Air Compressors, AC,  and Refrigeration System – An Important Guide

Air Compressors, AC, and Refrigeration System – An Important Guide

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 Most Frequently asked questions on Air compressors, Air Conditioning, and Refrigeration systems in MMDs. 

Air Compressors and Refrigeration System are subtopics in MEO Class 4 orals of Function 4 and Function 6 i.e, MEOL and MROL respectively. In this article, we have tried to provide you with the most important topics to be covered in Air Compressors and Refrigeration systems.
Air Bottle, Air Compressor, Refrigeration, Air Cond. & Evaporator:  
 

Air receiver:  

Air receivers and its safety devices

  • The total capacity of air receivers must be sufficient to give at least 12 starts for the reversible engine, and at least  6 starts for the non-reversible engine, without refilling the receivers. 
  • There must be two identical main air receivers and one emergency bottle for every vessel. 
 

Mountings:    

  • Fusible plug:  
    • Composition – Bismuth 50%, Tin 30%, Lead 20%,   
    • Melting point: 220΄F (104.4΄C).    
    • Fitted at the reservoir’s bottom or on the reservoir at shipside, when relief valve (safety valve) is not directly fitted on the reservoir. 
  • Atmospheric relief valve:  Provided for back-up of the fusible plug. In the case of ER fire when CO₂ flooding is required, this valve is opened before evacuating ER. 
  • Spring-loaded safety valve:  Setting pressure: 32 bar (for 30 bar working pressure),                    with >= 10% rise in accumulation of pressure. May be fitted directly or with extension. 
  • Compensation ring:  When a hole is cut or machined in pressure vessel, higher stresses will subject to the material around the hole, and to reduce this, compensation rings are fitted. It is a flange on which a valve or fitting is usually mounted. 
  • Manual Drain valve or Automatic Drain valve. 
  • Pressure gauges. 
  • Access doors. 
  • Main starting air valve, auxiliary starting air valve, filling valve, service air or whistle air valve. 
 

Internal surface coating:  

Graphite suspension in water, Linseed oil, Copal varnish or Epoxy coating having basic properties of anti-corrosive, anti-toxic or anti-oxidation. 
 

Safety devices on Main Air Bottle: 

  • Fusible plug. 
  • Pressure Relief Valve 
  • Atmospheric Relief Valve. 
  • Low Air Pressure alarm. 
  • Automatic or remote control Moisture Drain Valve.  

 Air Compressor: 

Air Compressors and its maintenance

LP Relief Valve opening: 

Causes: 
  • HP suction valve leaking. 
  • Intercooler choked. 
  • Relief valve, jammed by foreign particles, in the open position.

HP Relief Valve opening: 

Causes: 
  • HP discharge valve, in the closed position. 
  • Aftercooler choked. 
  • Relief valve, jammed by foreign particles, or spring sticking in the open position. 
  • [Relief Valves opening pressure are set at  ≯ 10% above stage pressure.]

Why Intercooler is fitted? 

  •  Reduce air temperature and volume, and increase air density for the next stage. 
  • Increase Compressor Capacity and Volumetric Efficiency. 
  • Better lubrication for cylinder and rings. 
  • Water and excess oil can be drained out, preventing fouling of Intercooler and pipes, Air Bottle corrosion, and starting airline explosion. 
  • Work done is saved. 
  • Metal stresses reduced, due to control of temperature.

Volumetric Efficiency: 

                                             The volume of Air drawn into Cylinder 
                                  VE   = ————————————–  
                                                   Stroke Volume of LP Piston  
                                                  
                                              The volume of Air discharged as ‘free air’ 
                                  VE   = —————————————     
                                                    Stroke Volume of LP Piston 
 
          Where ‘free air’ is, air at atmospheric pressure and temperature of  15°C. 

Why Multistage Air Compressor is built? 

  • To obtain near to ideal isothermal compression, the compressor is to be constructed of multistage with inter-stage cooling. 
  • Inter-stage cooling reduces air temperature and volume after 1st stage compression, thus increase the mass of air for the 2nd stage. 
  • Work done is saved and air compressor efficiency increased. 
Other advantages are: 
  • Easy to get high final air pressure. 
  • Easy to control air temperature. 
  • Easy to maintain correct lubrication. 
  • Better compressor balancing. 
  • Reduction in size.
  • Reduction in clearance volume loss. 

Capacity of air compressor: 

  • Capacity is checked upon how much filling time is lowered. 
  • Compressor should have enough filling capacity so that sufficient stopping time should be provided between fillings. 
                          12 consecutive starts: reversible engine. 
                            6 consecutive starts: non-reversible engine. 

