Refrigeration Circuit & Basics Of Refregiration Plant

Vapour-compression theoretical graphs

Absolute

temperature - Entropy

A-B, Isobaric Heat absorption in the evaporator

B-C, Isentropic compression in the compressor (frictionless adiabatic compression in ideal cycle)

C-D, Isobaric Heat removal in condenser

D-A, Constant enthalpy expansion in expansion valve

Heat energy equivalent of work done = Heat energy rejected- heat energy received

= Area ABCDA + Area under AD

Coefficient of performance = heat energy received/ Heat energy equivalent of work done

The coefficient of performance for freon is about 4.7

It should be noted that undercooling increases the heat received by moving point A to the left increasing the refrigerant effect.

The critical point is the poiunt above which

a.     the gas will not liquify by the action of pressure alone. This is an important temperature for refrigeration systesm which rely on the change of state for heat transfer.

b.    The gas will not liquify by cooling alone

p-h diagram (Mollier)

Typical system

The system shown above and described below is typical of that fitted on may ships other than it is more common to have two low temperature rooms rather than one.

Components

Cold rooms

Meat Room-Low temperature room typically working at -17^oC

Veg/ handling room-typically working at +4^oC

Compressor

Generally of the single stage, reciprocating type. Larger systems have multple cylinders with an unloader system using the suction pressure as its signal.

Refrigerant is compressed in the compressor to a pressure dependent upon the temperature of the cooling water to the condenser, and to a lesser extent the volume of gas in the system. As the temperature of the cooling water rises so does the minimum temperature of the refrigerant liquid rise, and with it the corresponding saturation pressure.

Compressor safety devices

The compressor is protected by three safety switches;

The OP switch or Oil Differential Pressure switch compares the measured lubricating oil pressure to the Suction (crankcase) pressure. Should the differential pressure fall below a pre-set minimum (about 1.2 bar) then the compressor will trip and require a manual reset to restart. A time delay is built into the circuit to allow sufficient time for the lubricating oil pressure to build up when starting before arming the circuit.

The HP or High Pressure switch, is fitted to the outlet of the compressor before the isolating valve. On over pressurisation (dependent on the refrigerant, up to about 24bar bar for R22) the switch will trip the compressor and a manual reset is required before restart.

The LP or Low Pressure switch when activated ( at about 1 bar for R22) will trip the compressor and require a manual reset before the compressor can be restarted.

Compressor control devices

This normally takes the form of an LP cut out pressure switch with automatic reset on pressure rise. The cut out set point is just above the LP trip point say at about 1.4bar. An adjustable differential is set to about 1.4bar to give a cut in pressure of around 2.8 bar. The electrical circuit is so arranged that even when the switch has reset, if no room solenoid valves are open the compressor will not start. This is to prevent the compressor cycling due to a leaky solenoid valve.

In addition to this extra LP switches may be fitted which operate between the extremes of the LP cut in and cut out to operate compressor unloaders.

Some modern systems contain a rotary vane compressor with variable speed (frequency changing) control

Oil Seperator

The purpose of the oil seperator, situated on the compressor discharge line, is to return oil entrained in the gas, back to the compressor sump.

The oil return may be float controlled as shown, electric solenoid controlled on a timer, or uncontrolled with a small bore capillary tube allowing continuous return.

With all of these methods a shut off valve is fitted between separator and compressor to allow for maintenance.

The oil gas mix enters the separator where it is made to change direction, the heavier oil droplets tend to fall to the bottom.

Condensor

Generally a water cooled tube cooler.A safety valve and vent are fitted. The purpose of the vent is to bleed off non-condensibles such as air which can enter the system when the suction pressure is allowed to fall below atmospheric or can be contained within the top up gas. The presence of non-condensibles is generally indicated by a compressor discharge pressure considerably above the saturation pressure of the refrigerant.

The coolant flow to the condenser is sometimes temperature regulated to prevent too low a temperature in the condenser which can effect plant efficiency due to the reduction in pressure.

