Q3 (16 Marks) Auxiliary Machinery
MEKG • Written Exam

Give reasons why each of the following features is considered desirable in air compressors (16)

(a) Smooth air passages with minimum obstruction at valves

(b) Minimum clearance volume

(c) Multi-tubular heat exchangers

(d) Generous size of suction filter

Appeared In: Jan 2024

Verified Model Answer (Text Solution)

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Part (a)

Smooth air passages with minimum obstruction at valves:

  • Smooth air passages reduce the buildup of impurities such as oil and water, which could lead to corrosion and wear.
  • Obstructions or dirt accumulation restrict the flow of air, lowering volumetric efficiency and reducing compressor performance.
  • Blockages at valves can cause localized pressure and temperature surges, potentially leading to the failure of critical components.
  • High temperatures combined with moisture in the air can oxidize lubricant oil, degrading its properties and leading to inadequate lubrication and potential failures.
Part (b)

Bumping clearance is the distance between the piston top and the cylinder cover when the piston is at the Top Dead Center (TDC). For safe and efficient compressor operation, it is typically maintained at 0.5% to 1% of the cylinder diameter.

1. If Bumping Clearance is Too Small:

  • The piston may physically strike the cylinder cover, leading to mechanical damage, including deformation of the piston, cylinder head, or connecting rod.
  • Frequent breakdowns and repairs may occur due to damage caused by insufficient clearance.
  • Piston collisions can result in overheating or failures, posing safety risks.

2. If Bumping Clearance is Too Large:

  • Larger clearance volumes increase the amount of high-pressure air trapped at the end of the compression stroke. This trapped air expands during the suction phase, reducing the effective stroke for fresh air intake.
  • To achieve the required compression pressure, the compressor must run for a longer time, consuming more energy and reducing operational efficiency.
  • The increased clearance volume causes a drop in compressor performance, as shown in the graph of volumetric efficiency vs. clearance volume.
Part (c)

Multi-Tubular Heat Exchangers:

  • Multi-tubular designs provide a larger surface area, facilitating more effective heat transfer compared to other designs.
  • Any leakage in the tubes can be easily identified during inspections.
  • Leaking tubes can be plugged temporarily without affecting the operation until they are repaired.
  • Straight tubes are simpler to clean and maintain, which extends the equipment's lifespan and ensures consistent performance.
Part (d)

Generous Size of Suction Filters:

  • A larger filter ensures an adequate air supply during the suction stroke, enhancing the compressor's performance.
  • Large suction filters minimize pressure loss across the filter, improving the efficiency of the compressor.
  • With reduced flow restrictions, more air is inducted into the system, increasing volumetric efficiency.
  • Generous-sized filters also function as silencers, reducing noise levels during operation.
  • Larger filters accumulate dirt less rapidly, allowing for longer intervals between maintenance and reducing downtime.
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