What is Free Air Delivery, FAD? 

  • Capacity of Air Compressor is stated in terms of [ m³/ hr ]. 
  • Volume of air actually discharged in 1 hour, that would occupy if expanded down to atmospheric pressure and cooled to atmospheric temperature. 

Safety devices on Main Air Compressor: 

  • Bursting Disc on Intercooler:  (At waterside)  
  • Bursting Disc and Fusible Plug (121°C) on Aftercooler  
  • Relief valves on LP and HP stages.  (Set to lift at 10% rise above normal stage pressure.) 
  • Automatic Moisture Drain Valve. 
  • Cooling water supply failure alarm. 
  • Low LO pressure alarm. 
  • Relief valve on the crankcase LO pump. 
  • Delivery air HT cut out and alarm on Aftercooler outlet.  (Max. 93°C) {LP discharge pressure 4 bars:  HP discharge pressure 30 bars:  Intercooler inlet air 130°C      Intercooler outlet air 35°C:   Aftercooler inlet air 130°C:   Aftercooler outlet air 35°C:      Intercooler is single-pass type:  Aftercooler, double pass U-tube type:} 

Uses of Compressed Air: 

  • Engine Starting: 20 to 25 bar 
  • Boiler Soot Blowing: 20 to 25 bar
  • General Service (Whistle, Pneumatic Tools, Lifeboat and Pilot ladder): 7 to 10 bar 
  • Instrumentation and Control: 1.5 bar 

Air Filter: 

  • Material:  Felt, Metal gauze or Nylon strands  
  • Removes contaminants from the air. Dirt and dust act as abrasives and increase wear.  
  • Contaminant deposits on valves can become hot and source of ignition.
Hazards of Dirty Filter:  
  • A very dirty filter or muffling filter results in a large pressure drop. 
  • Air has to be compressed over a higher range. 
  • In an extreme case, the discharge air temperature may exceed flashpoint, or auto-ignition temperature resulting in an explosion. 
  • As for safety against explosion, air temperature is limited to 93°C. Fusible Plug (121°C) or a High Temperature cut out (93°C) is provided on Compressor.   

Pressure Test on Air Compressor: 

Cylinders, cylinder cover, Inter & Aftercoolers are hydraulically tested to: 
  • Air Side: 1.5 x max. Working Pressure.   
  • Water Side: 4 bar or 1.5 x max. WP (whichever is greater) 

Refrigeration: 

Refrigeration System diagram

What are Primary Refrigerants? 

  • Mostly volatile liquids and employed inside direct expansion closed system. 
  • Evaporate at low temperature and at a reasonable pressure. 
  • Condense at SW temperature at a reasonable pressure.  

 What are Secondary Refrigerants? 

  • Non-volatile and employed at large, complex installation, to avoid circulation of expansive Primary Refrigerant in large quantities. 
  • Cooled inside Refrigeration Machinery Room and pumped around the ship to batteries in each cargo space.   

Thermostatic Expansion Valve: TEV: 

 Main Functions: 
  • Automatic and prompt response of Refrigerant Flow to match Evaporator Load. 
  • Prevention of Liquid flow into Compressor. 
  • Appropriate amount of Refrigerant maintained at HP and LP sides. 
TEV construction:  
  • Small quantity of Vapour Refrigerant is sealed in a bulb or phial, and attached to Compressor suction pipe, just coming out from Evaporator. 
  • Other end is connected by Capillary Tube to the chamber above Flexible Bellow in the valve body. 
  • The space below the Bellow is in communication with Evaporator outlet pressure.  
  •               [This is called Equalising Line.] 
  • If no further action is taken, pressure above and below the Bellow will be equalised and hence no superheat is obtained. 
  • This is overcome by providing adjustable Bias Spring under the Bellow, and Bias Spring pressure is proportional to required superheat. 
Operation: 
  • Refrigerant Liquid from Condenser enters into TEV via Dryer, it expands to Evaporation Pressure, and some flash gas is formed. 
  • Flash Gas amount varies between 25 – 35%, depending on refrigerant type, plant capacity and ambient temperature. 
  • Mixture of this expanded gases and some part of the liquid passed into Evaporator, where complete Evaporation takes place. 
  • Evaporator outlet pressure plus Spring pressure tends to close the valve and is opposed by the pressure above the Bellow, trying to open it. 
  • This pressure above the Bellow is in relation to temperature in Compressor suction pipe. 
  • Equilibrium condition is reached when Superheat is correct at the phial attachment point. 
  • Starved condition in Evaporator will result in greater Superheat, so expansion of Vapour Refrigerant in the phial will tend to open the valve further, to increase the flow. 
  • Flooded condition in Evaporator will result in lower Superheat, so contraction of Vapour Refrigerant in the phial will tend to close the valve further, so decrease the flow. 
  • Superheat Temperature adjusted at 3 – 6°C, by Bias Spring pressure.  