Below the condenser, or sometimes as a separate unit, is the reservoir. Its purpose is to allow accurate gauge of the level of refrigerant in the system. In addition to this it also allows a space for the refrigerant liquid when the system is 'pumped down'. This refers to the evacuation of the refrigerant gas to the condenser to allow maintenance on the fridge system without loss. For systesm not fitted with a reservoir, a sight glass is sometimes incorpotated on the side of the condenser. Care should be given to ensuringthat the liquid level is not too high as this reduces the surface area of the cooling pipes available for condensing the liquid and can lead to increased discharge pressures.

Sight Glass

Often of the Bulls eye form. This allows the operator to ensure that it is only liquid, and not a liquid/gas mix going to the expansion valves. On some designs a water indicator is incorporated, this is a coloured ring in contact with the liquid, when water is detected it changes colour, typically from pink to blue.

Filter Drier

Can be either a compacted solid cartridge or bags of dessicant. The main purpose of this unit is to remove the moisture from the refrigerant.

Moisture cause two main problems. Firstly it can freeze to ice in the evaporator and cause blockage. Secondly it can form acids by reaction with the freon refrigerants. This acid attacks the copper in the lines and deposits its in other parts of the system. This can become particularly troublesome when it is deposited on the compressor mechanical seal faces leading to damage and leakage.

Fine particles which could possible block the expansion valve are removed.

Topping up the refrigerant

A filling connection is fitted in way off the filter dryer, either directly onto it or on the inlet line after the inlet shut off valve. This allows additional refrigerant to be introduced into the system via the dryer element.

The normal procedure is to shut or partially shut the inlet to the filter. The compressor is now sucking from the system and delivering to the condenser where the gas liquifies. The filter dryer is on the outlet from the condenser therefore with its inlet valve shut the liquid level begins to rise in the reservoir. As the only gas entering the system is now coming from the top up line the compressor will tend to reduce the suction side pressure as it evacuates the system into the condenser.

The inlet valve can be briefly opened to allow more refrigerant into the system.

Thermostat and Solenoid Valve

These two elements form the main temperature control of the cold rooms.

The Thermostat is set to the desired temperature and given a 3 to 4 degree differential to prevent cycling. When the temperature in the room reaches the pre-set level the thermostat switch makes and the room solenoid is energised allowing gas to the refrigerant liquid to the expansion valve.

A manual overide switch is fitted as well as a relay operated isolating contact which shut the solenoid when the defrost system is in use.

System operation

Assume that the rooms are all warm and the compressor is running with all the solenoid valves open supplying refrigerant to the respective expansion valve and evaporator.

Should one or two rooms be down to temperature the solenoids close thus reducing the volume of gas returning to the compressor. The suction pressure drops and the compressor unloads. If more rooms shut down then the suction pressure will drop to cut out point and the compressor will stop. When the rooms warm the solenoids open again, refrigerant passes back to the compressor, the suction pressure rises and compressor starts. With more rooms opening, the suction pressure increases and the compressor loads up more cylinders.

Thermostatic expansion valve-

The purpose of this valve is to efficiently drop the pressure of the refrigerant. It achieves this by passing the liquid through a variable orifice giving a constant enthalpy pressure drop. The refrigerant at lower pressure has a corresponding lower boiling point (saturation temperature). Undercooling in the condenser increases the efficiency of the plant by allowing more heat to be absorbed during the vapourisation process. In addition it also reduces the internal heat absorption process that occurs during the expansion stage which is due to a small degree of flash off as latent heat (of vaporisation) is absorbed from surrounding liquid to reduce the temperature of the bulk liquid to the new corresponding saturation temperature for the reduced pressure

By this process of boiling (vapouriation) and latent heat absorption i.e. change of state, the refrigerant removes heat from the cold rooms.

The expansion process is controlled by the action of the bellows and push pins acting on the orifice valve plate. The bellows is controlled by a bulb which measures the temperature of the gas at outlet from the evaporator. To ensure no liquid passes through to the compressor, the expansion valve is set so that the gas at outlet from the evaporator has 2 to 3 degrees of superheat.

