Maintenance of industrial refrigeration units

During operation the ambient energy (service and maintenance personnel) and the energy of internal sources, connected with compression of refrigerant and cooling medium, and of sources accumulated the energy, has the influence on the refrigeration unit. When transferring mechanical, electromagnetic, internal and chemical energy in the form of work and heat, the processes of different nature can occur in the refrigeration unit elements, these processes are accompanied by the forces action, which leads to some changes in their initial properties. For example, a mechanical energy is supplied to the compressor, pump and fan for committing a workflow. Here the whole kinematic chain is subject to the action of mechanical force. As a result, the properties of the refrigeration unit elements can change.

Condensing-unit-Bitzer

It is impossible to exclude the undesirable change of properties (aging) of refrigeration unit elements. But knowing the cause and the matter of aging we can use the unit so to keep a working order (or only working capacity) during the specified operating time.

Aging process causes the degradation of object technical state. But the properties change can have a reversible nature, for example, if it is connected with material elastic deformation, corrosion products deposit and oil decomposition, scale formation, filters clogging etc. Damages and failures caused by these effects can be rectified at the expense of rather simple and not labour-intensive operations called maintenance.

Maintenance usually includes the object technical state monitoring, preventive operations (cleaning, lubricating, adjusting and others) and replacement of defective elements.

Technical inspection, which purpose is a working order check (or only working capacity) at industrial refrigeration units, is carried out by the operating personnel who examine the unit state visually (i.e. by means of sense organs) or with the help of measuring means, who record periodically an operation mode in a daily log (when there is no automatic recording) and make a decision upon the unit control on the basis of parameters values and characteristics of functioning.

Compressing units

The technical state of compressing units is monitored by means of operating parameters measurement, visually or with the help of technical means by functioning features, for example, leaks presence, oil state, noise, vibration and others.

The refrigerant leak is detected with the help of indicators, leak detectors and gas-analyzers, and the place of leakage – by using an indicator or a leak detector. The gland leakproofness is checked as per the quantity of oil drops dripping out per time unit.

Mechanisms operation is followed by mechanical and acoustic vibrations. As a rule, some defined levels of vibration and noise correspond to the operating condition of equipment.

The information upon the state of conjugation parts (gland seals, sliding and rolling bearings) can be obtained by the measurement of casing surface temperature or lube oil temperature.

In operation process the lube oil is oxidized, partially decomposed and contaminated with wear particles and decomposition products. As a result, it loses its quality. For this reason the check of oil state is required for oil change when reaching the maximum permissible state mentioned in normative and technical documentation (NTD).

The working capacity of lubrication system is characterized by the number of characteristics given in NTD. For example, oil level in the crankcase of reciprocating compressor and in the oil separator of screw compressor, pressure difference upstream and downstream of the pump, oil temperatures in the crankcase of reciprocating compressor and at inlet and outlet from screw compressor, pump leakproofness and oil state.

Screw-compressors-Bitzer

Lubricants

Lubricants (oils) used for compressors lubrication can be mineral or synthetic. Mineral oils based on petroleum are the most widespread. Synthetic lubricants can be based, for example, on alkylbenzenes, polyglycols, polyethers and other substances.

Synthetic oils have the higher indexes of properties, but they are more expensive than mineral oils. Universal refrigeration lubricants that meet the conflicting requirements to oils to the same extent do not exist yet. The oil which dissolves with refrigerant under equal conditions is a preferable one.

At present the following oils are used: mineral oils when using refrigerants R717, R744, R290, R600a and new synthetic oils based on polyethers and polyalkyleneglycols when using hydrofluorocarbons (R134a, R32, R125), their binary (R507, R410A) and triple mixtures (R404A, R407C), which are dissolved in them.

Centrifugal pumps

At centrifugal pump operation the following is checked in general: discharge and suction pressure (or their difference), connections leakproofness, noise and vibration levels, lubricant presence, gland and bearings temperature.

The pressure difference, created by a pump or only discharge pressure, if suction pressure is constant, characterizes the pump volume. The decrease of pressure, which the pump develops, may occur due to the following reasons: the leakage increases owing to the impeller and casing wear-out; the hydraulic circuit resistance decreases.

The decrease of noise and vibration level of pump is usually connected with the air inflow through gland and suction pipeline leaks, with cavitation and shafts misalignment.

The raise of bearings temperature (higher than 60 °С) is usually caused by worsening of lubrication rate.

For centrifugal leakproof pumps the technical inspection is foreseen at intervals not less than once every three months.

Condensers

Maintaining of evaporating condensers includes the monitoring of following parameters:

  • condensation pressure and temperature;
  • temperature of feeding and cooling water;
  • condenser inlet air temperature and humidity;
  • connections leakproofness;
  • state of water-distributing device (nozzle spray angle, uniform watering of tubes bundle);
  • vibration and noise levels.

