consider installing insulated underground storage tank that would allow night operation of chiller when electrical demand is low. This reservoir tank would be a source of supply of chilled water for daytime operations. Chiller would not be operated during the day.
  • Install differential pressure-sensing switches to alarm when air flow drops significantly.
  • Reduce peak loads with electric load limiters.
  • It is important for building owners to be aware of heat recovery measures. However, it is not wise to install such equipment without first analyzing the energy characteristics of the building, performance of the hardware, and how it fits into the overall energy plan.

 

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Topics: CII Division, Frequently Asked Questions, HVAC Controls

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