For larger systems where a significant pressure drop exists across the evaporator it is necessary to fit a 'Balance line'. This is a small bore tube which feeds the outlet pressure back to the thermostatic valve 'motor' element. Therefore the measured temperature is directly related to the superheat temperature at outlet pressure.

Some systems are designed so 5% liquid is available through the evaporator to coat the internal surfaces of the tubes increasing heat transfer efficiency.

Author Note

Careful note should be taken that system temperatures are set by the room solenoid and not by the expansion valve which are generally factory set and do not require adjustment.

This may seem an obvious fact but you would be amazed as to the number of broken valve plates removed from compressors due to the mal adjustment of the superheat.

Adjustment of the back pressure valves- which if they have not been touched by ships staff should be unnecessary- can allow better system balance especially when certain rooms are being starved of gas.

Back pressure regulator valve

This valve is fitted to the higher temperature rooms, vegetable and flour (+5^oC) only and not to the Meat and Fish rooms (-20^oC).

They serve two main purposes.

Firstly when all solenoid valves are opened they act as system balancing diverters, that is they restrict the liquid flow to the rooms which can be kept at the higher temperature and deliver the bulk to the colder rooms.

Secondly they serve to limit the pressure drop across the expansion valve by giving a set minimum pressure in the evaporator coil. This in turn limits the temperature of the refrigerant thereby preventing delicate foodstuffs such as vegetables from being damaged by having air at very low temperatures blown over them. Ultimately they may also be set to provide a safety limit to the room temperature by restricting the pressure to give a corresponding minimum saturation temperature of 0^oC.

Oil rectifier

In some installations there is a tendency for oil to collect in the evaporator under certain conditions such as low load when the speed of movement and agitation of the evaporating refrigerant are insufficient to keep the oil moving. To prevent loss of oil from the sump to the system, an oil rectifier may be fitted. The oil is automatically bled from the evaporator to a heat exchanger in which liquid refrigerant mixed with the oil is vaporised. The heat for vaporising the refrigerant is obtained by passing warm liquid freon from the condenser, through the heat exchanger. Vapour and oil are passed to the compressor where oil returns to the sump while the freon passes to the compressor suction. The regulator is thermostatically controlled valve which operates in the same way as the expansion valve on the main system. It automatically bleeds the oil from the evaporator so that the gas leaves the heat exchanger in a superheated condition.

Defrost system

Moisture freezes onto the evaporator eventually causing a restriction and reducing the efficiency of the plant. This must be periodically removed. For Veg and Flour rooms, were not restricted to 0^oC minimum by the back pressure valve, this is carried out once per day. For the Meat and Fish rooms this has to be carried out two or more times. Due to the low temperature in the rooms it is necessary to fit a drain heater.

When on defrost the solenoid valve is shut and the fan is off. On some systems at end of defrost the solenoid valve is opened momentarily before the fan is started. This allows moisture to be snap frozen onto the surface of the element, creating a rough increased surface area and thereby increasing the heat transfer rate.

Author note

Care should be taken after loading any great quantity of stores especially into the vegetable rooms. The fresh stores tend to sweat and icing up of the evaporator can become rapid. The only solution is constant monitoring and defrosting as soon as necessary.

Effects of under and over charge

The effects of overcharge are a full condenser/receiver gauge glass. System pressures are not effected until highly overcharged when a possibility of excessive HP pressure exists. Undercharge causes failure to maintain cold room temperatures and compressor cycling. Compressor cycling is caused by there being insufficient gas to maintain the compressor loaded even with all room solenoids open. In extreme the compressor will cut in and out. Undercharge is detected by low levels in the condenser/receiver gauge glass/ bubbles in liquid sight glass, compressor cycling and low suction pressures.

Troubleshoot

A ship had real problems with the control of room temperatures, one room in particular. attempts to 'balance' the system using the back pressure valves usually resulted in rooms starved of gas and/or the compressor tripping on Low Pressure trip. It turned out that sag on one or two of the liquid line pipes allowed oil and debris to build up in this section and restrict flow.

