Most refrigerating plants are automatically operated to some extent, as manual control of the various refrigerating functions is difficult and not very accurate. Moreover very skilled operators or technicians must be employed for such tasks.
The following operations are more in need of automatic control than others: regulation of temperature and relative humidity in the cold chamber; feeding to the evaporator; adapting the refrigerating capacity of the plant to one heat load of the cold room at different ambient conditions; regulating condensing temperature; starting the compressor operation sequence; and initiating the sequence for the defrosting cycles.
Where control is automatic safety devices must be installed to protect the equipment against malfunction. These safety devices are not only for the refrigerating system but also for the electric circuit, though the latter concerns other specialists and is not within the scope of this handbook.
When a safety device is in operation it indicates there has been a breakdown or a malfunction. It is necessary to search for the cause of the trouble and repair it before the system is restarted, otherwise the safety device will operate again and the malfunction may cause further problems.
Safety devices only operate occasionally and some do not operate for long running periods. Consequently it is advisable to check periodically that they operate properly and adjust their operating ranges. These checks should be scheduled once a month for high-pressure, low-pressure and lubricating oil pressostats as these prevent any malfunction of controlled elements and they undergo frequent on-off operation during the running cycles.
Repairs and maintenance must be entrusted to competent refrigeration mechanics because of the risk to personnel, particularly in ammonia plants. Intensive training should be provided for local mechanics and technicians.
The following section describes the main safety devices and the elements they control.
HIGH-PRESSURE SAFETY PRESSOSTAT
High condensation pressure can be caused by any of the following.
Faulty cooling of the condenser, which may be because of: the failure of pumps in water-cooled condensers or of fans in air-cooled condensers; failure of the water circuit which does not circulate sufficient water or because the water temperature and the equivalent air temperature are too high; and finally, fouling of the condenser surface (scale, algae and dust in air-cooled condensers).
The heat exchange surface area is reduced by refrigerant liquid accumulation in the lower part of the condenser. This may be the result of an excessive refrigerant fluid charge in the system or irregular operation of the pressure control.
Non-condensable gases present in the circuit, particularly with ammonia installations. In this case a purge is necessary.
The automatic control of the condensation pressure is adjusted to an unusually high level.
A fire on the premises, producing a quick rise in circuit pressure.
The high-pressure pressostat controls most of the risks derived from an unusual increase in pressure. A high temperature level is usually considered to be in the 40–60°C range. However this safety device is not enough to protect the refrigeration plant and some other pressure cut-out device such as a safety pressure relief valve, a fusible plug, a rupture disk or a fuse are necessary.
Whenever high-pressure vapour breaks out it must be evacuated from the environment without any risk to personnel or people in the machine room or surrounding areas. Sometimes the high pressure is released into the lowpressure circuit, which should be provided with a leakage restrictor. A highpressure pressostat must always be fitted when water-cooled condensers are used, and the connection must be taken from a point in the discharge line where it cannot be inadvertently isolated.
The cut-out pressure should be set well above the maximum condensing pressure when the plant is operating at full load; 14 kp/cm2 gauge pressure is recommended as the cut-out setting. This is equivalent to 60°C for and aircooled halogenated refrigerant plant; 19 kp/cm2 gauge pressure is equivalent to 50°C for a water-cooled ammonia plant.
The pressure differential should be established according to the operation and bearing in mind that a small differential will result in compressor shortcycling that may cause electrical damage, while large differentials will result in too long off-cycles, leading to an abnormally high rise in storage temperature.
A high-pressure pressostat must be manually reset to prevent any risk of automatic on-cycling before the malfunction has been found and repaired.
LOW-PRESSURE SAFETY PRESSOSTAT
The low-pressure safety pressostat or cut-out operates when the pressure in the suction circuit decreases below a set level, with the risk of air entry into the circuit if it drops lower than atmospheric pressure. The most common risk is that of very low evaporation temperatures with possible damage to the stored produce and/or the refrigeration plant.
Low-pressure pressostats break the circuit to the compressor motor when evaporating pressure falls. There are several causes for low evaporating pressures.
The evaporator is not correctly fed or is not fed at all. This may be owing to the blockage of the expansion valve or its filter, or the solenoid valve upstream of the expansion valve is not in operation, generally because the coil is out of order and thus the automatic valve operated by the solenoid valve is not working.