Why Equalising Connection is fitted? 

  • In some plant having large Evaporator or Multi-circuit Evaporator, the excessive pressure drop across Evaporator occurs, and always tend to starve the Evaporator and increase the Superheat. 
  • To counteract this, if the pressure drop across Evaporator, exceeds 0.3 bar, an Equalising Connection must be provided at TEV. 
  • A direct connection between underside of Bellow and suction piping of Compressor, preferably between phial and Compressor. 

 Safety devices on Refrigeration Plant: 

  • LP cut-out switch:  Set at a pressure corresponding to 5°C below the lowest expected evaporating gauge reading. 
  • HP cut-out switch: Set at a pressure corresponding to 5°C above the highest expected condensing gauge reading. 
  • LO LP cut-out:   Oil pressure usually set at   2 bar above crankcase pressure. 
  • Condenser cooling water LP cut-out. 
  • Liquid shock valve on Compressor cylinder head. 
  • Bursting disc on the cylinder head, between inlet and discharge manifold. 
  • Bursting disc on Condenser, [if fitted]. 
  • Relief valve on Condenser. 
  • Master solenoid valve: To prevent liquid being entered into Compressor, when the plant is standstill, especially in Large Plant. 
 

Refer plant survey: 

  • General examination of machinery and testing under working condition.  
  • The log examined, to ascertain successful operation during voyages. 
  • Compressor and the prime mover to be open-up and examined. 
  • Primary system to be leak-tested to their w. p. and brine cooling coils are to be hydraulically tested to  6.3  kg/cm². 
  • Survey is done at 1 year from the date of installation, and special periodical surveys are to be carried out at 5 years intervals.  ( 1+ 5 ) 
 

System errors: 

Refrigeration System diagram
1. Air in the system:  
Indication:   
  • Abnormal and shaking of Compressor discharge pressure gauge reading.            
  • Sight glass shows small air bubbles.   

Remedy: 

  • Close liquid stop valve at Condenser outlet. 
  • Pump down the entire charge into Condenser, until suction pressure is just above zero, and then stop Compressor. 
  • Shut Compressor discharge valve. 
  • Cooldown the Condenser content, by running cooling water for some period. 
  • Then purge air at the top of Condenser, through purging valve until refrigerant gas appears at the valve. 
2.Moisture in the system:  
Indication:  
  • Blockage at Expansion Valve. 
  • Compressor tends to stop by H.P. cut-out.   
Remedy: 
  • By renewing Drying agents. Common drying agents are Silica gel, Activated Alumina, Calcium oxide, Calcium chloride. 
  • If for the reincarnation of Silica gel and Activated Alumina, they must be baked at 140°C and can be used again. 
 
3. Oil in the system: 
 Indication:  
  • Incorrect Condenser and Evaporator temperature differentials. 
  • The compressor will be running longer than normal. 
  • Very difficult to cool down the room temperature due to excessive oil in the piping system. 
 Remedy: 
  • Pump down the system charges into the reservoir and totally shut down the whole system. 
  • Then blow out the collected oils inside piping and evaporator. [If necessary, renew Compressor piston rings or Oil separator, and replenishment of oil]. 
 
4. Overcharge: 
  Indication: 
  • Very high Condenser pressure gauge reading, and full sight glass. 
  • Liquid may flow back to the compressor suction. 
 Remedy: 
  • Pump down system charges into the reservoir and purge out excessive refrigerant from the vent valve. 
 
5. Undercharge: 
  Indication: 
  • Low Condenser pressure gauge reading.  
  • Appearance of large bubbles in the sight glass. 
  • Hot Compressor discharge pipe. 
 Remedy: 
  • Test leak points by Halide torch,  Soap bubble solution, Dye refrigerant, Electronic detector,  Sulphur candles,(anyone) which gives off white dense smokes when contact with Ammonia. 
  • After rectification of leak points, recharging is necessary. 

 

6. Short cycling: 
  • Repeated running and stopping of Compressor due to L.P. cut-out. There may be high leak points in the system. 
 