On another ship the lagging around a penetration piece had been damaged and water had got behind it into the insulation. This liquid had frozen and exerted a crushing force on the pipe sufficient to severely restrict the flow. This was only found after some searching as before the lagging was removed nothing wrong could be seen.

Circuit Breakers

The main purpose of a circuit breaker is to interrupt fault current as quickly as possible and so keep the damage to the other pieces of equipment to a minimum.
Generator circuit-breaker and other large circuit-breakers (600-6000A) on board ship are usually of the air break type. This means that the circuit-breaker contacts separate in air. (Ashore, comparable size circuit-breakers are often immersed in oil (OCB) and larger circuit-breakers for high voltage operation are either air blast, or have a special gas filing or a vacuum break).


Air Circuit Breakers (ACB) are mounted on special rails in the main switchboard cubicle, and must be racked out and isolated from the bus-bars for maintenance and testing. The ACB and its slide rails are usually mounted in a special cassette bolted into the switchboard cubicle and electrically connected to the bus bars. If repair demands that the ACB be completely removed from its cassette, then usually a special hoist or ‘fork-lift’ is required for large, heavy-duty breakers.
The action of withdrawing the ‘breaker’ causes a safety shutter to cover the live bus-bar contacts.
Mechanical linkage in the circuit-breaker is quite complex and should not be interfered with except for maintenance and lubrication as specified by the manufacturer.
The main fixed and moving contacts are of copper (sometimes of special arc resistant alloy or silver tipped) and most often silver coated. Main contacts should not be scraped or filed. If the main contacts suffer severe burnings they probably require realignment as specified by the manufacturer. Arcing contacts normally suffer burning and may be dressed by a smooth file as recommended by the manufacturer. Carborundum and emery should not be used – the hard particles can embed themselves in the soft copper contacts and cause future contact troubles.
The arc chutes or arc splitter boxes confine and control the inevitable arc, and to accelerate arc extinction. These must be moved and inspected for broken parts and erosion of the steel splitter plates.
The circuit breaker must b closed against powerful ‘throw-off’ springs which are later used to open the contacts in the tripping operation. In addition, if the circuit breaker is closed onto a fault, the electromagnetic effect of the fault current will attempt to open the contacts and will therefore act in opposition to the closing force. For safe operation when closing against a fault the contact should fully close before opening.
Consequently, as the fault rating of the circuit breaker increases, the closing force must also be increased. For high fault level equipment an operator may be unable to produce the force necessary to ensure correct closure, and all modern designs of ACBs now use either springs or solenoids.

Various types of closing mechanism may be fitted.

(a) Independent Manual Spring – The spring charge is directly applied by manual depression of the closing handle. The last few centimetres of handle movement releases the spring to close the ‘breaker’. Closing speed is independent of the operator.
(b) Motor Wound Stored Charge Spring – Closing springs are charged by a motor/gearbox unit. Spring recharging is automatic following closure of the ‘breaker’. Breaker closure is operated by a push button. This may be a direct mechanical release of the charged spring or it may initiate an electrical release via a solenoid latch.

c) Hound Wound Stored Charge Spring – This is similar to (b) but with manually charged closing springs.
d) Solenoid – The ‘breaker’ is closed by a dc solenoid energised from the generator or bus bars via a transformer/rectifier unit, contactor, push button and, sometimes, a timing relay.No index entries found.

WARNING – Circuit breakers store energy in springs;
(a) in store-charge mechanism in the closing springs and
(b) in contact and kick-off springs.
Extreme care must be exercised when handling circuit breakers with the closing springs charged, or when the circuit breaker is in the ON position.
Isolated circuit breakers when racked out for maintenance should be left with the closing springs discharged and in the OFF position.
Circuit breakers are held in the ‘closed’ or ON position by a mechanical latch. The breaker is tripped by releasing this latch allowing the kick-off springs and contact pressure to force the contacts open.