Compressor capacity clearly exceeds evaporator capacity. This may be because of malfunctioning of the compressor capacity control device; the evaporator fans may have stopped accidentally, reducing heat exchange; there may be excessive frost deposit on the evaporator leading to the same heat exchange problem; or the constant pressure valve regulating the evaporating temperature may not be properly adjusted. If there is a very small difference between room and evaporator temperatures the suction pressure may be too low.
The refrigerant fluid charge in the circuit is too small, which may be because of an installation error or a small leakage difficult to discover.
Sudden and unusual oil dragging into the low-pressure circuit, which both diminishes heat exchange and increases pressure loss in the suction line.
The thermostat is blocked in an open position, keeping the compressor in operation while the thermal load of the evaporator decreases continually.
The pressostat must be set to cut-out at a pressure well below the lowest normal evaporating pressure when the evaporator is working at minimum load, but above a certain pressure to avoid freezing or cold damage to the produce.
Small differentials will result in compressor short-cycling so they must be avoided to prevent equipment damage. On the other hand large differentials will result in long off-cycles.
When the low-pressure safety device is in operation a limited risk is avoided, which does not represent any great danger to the refrigerating plant. Therefore the pressostat may or may not be manually reset, though for high-capacity machines manual resetting is recommended.
These safety devices can suffer from such mechanical failure as leakage of the pressure bellows (they must be replaced), obstruction or partial blockage of the small connecting pipe of the control element with pipe scale and/or sludge (isolate and remove the pipe, flush with refrigerant R.11) or the linkages may be worn, damaged, stuck or loose and must be replaced or readjusted.
OIL-PRESSURE CONTROL
Faulty lubrication of the compressor can be caused by either of the following.
Not enough lubricating oil in the compressor crankcase. This may be because of error in the quantity of oil charge to the circuit, especially when halocarbon refrigerants are used; entrapment of oil in the circuit, which does not return at a proper rate to the crankcase; some oil leaks are present and the oil purge from the oil and liquid separators is not correctly conducted.
The oil pump is accidentally out of order, probably because of breakdown. The suction and/or delivery oil pipes are obstructed and the pressure of the pump is not enough to overcome this.
The oil-pressure control stops the compressor when the useful pressure developed by the pump falls below a fixed minimum or the oil pressure does not rise to the minimum safety level within an established period. Useful pressure is the difference between total pressure and suction pressure. Therefore the oil-pressure switch, which is activated by useful pressure, must be connected by two pressure bellows to the crankcase and the oil pump discharge.
A time delay relay is incorporated in the oil-pressure control to allow the compressor an operating period of about 30–120 seconds with oil pressure below safety level. If the control starts operating as a result of malfunction in the compressor lubricating system, a lamp and/or a hooter (visibleaudible alarm) will be activated. The latter can be manually disconnected. After failure and before the compressor can be restarted the oil-pressure control must be manually reset because of the high risk involved in its operation.
The compressor manufacturer's instructions usually set the cut-in and cutout pressures for the oil-pressure control, but whenever such data are not available they should be set at about 0.35 kp/cm2 absolute pressure below useful pressure level and approximately the same quantity below cut-in pressure.
DELIVERY THERMOSTAT
This safety device is used only for ammonia machines as there is the risk of increase in compressor delivery temperature above 130°C. This temperature is dangerous as the lubricating oil may break down, notwithstanding high temperature decomposition, and form corrosive acids and sludges. There are several causes for an excessively high delivery temperature.
The system is operating under an excessive compression ratio (absolute discharge pressure divided by absolute suction pressure) especially when it is linked to a refrigerant fluid with a high polytropic compression exponent, for instance ammonia.
The suction pressure is very low because the constant pressure valve is not correctly regulated. The lower the suction pressure the higher the discharge temperature, other circumstances remaining constant. Again the two-stage compression systems offer an advantage, as they avoid high discharge temperature.
Suction superheat is excessively high due to poor insulation of the suction pipe and/or because it is too long. In an ammonia refrigerant system an automatic humidifier is an advantage where the vaporization of partly vaporized liquid ammonia injected in the suction line from the liquid receiver provides the necessary desuperheating of the vapour.
The delivery superheat thermostat cycles-off the compressor if the discharge temperature is too high. When in operation it avoids serious risk to the compressor, so it must be manually reset.
