MARINE AUXILIARIES

Figure 6.8 Double-entry centrifugal pump

example of single and double entry suction impeller design

Other configurations of centrifugal pumps are used for particular duties or to meet system requirements. 1. A vertical single stage double entry centrifugal pump is shown in Figure 6.8. The incoming liquid enters the double impeller from the top and the bottom and passes into the volute casing for discharge. 2. A vertical multi-stage single-entry centrifugal pump is used for deep-well cargo pumping is shown in Figure 6.9. This can be considered as a series of centrifugal pumps arranged to supply one another in series and thus progressively increase the discharge pressure. The pump drive is located outside the tank and can be electric, hydraulic or any appropriate means suitable for the location. A diffuser is fitted to high-pressure centrifugal pumps. This is a ring fixed to the casing, around the impeller, in which there are passages formed by vanes.

Figure 6.9 Multi-stage centrifugal pump

The passages widen out in the direction of liquid flow and act to convert the kinetic energy of the liquid into pressure energy. Hydraulic balance arrangements are also usual. Some of the high-pressure discharge liquid is directed against a drum or piston arrangement to balance the discharge liquid pressure on the impeller and thus maintain it in an equilibrium position.

What is a priming? Most centrifugal pumps are not self-priming. In other words, the pump casing must be filled with liquid before the pump is started, or the pump will not be able to function. If the pump casing becomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping. To ensure that a centrifugal pump remains primed and does not become gas- bound, most centrifugal pumps are located below the level of the source from which the pump is to take its suction. The same effect can be gained by supplying liquid to the pump suction under pressure supplied by another pump placed in the suction line. The process of filling the pump with liquid is called priming.

What is the function of ejector?  For priming of centrifugal pumps installed above the level of the liquid to be pumped. How does it works?  Through the use of compressed air, the ejector produces inside the vacuum which removes the air contained in the pump casing and in the suction pipe, allowing the liquid to rise up to the pump.  A built in level sensor gives the start to the pump avoiding the dry-running. When the delivery pressure of the pump reaches the set point of the pressure switch, the ejector is automatically deactivated stopping the flow of compressed air and isolating the connection which aspires from pump.  In the case where the discharge pressure of the pump drops due to loss of priming, the ejector automatically is activated repeating the cycle of priming.





Malaysian Institute of Marine Engineering Technology (MIMET) Marine Auxiliaries / WAS / Jan 2020

 The term refrigeration means cooling a space, substance or system to lower and/or maintain its temperature below the ambient one (while the removed heat is rejected at a higher temperature).  In other words, refrigeration is artificial (human-made) cooling.  Energy in the form of heat is removed from a low-temperature reservoir and transferred to a high- temperature reservoir.  The work of energy transfer is traditionally driven by mechanical means

• Ship refrigeration systems play a vital part on ships carrying refrigerated cargo and victuals for the crew or passengers. Refrigeration prevents any damage to the cargo or decay of perishable foods, like growth of micro- organisms, oxidation, fermentation and drying out of cargo. The temperature of the sometimes sensitive cargo is controlled by the ship’s refrigeration plant. As the main purpose of vessels carrying refrigerated cargo is to ensure the cargo is transported in good and healthy condition, it is of uttermost importance that the refrigeration system works perfectly at all times. • Refrigerators are reverse heat engines that pump energy out of the inside of the fridge and into the surrounding area by the way of an electric motor. In other words, the motor takes some quantity of energy from the inside of the fridge and brings it to the outside.

• The heat engine produces work by absorbing heat from the source and liberating some heat to the sink. • The reversed heat engine absorbs the work and transfers heat from the sink to the source

Refrigeration is a process in which the temperature of a space or its contensts is reduced to below that of their surrounding. Air conditioning is the control of temperature and humidity in a space together with the circulation, filtering and refreshing of the air in a space without necessarily a change of temperature. Ventilation Process by which ‘clean’ air (normally outdoor air) is intentionally provided to a space and stale air is removed. This may be accomplished by either natural or mechanical means.



Main Component Of Refrigeration System and its function Compressor - Reciprocating single or two stage compressor is commonly used for compressing and supplying the refrigerant to the system. Condensor - Shell and tube type condenser is used to cool down the refrigerant in the system Expansion Valve Regulator/Thermal Expansion Valve - An Expansion valve regulates the refrigerants to maintain the correct hold or room temperature. Evaporator - The evaporator unit act as a heat exchanger to cool down the hold or room area by transferring heat to the refrigerant.

Example of Condenser.

