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 point above which
p-h diagram (Mollier)
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.
Meat Room-Low temperature room typically working at -17oC
Veg/ handling room-typically working at +4oC
Generally of the single stage, reciprocating type. Larger systems have multiple 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
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.
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.
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.
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.
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.
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 (+5oC) only and not to the Meat and Fish rooms (-20oC).
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 0oC.
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.
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 0oC 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.
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.
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.
Compressor bodies are normally of close grained castings of iron or steel. Modern valves are of the reed or disc type mounted in the head and are of high grade steel on stainless steel seats with a usual lift of about 2mm. Connecting rods are aluminium with steel backed white metal big ends. The crankshaft is spheriodal graphite iron.
The pistons are made from cast iron in older units, and of aluminium alloy more recently. The piston is attached to the crankshaft by con rod in the normal manner. It should be noted that the crankcase is full of refrigerant gas at suction pressure.
Liners are made from high tensile cast iron. Lubrication is generally splash only for smaller compressors with a crankshaft driven gear pump supplying bearings on larger machines.It is important to understand that actual pumped lube oil pressure is the indicated pressure less this crankcase pressure.
The properties of the Lubricating oil used in are compressors are critical and specific to the refrigerant gas used. The properties of this oil will be dealt with in the tribology section.
By the nature of the system a possibility exists whereby liquid may be passed to the compressor suction. To prevent serious damage, some form of unloading device is normally fitted. In this case the suction valve assembly is held on the liner by a heavy gauge spring. In the event of liquid passing to the compressor the suction valve will lift against this spring.
Should water enter the system, acids may be formed by the reaction with the refrigerant gas. This is especially true for freon systems. These acids attack the copper in the system- typically the pipework- and allow it to be transported through the system. It is not uncommon to find this deposited on the suction valve plate. More troublesome is when the deposit finds its way to the crankcase seal destroying the running face.
Thus the importance of maintaining filter dryers in good condition can be seen. These should be changed at least on a schedule determined by the ships planned maintenance system. In addition to this it is common to have liquid line flow bullseye which incorporate a water detection element. Blockage of the filter dryer can be gauged by feeling the filter. If it is cooler than the surrounding pipework then the gas is being throttled through it.
Although not considered good practice in an emergency I have 'dried' the filter drier element in the galley oven although this practice is not recommended but does work.
It should be noted that for this design the carbon seal and flexible bellows is fixed in way of the mounting plate and the hard running surface is allowed to rotate. This is the opposite to the set up for seals mounted on pumps.
The finish of the running surface of the seal is extremely fine. However, in extenuating circumstances i.e. when the surface has been damaged say by the deposit of copper, it is possible to lap the face of the carbon. The method I would recommend is metal polish such as brasso, on a true flat surface on which is laid chart paper. The chart paper absorbs the wear particles as they are removed an a reasonable finish is possible.
Such compressors are used mainly in house hold applications but modern practice sees there use in cargo conditioning.
A variation on this is the multi blade type where the rotor has slots cut in it, fitted to which are spring loaded blades. Alternately the blades may rely on centrifugal force.
With both these types , when the compressor is stopped the sealing pressure and oil film are broken and there fore the suction and discharge are common. This reduces starting loads but requires a suction non return valve to be fitted.
Where these are fitted to large refrigeration systems it is possible to use variable speed thyristor controlled electric motors. Thereby the compressor can run at optimal revolution to maintain plant efficiency