WATER-REGULATING VALVE
As well as the high-pressure pressostat already described, water-cooled condensers must be provided with a complementary device for controlling water flow. In a waste water system the regulating valve is installed at the inlet of the condenser and is activated by the compressor discharge. The valve modulates the flow of water through the condenser in response to changes in condensing pressure, tending to maintain the condensing pressure constant, though considerable differences occur with changes in the operating cycle (higher pressures during periods of peak loading). When the compressor is shut off, the condensing pressure decreases rapidly and the water flow is also completely cut off with a well-adjusted valve. This results in water and power savings. The water valve shut-off pressure should be set at the refrigerant saturation pressure corresponding to the maximum ambient temperature of the hottest expected period at the refrigeration plant location. The opening pressure of the valve should be set at about 0.5kp/cm2 above shut-off pressure.
When several condensers are installed each must be provided with a regulating valve that will modulate water flow. Installation costs are high, so the refrigerating plant should be equipped with just the necessary number of condensers. The water-regulating valve works well, does not usually present any difficult and its mounting in the water line is simple. Two problems may arise with water flow, which may be insufficient or too high. The causes of insufficient water flow can be poor adjustment of the valve, the strainer and/or the valve seat are dirty and require cleaning or the valve bellows leak and must be replaced. If the
water flow is too high the reason may be poor adjustment of the valve, the return spring is broken, water pressure is too high and needs to be reduced or the valve seat is dirty and requires cleaning or it is worn and needs replacement.
water flow is too high the reason may be poor adjustment of the valve, the return spring is broken, water pressure is too high and needs to be reduced or the valve seat is dirty and requires cleaning or it is worn and needs replacement.
EXPANSION VALVES
The expansion of the liquid refrigerant fluid from high to low pressure is achieved by passing it through a device known as an expansion valve. Expansion valves have two functions: metering the liquid refrigerant into the evaporator and maintaining a pressure differential between the high-and low-pressure sides of the refrigerating circuit.
There are several types of expansion valve, all automatic. In industrial plants it is advisable, regardless of the type installed, to set up a handoperated controller in parallel with the main expansion valve to avoid plant shut-down if the valve should break down. A solenoid valve is mounted upstream of the expansion valve; in industrial plants both elements as a whole are placed between two manually operated valves to isolate them for maintenance or repairs. This is not done in medium- or small-capacity installations.
Thermostatic expansion valve
A solenoid valve in the high-pressure liquid line to protect the expansion valve is a compulsory element when a thermostatic expansion valve is installed and it can also serve to protect any other flow control element.
The thermostatic expansion valve is fundamentally a needle valve activated by a bellows or diaphragm and a remote bulb opening on to the valve side through a capillary tube. With a few exceptions the fluid in the remote bulb is the same refrigerant as that used in the refrigerating circuit.
The position and installation of the remote bulb are of paramount importance for the accurate functioning of the valve. It must be firmly clamped to the suction line at the outlet of the evaporator, the groove of the bulb fitted against the side of a vertical pipe and on a horizonatal pipe attached to the top at a 10 or 2 o'clock position; it should be far from any fitting or point where liquid can accumulate. The outer surface of the suction pipe must be thoroughly cleaned, removing all grease and moisture. The bulb and part of the suction pipe must be insulated, usually with rubber foam.
The amount of superheat required to bring a thermostatic expansion valve into equilibrium is fixed by adjusting the tension of the spring in the valve, called the superheat adjustment. A high degree of superheat is usually undersirable as the effective heat transfer area of the evaporator is reduced; conversely, if the superheat is set too low the valve will lose control of the refrigerant flow and the evaporator will alternatively starve and overfeed, creating very different operating conditions. The valves are usually correctly adjusted for a superheat of 5°C. The main problem with the thermostatic expansion valve is that it cannot maintain a certain evaporator temperature and pressure; as these are influenced by the thermal load of the evaporator the refrigerant boiling temperature decreases with room temperature.
The main operating faults of thermostatic valves are the passing of too much or too little liquid. When the valve does not control liquid flow properly and it is too high, the fault is easily noticeable as the suction pipe sweats excessively and even accumulates frost, the compressor cylinder heat is cooler than normal and evaporating pressure is higher than normal. This may be due to poor valve adjustment, to wax or ice crystal deposits on the valve seat that block its closing or to the valve needle being stuck in the open position. A refrigeration engineer can easily fix these faults by readjusting the valve, cleaning it and ensuring the oil is suitable for the evaporating temperature, and replacing damaged valves. The same problem of liquid flow appears when the bulb is not secured properly to the pipe or is not correctly insulated. The bulb should be tightly refitted and/or the insulation changed.