Refrigeration system accessories Oil Seperator - separate oil from the hot gas in the discharge line and return it to the compressor crankcase or to the oil reservoir in systems Drier - It absorbs moisture that usually present in the refrigerant circulating to entire system. Also used for filtering and collecting foreign materials during charging refrigerant Liquid receiver - Stores unused or ecess refrigerant returning from the condensor. Also provide a place to store refrigerant when pumping out the evaporator during maintenance operation Pressure gauge - indicate the working pressure exerted into the system Circulating fan - To circulate air evenly cooling while in operation in order to absorb heat Cooling water pump - Supply cooling water into the condensor directly

The pressure of the refrigerant gas is increased in the compressor and it becomes hot. This hot, high-pressure gas is passed through into a condenser. The refrigerant gas will be cooled either by air or water, and the gas will condense at high pressure. The liquid refrigerant is then distributed through a pipe network until it reaches a thermal expansion valve (TEV) and to the evaporator where the cooling is taking place. The system must be designed that the liquid refrigerant in the evaporator should be boiled off (to become gas ) and the gas slightly superheated before it returns to the compressor at a low pressure to be compressed in the compressor. The heat that is transferred from the air to the evaporator and then pumped round the system until it reaches the condenser where the heat is transferred or rejected to the ambient air or water (normally use sea water)







• The compressor acting as a circulation pump for refrigerant has two safety cut-outs- Low pressure (LP) and High Pressure (HP) cut outs. When the pressure on the suction side drops below the set valve, the control unit stops the compressor and when the pressure on the discharge side shoots up, the compressor trips. • LP or low pressure cut out is controlled automatically i.e. when the suction pressure drops, the compressor stops and when the suction pressure rises again, the control system starts the compressor. HP or high pressure cut out is provided with manual re-set. • The hot compressed liquid is passed to a receiver through a condenser to cool it down. The receiver can be used to collect the refrigerant when any major repair work has to be performed. • The master solenoid is fitted after the receiver, which is controlled by the control unit. In case of sudden stoppage of compressor, the master solenoid also closes, avoiding the flooding of evaporator with refrigerant liquid.

• The room or hold solenoid and thermostatic valve regulate the flow of the refrigerant in to the room to maintain the temperature of the room. • For this, the expansion valve is controlled by a diaphragm movement due to the pressure variation which is operated by the bulb sensor filled with expandable fluid fitted at the evaporator outlet. • The thermostatic expansion valve supplies the correct amount of refrigerants to evaporators where the refrigerants takes up the heat from the room and boils off into vapours resulting in temperature drop for that room. • This is how temperature is maintained in the refrigeration plant of the ship.

Refrigerants are usually substances that evaporate quickly. In the process of evaporation they draw heat from surrounding substances. Divided into:- 1) Primary refrigerants 2) Secondary refrigerants 3) Environmentally friendly refrigerant. 1) Primary refrigerants Primary refrigerant: This is the refrigerant used in the compressor, condenser and evaporator system . For example it will boil off or evaporate at a low temperature, and it will condense at a temperature near normal sea water temperature (about 35degress)

Characteristics of primary refrigerant. It must be free from; 1. Toxic 2. Explosive 3. Flammable 4. Corrosive Some refrigerants have critical temperatures above which the refrigerant gas will not condense.

Type of Primary Refrigerants 1. Carbon dioxide as refrigerant.  Used for many years on ships.  Ships operating in areas with very high sea-water temperatures had difficulty in liquefying the carbon dioxide without some additional sub-cooling system.  carbon dioxide was the very high pressure, at which the system operated, resulting in large and heavy machinery.  Between the carbon dioxide era and the present refrigerants, Methyl Chloride and ammonia were used.

2. Methyl chloride – Disadvantage.  Due to its explosive properties is now banned for shipboard use. 3. Ammonia  Is still employed, but requires special ventilation. (toxic and hazardous) 4. Refrigerant 11 – Disadvantage.  Very low-pressure refrigerant which requires a large circulation for a cooling effect.  It has low power consumption- Advantage  Non-corrosive -Advantage 5. Refrigerant 12- Disadvantage air and moisture.  The evaporator pressures are below atmospheric and any system leaks draw in

4. Refrigerant 11 – Disadvantage.  Very low-pressure refrigerant which requires a large circulation for a cooling effect.  It has low power consumption- Advantage  Non-corrosive -Advantage 5. Refrigerant 12- Disadvantage.  The evaporator pressures are below atmospheric and any system leaks draw in air and moisture.

