Energy Efficiency Reference/Refrigeration/Recommendations/Install ASD on Condenser Fans

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Install ASD on Condenser Fans[edit]

Summary: Install Adjustable Speed Drives (ASDs) to reduce part-load fan power.

When to Apply: Single-speed to two-speed fans that cycle often to maintain minimum discharge pressure.

Key Engineering Concepts:

  • Ideal fan power is proportional to the cube of fan speed with an ASD
  • Actual fan power is greater due to ASD inefficiencies
  • Fan energy is directly proportional to use factor (UF) with on-off fan control
  • Fan energy is inversely proportional to the condensing approach temperature difference

Preparation:

Tool Required

  • Power Meter
  • DMM
  • Clamp-on Ammeter
  • Stopwatch
  • Thermometer
  • Psychrometer
  • Refrigeration data sheets

Data Required

  • Minimum condensing temperature (Tm)(saturated temperature for condenser fan set-points)
  • Fan power (FP)(measure)
  • Wet and dry bulb temperatures (measure)
  • Condenser MATD - (measure with all fans on)
  • Fan Use Factor, may be available from trend logs on electronic control systems.

Analysis process: 1) Calculate Condenser Fan Use Factor 2) Calculate Current Fan Energy Use Calculate current fan energy use differently than proposed energy use due to the change in control strategies. The existing control strategy is on-off control.

Calculate fan energy for each bin temperature with the fan power, use factor, and bin hours. Sum the energy for each bin temperature, normalized with operating hours, for total fan energy.

  • fan energy = sum of(bin hrs x use factor x fan power) x (operating hrs / total bin hrs)

3) Calculate Proposed Fan Energy Use Power and fan capacities are related cubically for ASD motors.

Proposed Fan Energy Use Estimate energy for each bin temperature with fan power, percent capacity, bin hours, and the following cubic relation shown is a curve fit based on measured fan data that relates the fan percent power to the bin specific use factor. Sum energy for each bin temperature, normalized with operating hours for total fan energy.

  • %power = curve fit based on percent capacity
  • fan energy = sum of (total power x % power x bin hrs) x (operating hrs / total bin hrs)

The curve fit is based on data collected by a former OSU graduate student.

4) Calculate Fan Energy Savings Calculate fan energy savings:

  • fan energy savings = difference between existing and proposed energy use

5) Calculate Cost Savings

  • Calculate cost savings by multiplying energy savings by the energy cost from the utility bills.
  • Estimate implementation cost
  • Adjustable speed drives vary in coast due to size and type:
  • Get a quote
  • They may be additional costs for ASD rated motors, isolation transformers, and / or harmonic filters, if needed
  • Determine payback
  • Calculate simple payback by dividing the implementation cost by annual cost savings.