Differential Pressure Bypass Setting

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Differential Pressure Bypass Setting
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• Positive displacement machines such as rotary
  gear pumps, piston pumps, screw pumps, screw
  compressor, or scroll compressors, reciprocating
  compressors are constant volume machines.
  Likewise, we may also take evaporators and
  condensers as constant flow rate equipment.

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• A fairly constant water flow rate is needed for
  evaporator for two reasons -
• To prevent damages caused by icing.
• To maintain good heat transfer.

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• Manufacturers typically recommend water flow
  above 40 to 50 of nominal design value to ensure
  optimum heat transfer.

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• Differential pressure bypass control is to ensure
  that, during part load, unused chilled water is
  diverted back to the chiller to maintain fairly
  constant flow rate through the evaporator. While
  flow switch is used as the last line of defense,
  tripping the chiller when flow drops to 40
  approximately, ?P control must operate when flow
  drops below 70 to 80 of design value.

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• Unless all load side equipment are equipped with
  3-way valves, chilled water flow demand is
  reduced during part load, thus the head or
  pressure as seen by the pump/chiller increases.
  This head is represented by the ?P across the
  pump suction header and the chiller supply
  header. It is also called residual pump
  pressure.

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Example on ?P control setting

• 100TR chiller, 7C/12.5 supply/return
• Pump pressure
• ?P
• Chilled water side pressure drop of chiller, plus
  pipe, valves and fittings from pump suction
  header up to chiller supply header is 100 kPa.
• Differential pressure control shall be able to
  maintain 80 of design flow.

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Answer

• Design flow rate
• Pump pressure at 15.2 l/s
• ?P
• Pump pressure at 80 flow,
• i.e. 0.8 x 15.2 12.16 l/s
• ?P

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Answer

• At 80 flow chilled water side pressure drop of
  chiller, pipes, valves and fittings from pump
  suction header up to chiller supply header is
  (0.8)2 x 100 64 kPa.
• Residual pump pressure (differential pressure) at
  full load is 250 100 150 kPa.
• Residual pump pressure (differential pressure) at
  80 flow is 285.8 64 221.8 kPa.

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Answer

• Therefore, differential pressure bypass valve
  must be fully opened at 221.8 kPa and fully
  closed at 150 kPa.