When maintaining the water-cooled condensers, the following should be periodically done: measuring of condensation temperature and pressure, of water temperature at inlet and outlet from the device; checking of connections leakproofness, including tubes in the device when there is a refrigerant in the condenser outlet water; periodical removal of the oil from ammonia condensers.

When maintaining the air-cooled condensers, the following should be periodically checked:

  • condensation pressure and temperature;
  • air temperature at inlet and outlet from the device;
  • vibration and noise levels;
  • connections leakproofness.

During the quarterly maintenance inspection, the state of fans, fittings and instrumentation equipment is checked, the evaporator sump and the cover of shell-and-tube condensers is cleaned from dirty, the surface of air-cooled condensers batteries is washed with a solution.

Condensers-Bitzer

Evaporators

During the operation of evaporators intended for refrigerant cooling, the following should be periodically done: recording of boiling temperature and pressure, evaporator outlet vapour temperature; refrigerant inlet and outlet temperature, and also checking of refrigerant and cooling medium levels, connections leakproofness whether there is a refrigerant in cooling medium, the cooling medium concentration in a solution.

Depending on maintenance conditions the corrosion inhibitor may be periodically brought to the cooling medium; the oil is removed from the refrigerant cavity after the device warming-up; the air is discharged from closed evaporators through air cocks on the device covers.

The regulated technical inspection to be carried out every three months provides for the check of the instrumentation equipment operability, the state of fittings and protectors for electrochemical corrosion protection and mixers in open evaporators and accumulators.

Cold-producing devices

When maintaining the cold-producing devices, filling them with a refrigerant (or with a cooling medium) is visually checked by presence of frost and its thickness.

The cold-producing devices operability is provided by periodical defrosting and oil removal.

The regulated maintenance of air coolers, carried out once every three months, includes the check of the electric motor winding resistance (not less than 0.5 Mohm), the strength of earthing wire fastening, the presence of consistent grease in bearings.

Capacitive tanks (containers)

When maintaining the linear, drain, protective, circulation receivers and intermediate containers the following should be periodically checked: the refrigerant pressure and the device outlet vapour overheat in intermediate containers, and the liquid refrigerant level.

The oil is periodically removed from the devices, if it is not dissolved in the refrigerant, and the air is purged from the linear receiver, if there is no automatic air separator, the devices tightness is checked. The regulated technical inspection to be carried out every three months provides for the check of the instrumentation equipment state and the state of devices fittings.

Reseivers-Bitzer

Technological pipelines

The technical state of inter-plant pipelines is periodically checked by the maintenance personnel who visually check the state of welded seams, flange connections, supports, suspensions, heat insulation, anticorrosion protection etc.

Commercial refrigeration equipment (CRE)

In the process of CRE operation the following is checked: the air temperature in the refrigerated enclosure, the vibration and pulsation level when there is a built-in refrigeration unit, operating rate of the refrigerated enclosure. Ordinary CRE is only meant for storage of pre-cooled or frozen products. Loaded products in the open equipment should not exceed the refrigerated enclosure limits to prevent the disturbance of air screen operation.

Carry out the technical inspections of the open high-temperature and medium-temperature equipment – weekly, of the closed equipment – monthly, and of low-temperature equipment – quarterly.

Deenergize the CRE. Discharge the products. Remove the stock which is easy to take away (shelves, grilles and others) and dirty accumulated in the sump bottom. Wipe the equipment outer surfaces painted or made of non-corrosive metal using the cloth wetted in a washing solution and then wipe them with dry cloth. At first wipe the aluminum surfaces with wet cloth and then with dry cloth. Wash the inner surfaces of equipment and stock with a solution, rinse with water and dry.

Oil-separators-Bitzer

Defrosting of cold-producing devices

Frost formation on a heat-transmitting surface of cold-producing devices leads to the increase of transferable heat flux within the first hour of operation. In the following operation hours as the frost layer thickness grows the transferable heat flux reduces according to the exponential law. That is why the cold-producing devices are required to be defrosted for keeping the acceptable value of heat flux.

When considering the cycle operation of the compartment to be cooled in rather long period of time, you can notice that frequent defrosting improves the heat transfer of cold-producing devices thus it increases their heat flux and lessens costs connected with functioning of cold-producing devices. However the expenditure of energy and unproductive time connected with defrosting grows. And vice versa, less frequent the cold-producing devices are defrosted, less the heat flux and more the expenditure of energy and unproductive time is.

There is defrosting optimized frequency, at which, for example, the minimum efforts for operation or maximum heat flux of the compartment to be cooled are provided. It is difficult to solve this optimization problem due to the complications of frost formation process investigation. Therefore, in practice be guided by the following principle – not lower than the specified level of effectiveness.

It is supposed that the reduction of heat-flux density of cold-producing devices due to the frost formation should not exceed 15…20 % from its maximum value. And the frost layer thickness (approx. 2 mm for air separators) or pressure drop in the air flow duct of air cooler (approx. 0.15 kPa) is a particular characteristic of defrosting process.