7. Excessive icing up at Compressor suction: 
Causes: 
  • Abnormal operation of TEV. 
  • Overcharge of the system. 
  • Moisture in the system owing to dirty Dryer. 
8. Defective Suction valve: 
    Indication: 
  • Continuous running of Compressor. 
  • Insufficient cooling effects. 
  • Noisy operation. 
  • High suction pressure. 

 

9. Defective Discharge valve: 
    Indication: 
  • Continuous running of Compressor. 
  • Insufficient cooling effects. 
  • Noisy operation. 
  • High suction pressure during running. 
  • Low discharge pressure during running. 
  • Suction pressure rises faster after Compressor is shut-down. 
  • Warm cylinder head. 
 
10. Choked Expansion valve:   
         Causes:     
  • Due to dirt and freeze-up of water present in the system. 
        Effects:      
  • Starved Evaporator 
  • High superheat temperature. 
  • Rapid Condenser pressure rise can cause stopping of Compressor,  
        Remedy:    
  • Clean Expansion valve and filter 
  • Renew Dehydrator. 

 

CFC: Chlorofluorocarbon 
  • Due to damaging effects on the OZONE layer and causing Global Warming, most CFCs are now replaced by HFCs, Hydrofluorocarbon. 
  • HFC 134a has Ozone Depletion Potential, ODP ‘0’ and Global Warming Potential, GWP ‘0.28’.   
 
Defrosting:   
  • A method of removal of frost, built-up on Evaporator coils. Defrosting should be done before snow thickness exceeds ¼”.  
 
 Reasons for defrosting: 
  • Affecting heat transfer properties. 
  • Affecting airflow and circulation. 
  • Liquid back to Compressor. 
 
Defrosting Systems: 
  • Water wash defrosting 
  • Hot gas defrosting 
  • Electric defrosting 
  • Manual shut down defrosting 
  • Warm brine defrosting 
 
Various methods to defrost Brine System: 
 
  • Hot brine thawing:   Best and fastest method, used powerful brine heater with separate thawing system. Watertight trays under the pipes collected the dripping water. 
  • Hot air from the atmosphere: It is important that isolating doors in air trunks are perfectly tight, so as to prevent hot air going into cargo spaces. 
  • By shutting off brine:   Allow the snows to be melted by the heat of the air in circulation. Very slow operation and tends to throwback a great deal of moisture into cargo space. 
 
Notes: 
  • Direct expansion grid system:     Hot gas defrosting. 
  • Battery cooling system:               Water spray, electrical or steam heater. 
  • Brine cooling:                              Hot brine thawing. 
 

Refrigerant Charging: 

  • Normally refrigerant in the liquid state is charged at the high-pressure side. 
  • Weigh the bottle with a spring scale. 
  • Connect the charging pipe between the liquid valve of bottle and charging valve. This pipe must be tightened after purging out air until refrigerant comes out. 
  • Fully open the bottle liquid valve, charging valve still closed. 
  • Close main liquid stop valve from the condenser and run the Compressor. 
  • Slowly open the charging valve ensuring that the frost must not be formed on the suction pipe. 
  • After filling the Compressor are shutdown and cooling water kept for some hour. 
  • Then air can be purged out from the air vent valve of the condenser. 
 
Prevention of Liquid Flow Back: 
  • Liquid shock valve (on cylinder head). 
  • TEV. 
  • Master solenoid valve (when the plant is standstill, especially in Large Plant). 
  • Defrosting. 
  • Bursting disc(on the cylinder head, between inlet and discharge manifold). 

 Air Conditioning: 

Objectives:  
  • To extract excess heat.   
  • To raise air temperature when required.   
  • To add or reduce moisture when required.   
  • To maintain sufficient oxygen and airflow.              
  • To remove dust. 
 
Relative Humidity: 
 
  • The ratio of the amount of water vapour in a given volume of air, to the maximum amount of water vapour that can be present before precipitation occurs.  
Control of temperature: 
  • Comfortable temperature range is about 22°C and RH about 60%, (usually 40 ~ 70%). 
       
 All zone temperature:     
  • Controlled by compressor suction pressure, via solenoid valve as step controlling. Thermostat, placed at some accommodation space actuates the Master Solenoid Valve of the plant, which will stop the Compressor when the pre-set temperature is reached. 
  • Capacity Unloader of Compressor units does last step controlling, as required. 
 
Particular zone temperature:  
  • Controlled by a flap valve fitted in each zone loop. 
  • The local cabin temperature can be adjusted by volume control at the delivery point of air duct controller. 

 

 

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