6. Refrigerant 22 -Advantages 1) The most common refrigerant now. 2) It provides a considerable range of low-temperature operation before the evaporator pressure drops below atmospheric conditions. 3) There is also a space saving as the compressor displacement is about 60% of that required for Refrigerant 12. 4) Non-corrosive.

7. Refrigerant 502- Advantages  The main advantages are that the displacement required is similar to that of Refrigerant 22.  Gas delivery temperatures from the compressor are greatly reduced. Therefore less likely to be a break-up of the lubricating oil and cause stressing of the delivery valves.  Non-corrosive. Disadvantage:  At present Refrigerant 502 is still an expensive gas and not readily available worldwide.

2. Secondary refrigerants. Use for;  Large air conditioning plant  Cargo cooling systems. The primary refrigerant evaporator will be circulated with the secondary refrigerant, which is then passed to the space to be cooled. Secondary refrigerants are employed where the installation is large and complex to avoid the circulation of expensive primary refrigerants in large quantities.

Disadvantage: The primary refrigerants can easily escape through minute (small) clearances, so keep the number of leaking points to a minimum. The common type of secondary refrigerant used on large cargo installations is a calcium chloride brine to which is added inhibitors to prevent corrosion. In case of large air conditioning plant, fresh water is the normal secondary refrigerant, which may or may not have a glycol solution added to the water. Glycol, any of a class of organic compounds belonging to the alcohol family Ethylene glycol is widely used as antifreeze in automobile cooling systems.

The common type of secondary refrigerant used on large cargo installations is 1. Calcium chloride brine to which is added inhibitors to prevent corrosion. In case of large air conditioning plant, 2. Fresh water is the normal secondary refrigerant, which may or may not have a glycol solution added to the water.

C. Environmentally friendly Refrigerant Freon 134A, 404A, CO2 and Ammonia. Most of modern refrigerant now such as Freon 12, 22 are ChloroFluoroCarbons (CFCs) which have damaging effect on the ozone layer and can causing global warming, and they are gradually be phasing out and alternatives refrigerants are being manufactured for shipboard use.  Freon 12 and Freon 11 are readily to be replaced by Freon 134A Hydrofluorocarbons (HFC). which causes zero ozone depletion and alternatives to CFC11, HCFC22 and the meantime the R502 are also being developed. Other choices include the use of carbon dioxide or ammonia, both previously used as refrigerants but potentially dangerous if incorrectly handled. Lloyds Register (Classification Societies) has issued Guidance Notes for Marine Ammonia Plant as a supplement to the Rules, owing to the resurgence (revival) of interest in this refrigerant.

Hydrofluorocarbons (HFC) Hydrofluorocarbons, organic compounds that contain only one or a few fluorine atoms, are the more common type of organofluorine compounds. Used as refrigerants because they do not harm the ozone layer as they do not contain chlorine or bromine.



The challenge in refrigeration (and air conditioning, etc.) is to remove heat from a low temperature source and dump it at a higher temperature sink. Compression refrigeration cycles in general take advantage of the idea that highly compressed fluids at one temperature will tend to get colder when they are allowed to expand. If the pressure change is high enough, then the compressed gas will be hotter than our source of cooling (outside air, for instance) and the expanded gas will be cooler than our desired cold temperature. In this case, we can use it to cool at a low temperature and reject the heat to a high temperature. An ideal refrigeration cycle looks much like a reversed Carnot heat engine or a reversed Rankine cycle heat engine. The primary distinction being that refrigeration cycles lack a turbine, using a throttle instead to expand the working fluid

CONT' The cycle operates at two pressures, Phigh and Plow, and the statepoints are determined by the cooling requirements and the properties of the working fluid. Most coolants are designed so that they have relatively high vapor pressures at typical application temperatures to avoid the need to maintain a significant vacuum in the refrigeration cycle. Reverse rankine cycle











3D6ME - UniKL MIMET Compressors: There are mainly three types of compressors in use: 1. Reciprocating compressor 2. Rotary compressor 3. Centrifugal compressor • Reciprocating and rotary compressors are more commonly used for small capacities but for lower evaporator temperatures. • Centrifugal compressors are essentially large discharge capacity compressors. These are preferred for higher evaporator temperatures applications like, water chillers and air conditioners. They offer high efficiency but are expensive.

3D7ME - UniKL MIMET 1. Reciprocating Compressor:

3D8ME - UniKL MIMET CONT'

3D9ME - UniKL MIMET 2. Rotary Compressor:

CONT' rotary vane GIF 40 DME - UniKL MIMET

4D1ME - UniKL MIMET 3. Centrifugal Compressor