Tripping can be initiated by:

(a) Manually – a push button with mechanical linkage trips the latch.
(b) Undervoltage trip coil (trips when de-energised).
(c) Overcurrent/Short-circuit trip device (trips when energised).
(d) Solenoid trip coil – when energised by a remote switch or relay (such as an electronic overcurrent relay).

Mechanical interlocks are fitted to ACBs to prevent racking out if still in the ON position. Care must be taken not to exert ‘undue force’ if the breaker will not move – otherwise damage may be caused to the interlocks and other mechanical parts. Dangers of explosion and fire may also result from such action.
Electrical interlock switches are connected into circuit-breaker control circuits to prevent incorrect sequence operation, e.g. when a shore-supply breaker is closed onto a switchboard. The ship’s generator breaker are usually interlocked OFF to prevent parallel running of a ship’s generator and the shore supply.
Even an experienced operator can sometimes attempt to carry out these operations in an incorrect manner and interlocks are usually provided to prevent this.
Local and remote electrical indication will often be employed to show the state of a circuit breaker. However, whether or not electrical indication is provided mechanical indicators should be included in the circuit breaker design. These are used to show whether the circuit breaker is open, closed or isolated.
This function is sometimes provided by minim diagrams on the front of the switchboard an arrangement which is preferred by many engineers. These diagrams are of single line schematic type, showing the particulate circuit with ‘windows’ at the breaker position. A mechanical indicator behind the window shows the state of the circuit. When a spring charging mechanism is used the state of the spring should be indicated: either ‘charged’ or ‘free’.
For medium size motors moulded case circuits breakers (MCCB) are used. These are small, compact air circuit-breakers fitted in a moulded plastic case. They have a lower current rating (30 – 1500A) than air circuit breakers and generally a lower breaking capacity.
MCCBs usually have an adjustable thermal overload setting and an adjustable or fixed magnetic overcurrent trip for short-circuit protection built into the case. An undervoltage trip coil may also be included within the case.
MCCBs are usually closed by a hand operated lever but motor closing can also be fitted. MCCBs are claimed to be reliable, trouble free and require negligible maintenance. If the breaker operates in the ON position for long periods it should be tripped and closed a few times to free the mechanism and clean the contacts. Terminals should be checks for tightness otherwise overheating damage will develop. The front cover of larger MCCBs (around 400A rating) can usually be removed, interior dust blow out and the contacts dressed with a file if required. Following tripping under a short-circuit fault the breaker should be inspected for damage, checked for correct operation, and its insulation resistance measured. A reading of at least 4MΩ - 5MΩ is usually required. Any other faulty operation usually requires replacement or overhaul by the manufacturer.
MCCBs can be used for every application on board ship from generator breakers to small distribution breakers. The limited breaking capacity may demand that ‘back-up’ fuses be fitted for very high prospective short-circuit fault levels.
For small loads as is usually found in lighting distribution boards, and the like, miniature circuit breakers (MCB) are used.
MCBs are very small air circuit breakers fitted in moulded cases. They have current ratings of 5-100A and generally thermal overload and magnetic short-circuit protection. They have a very limited breaking capacity (3000A) and are commonly used in final distribution boards instead of fuses. The distribution board is supplied via a fuse of MCCB with the required breaking capacity.
MCBs must be replaced if faults develop – no maintenance is possible.
Handles for opening the doors on switchboard cubicles are usually linked (or interlocked) to an isolating switch. This ensures that supplies to components in the cubicle are switch off before the door can be opened.
Fused isolators are isolating switches that incorporate fuses. The action of opening the switch isolates the fuses so they can be replaced safely.
Fused isolators can also be interlocked to the cubicle door handle. Motor starters frequently incorporate this arrangement
One type of interlocked fused isolator can be completely withdrawn and removed to ensure complete safety when carrying out maintenance on equipment.

Maintenance on fused isolators consists of periodically checking the operating mechanism. Contacts must be inspected for damage and lightly greased with an electrical lubricant. The interlock mechanism (if fitted) should also be examined for correct, safe operation.