If the evaporator pressure is lower and the compressor cylinder head is warmer than normal, the liquid refrigerant fed to the evaporator will not be sufficient. This may be due to several problems with the valve itself or with the bulb charge. The valve may be poorly adjusted; if so it must be readjusted. The inlet filter may be dirty, preventing liquid refrigerant from flowing freely through it, in which case it should be cleaned. There may be wax or ice crystal deposits preventing the valve from opening, so it must be cleaned. If the valve is stuck in the closed position it must be dismantled and repaired or discarded.
To overcome pressure drop in the refrigerant in the evaporator, which results in a considerably lower saturation temperature at the evaporator outlet than at the inlet, an externally equalized thermostatic expansion valve should be installed if the refrigerating plant is of high capacity, as evaporators produce an excessive pressure drop.
Automatic expansion valve
The automatic expansion valve maintains a constant pressure in the evaporator by more or less looding the evaporator surface, depending on the heat load of the cold chamber. The main disadvantage of this expansion valve is its relatively poor efficiency. It has been mainly used in store applications where the cooling load is fairly constant and low evaporating pressures must be avoided. However, it is seldom used nowadays. Its operating faults, their cause and remedy are practically the same as those encountered with the thermostatic valve, with the exception of those corresponding to the bulb. Sometimes a too large valve is installed leading to erratic operation. This can only be corrected by fitting the correct capacity valve.
Low-pressure float valve
The low-pressure float valve is used in industrial plants to maintain a constant liquid level in the accumulator. The evaporator is therefore constantly filled to the desired level with liquid refrigerant in all conditions of heat load and independently of evaporator temperature and pressure. The vapour pressure in this flow controller and in the accumulator is the evaporation pressure.
If the float valve allows too much liquid to pass through its control evaporating pressure will be higher than normal and/or unevaporated liquid will return to the compressor, causing pipe sweating and eventually knocking. Faulty operation may be due to the valve seat not being properly cleaned, or to the ball float being punctured in which case it must be replaced (there is the risk of high-pressure liquid being trapped inside the float, so careful handling is required) or finally, the operating linkage may be jammed and must be dismantled, cleaned and lubricated.
The float valve may not allow sufficient liquid to flow, the low-pressure circuit exhibiting a lower than normal evaporating pressure and the cooling effect being noticeably reduced. This malfunction may be due to a dirty inlet filter. The operating linkage may not be moving freely and if so it must be dismantled, cleaned and lubricated. Finally the accumulator chamber may be filled with gas, either because it is not properly insulated to maintain constant temperature and pressure or because there is an obstruction in the vent pipe.
High-pressure float valve
The high-pressure float valve is similarly a liquid-activated refrigerant flow control, located on the high-pressure side of the refrigerating circuit and controlling indirectly the amount of liquid in the evaporator by maintaining a constant liquid level in the receiver. It also ensures a continuous liquid flow toward the low-pressure circuit. The bulk of the refrigerant charge always remains in the evaporator, which is advantageous as the receiver can be small.
The operating principle of this valve is based on the perfect equilibrium of the refrigerating system as the vapour is always condensed in the condenser at the same rate that the liquid is vaporized in the evaporator. Therefore the high-pressure float rate will continuously and automatically feed the liquid back to the evaporator at the rate of vaporization and will close the circuit whenever the compressor stops.
To ensure correct feeding it is only necessary to install a single float valve for each evaporator. The surge drum for the flooded-type evaporator should have enough volume to hold the liquid. In order to avoid flooding back or slugging, a volume equal to at least 25 percent of the evaporator volume is recommended.
The refrigerant charge in the refrigerating plant is critical for this type of flow control. An overcharge will cause evaporator overfeeding, which risks liquid refrigerant flooding back to the evaporator. A more serious overcharge will impede the reduction of evaporator pressure by the compressor to the desired low level. If the system is undercharged the operation of the float valve will be erratic and the evaporator will be starved. When the refrigerant charge is seriously reduced (leaks not detected) the total amount of liquid is not enough to reach the minimum level that opens the valve and installation capacity will be nil.
The high-pressure float valve can be installed either below or above the evaporator level and it should be as close to the evaporator as possible. As it is generally placed close to the receiver the piping to the evaporator, which is very long, must have proper thermal insulation. Also, an intermediate pressure-reducing valve should be installed in the liquid line at the evaporator inlet so high pressure will be maintained in the line. This pressure-reducing valve is used in industrial systems when the thermal load is markedly constant.
