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Energy Efficiency Reference/Refrigeration/Recommendations/Use Thermosyphon Oil Cooling

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Recommendation: Use Thermosyphon Oil Cooling

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Summary: Install thermosyphon oil cooling to avoid power and energy costs associated with liquid injection oil cooling. Liquid injection oil cooling typically consumes 5% to 15% of the compressor power to recompress injected refrigerant. Thermosyphon cooling does not consume compressor power.

When to Apply:

  • When liquid injection is used to cool large compressors.
  • When reducing refrigerant, condensing pressure is limited by liquid injection cooling without installation of a liquid injection pump. The pump uses energy and improves the economics of thermosyphon oil cooling.

Key Engineering Conceptets:

  • Liquid injection oil cooling works by expanding high-pressure refrigerant liquid to an intermediate pressure and injecting it into the compressor.
  • Some liquid refrigerant flashes to vapor during expansion and the rest evaporates as it cools the compressor.
  • Thermosyphon oil cooling uses a heat exchanger near the compressor sump to cool the oil.
  • A high pressure pilot receiver collects refrigerant liquid which falls by gravity into the oil heat exchanger.
  • The refrigerant evaporates and buoyancy drives the flow to the condenser.
  • The thermosyphon oil cooler may require an additional condenser.
  • Condenser fans will use more energy, but much less than compressor.

Preparation:

Tools Required:

  • Refrigeration Data Sheets
  • Power Meter
  • DMM
  • Clamp-on Ammeter

Data Required:

  • Compressor Nameplate Data
  • Refrigerant Type
  • Proposed Suction and Discharge Pressures
  • Compressor Power Profile (CP) - (measure)

Special Considerations: To size the thermosyphon heat exchanger, you will need to know the refrigerant mass flow rate, which you can calculate from.

  • Compressor Volumetric Flow Rate
  • Intake refrigerant density

Analysis Process: 1) Calculate Power Savings Liquis injection oil typically increases compressor power by 5% to 15%. Use operating hours to calculate energy use.

Compressor Power Savings: Conservatively estimate the liquid injection power as 5% of compressor power. Measure compressor power or calculate from measured volts and current

Compressor Power Savings are:

  • CPS = 5% x compressor power

If the liquid injection system currently pumps liquid refrigerant, there will be further pump power savings (PPS) from turning off the pumps.

Total Power Savings (TPS) is combination of a decrease in pump and compressor power.

  • TPS = compressor savings + pump savings

2) Calculate Energy Savings Calculate energy savings from refrigeration system power savings and operating hours (OH).

Energy Savings: The operating hours of the refrigeration system may change with time of day or year. Pay attention to the operating schedule when calculating the following.

  • Energy savings = total power savings x operating hours

3) Calculate Cost Savings. Calculate cost savings by multiplying energy savings by energy cost from utility bills.

Estimate implementation cost

Use the refrigeration mass flow rate, the discharge pressure of the refrigerant, and compressor power to size the thermosyphon heat exchanger. Calculate mass flow from compressor volumetric flow rate and refrigerant density at suction conditions.

4) Determine payback Calculate simple payback by dividing implementation cost by annual cost savings.

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