The most common operating faults of high-pressure float valves are, first, too much liquid passing to the evaporator due to an overcharged system. The refrigerant must be slowly purged until correct operating conditions are restored. Second, if insufficient liquid passes to the evaporator the evaporating pressure becomes lower than normal and the cooling effect decreases. The causes are similar to those with low-pressure valves (inlet filter dirty, ball float punctured or operating linkage jammed) and can be similarly solved. The system may also be undercharged because of refrigerant fluid leakage. Leaks must be traced and repaired and the system then carefully recharged to prevent overcharging. There may be a complete loss of cooling. This is indicated also by the equilibrium of the evaporation and condensing pressure. This is caused by the valve remaining open, usually because of dirt accumulation in the valve seat or because the linkage is jammed. The valve must be repaired, though generally cleaning and lubricating are enough.
A high-pressure float valve can be used in the oil separator to assure the automatic return of the lubricating oil to the compressor crankcase.
SOLENOID VALVES
Solenoid valves are widely used in refrigerating plants to control automatically not only fluid refrigerant flow (liquid line and evaporator outlet) but water, brine or any other liquid flow, for instance in the condenser water cooling line. When gas defrosting is used solenoid valves are fitted in the refrigeration circuit to reverse refrigerant flow.
A solenoid valve is simply an electrically operated valve consisting of an electromagnetic coil which when energized draws a plunger which opens the valve port. Closing action is achieved by gravity when the coil is deenergized. Solenoid valves controlling the refrigerant flow in the liquid line feeding the evaporators are thermostatically activated.
Solenoids vary according to permitted pressure differences across the valve, drop in pressure through the valve, the desired flow rate and the state of the circulating fluid. The valves can be direct acting or pilot operated. Small solenoid valves are usually direct acting; they are fitted in smalldiameter pipes and their power demand is low, about 15 Watts.
Pilot-operated solenoid valves are used in large-diameter pipes as the pressure differences across the valve provide the force to carry out the closing and opening actions.
Solenoid valves are simple and robust, and their functioning is generally reliable and accurate as they usually operate in an on-off mode. This is also true when they work in a modulated fashion (applying a changeable voltage to the electromagnetic coil of the valve).
They have some drawbacks. They are not gastight, particularly for large pipe sections. Their most usual failure is coil breakdown, which is not foreseeable so it is impossible to detect during maintenance operations. Unless they are vapour-tight condensation of water vapour on their cold side may occur, particularly when they are installed in rooms with high humidity and working in medium-temperature conditions. There is also some risk of causing a “water hammer” effect in the liquid line.
Solenoid valves must be correctly installed. They must always be mounted in a vertical position with the coil on top, unless they are especially. designed for horizontal installation. They must also be mounted in line with the direction of flow, usually shown by an arrow on the valve body.
Some operating faults may appear when a solenoid valve is at work. If the valve fails to open the cause could be mechanical or electrical. Mechanical problems follow when the valve is not mounted level on the pipe or when the plunger is stuck to the valve seat. Electrical failures could be because of dirty or loose electrical connections, no power supply, and/or a damaged coil. It is advisable to have several coils in stock for each type of solenoid valve. If the valve does not fully close the valve seat may be either dirty or worn. The valve should be dismantled and the valve seat and plunger cleaned or renewed, or a new valve should be fitted. When the valve is noisy during operation it has obviously not been correctly installed and is not level on the pipe; an alternative could be that the electrical connections are not sound.
THERMOSTATS
Although the fundamentals of these elements and the role they play in room temperature control have already been discussed, it is useful to review them briefly regarding faulty operation, its causes and remedies.
A problem with the pressure bellows type of element is they may lose their fluid charge. To check this compress the bellows by hand. If they move under finger pressure the fluid charge has been completely or partially lost. The bellows assembly must be replaced, if accessible, otherwise the thermostat must be changed.
Electric failure of the thermostats may be due to poor electrical connections (clean and tighten them for correct operation) or to worn, pitted or corroded contact points (replace them). These mechanical or electrical failures will cause the thermostat to remain either open or closed. When it stays open the compressor will shut down even if the room temperature is above the desired level. The use of a jumper lead across the terminals should immediately restart the compressor.
When the thermostat remains closed the compressor will continue running even if the room temperature is below the established level. Test by turning the control knob to a higher temperature.
Faulty operation may also be traced to the incorrect installation of the sensing element, usually the fluid bulb. If it is not securely attached to the evaporator surface and/or it is not sufficiently clean and dry for good thermal contact, an excessive temperature difference is necessary before the thermostat operates.
When the thermostat directly controls space temperature the bulb is fixed to the chamber wall with a metallic bracket in one of the positions already described. If there is no thermal insulation between the bulb and the bracket the equilibrium temperature of the bulb will be higher than that of the room space.
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