Defrosting of cold-producing devices of industrial enterprises chambers is mainly carried out by using hot vapour of compressors discharge refrigerant. Defrosting is performed by the maintenance personnel of compressor shop in compliance with the approved schedule according to the peculiar instruction.

Thus prior to batteries defrosting, cover the load located under them (for example, with a tarpaulin) to prevent the worsening of its appearance and to simplify the further removal of melt water and frost.

Turn off the cooling mode of chamber batteries having closed the proper valves on liquid and vapour manifolds. Reduce the pressure in a drain receiver having opened the valve on the pipeline which connects it with a circulation (or protective) receiver (CR). The valve remains in open position during the entire defrosting process, if, for example, the high pressure float regulator (HPFR) is installed in the drain pipeline. The HPFR excludes the supply of high pressure vapour from cold-producing devices in the drain receiver and provides the condensate drainage as it accumulates in HPFR casing.

After pressure decrease in the drain receiver, open the shutoff valves on the drain pipeline and hot vapour supply pipeline. During defrosting of cold-producing devices the pressure being shown on a pressure gauge placed on a defrosting manifold (DM) should not exceed the value of test pressure set for cooling batteries data.

The defrosting process is finished when the heat-transmitting surface of cold-producing devices is free of frost.

After defrosting, stop the hot vapour supply and the condensate drainage having closed the respective shutoff valves.

The refrigerant collected in the drain receiver is held for some time in order to raise the temperature and stratify the refrigerant and the oil. The oil is removed from the drain receiver into the oil receiver and the remaining liquid refrigerant is forced over in cold-producing devices through the distributive manifold of control station by closing the valve in the line of liquid refrigerant supply from the linear receiver.

Perform the air coolers defrosting with the refrigerant hot vapour like the batteries defrosting. During defrosting the suction and discharge valves must be closed, the electric motors of air coolers fans must be switched off, and the valve on defrosting and drain pipelines must be open. The sump and the pipeline, along which the water formed from melted frost is removed from the sump, are heated slower than heat-transmitting tubes. That’s why start heating of water removal pipeline, usually heated with flexible electric heaters, 15-20 min. earlier than heating of tubes. Supply the hot vapour into the sump coil first and then in tubes.

Perform the air coolers defrosting with the help of electric heaters in the following sequence. Decrease the pressure in drain the receiver having connected it with a circulation (protective receiver). Switch on the air coolers defrosting mode – disconnect them from evaporation system, switch off the fans electric motors, connect with the drain receiver and switch on the electric heaters. After defrosting, switch on the air coolers cooling mode carrying out the operation in the reverse sequence. And after a while remove the oil and the refrigerant from the drain receiver.

Corrosion protection

Refrigeration unit elements, contacting with the contaminated atmospheric air, cooling medium, water, soil, are subject to destructive effect of various types of corrosion which shortens their service life, and the corrosion products depositing on heat-transmitting surfaces increase the thermal resistance in heat exchangers.

Corrosion rate, usually measured by the thickness of destructed material (mm), depends on material type (its standard equilibrium potential), medium composition and external conditions (temperature, pressure, motion speed). For example, the atmospheric corrosion rate increases as atmospheric moisture increases where there are gaseous (НСl, SO2, NH3, Cl2) impurities. Electrochemical corrosion rate increases in acidic medium at the temperature growth and the medium motion speed, under the action of ground current and when there is a contact with other metals.

Different methods are applied for protection of refrigeration unit elements against corrosion: the metal isolation from corrosive environment by coating its surface with a corrosion-resistant material layer; the reduction of environment corrosion activity; the use of an inhibitor (a substance which slows down the corrosion rate); the change of corrosion (standard) metal potential.

Metal protection from corrosion by means of applying a layer of prime coat, paint, lacquer and enamel is widely used. The paint-and-lacquer coating fulfils a protective function if the layer is continuous that is not always possible. The synthetic resin coating (phenolic, silastic) of a shallow thickness is harder and more durable than the paint-and-lacquer coating and is more often applied.

The surface of cold-producing devices, air and vapour condensers is galvanized, the surface of heat-transmitting tubes of air coolers and condensers is sometime plated (coated) with aluminum layer. Such coatings protect the steel when the layer integrity is disturbed as they are protectors affecting a steady metal potential.

Environment corrosion activity can be reduced as follows:

  • maintaining of the appropriate value of hydrogen-ion concentration (рН = 7-12 for carbon steel, рН = 7 for aluminum);
  • reducing of concentration of О2, Н2, ions of heavy metals, halogens and others;
  • reducing of air humidity, preventing of moisture condensation on a surface;
  • decreasing of temperature, pressure and motion speed.

Inhibitors are mainly applied in refrigeration systems with stable or rarely renewed amount of corrosive medium. For example, to prevent corrosion of the heat-transmitting surface of devices and pipelines from water and cooling medium circulating along the closed circuit. Organic (amines, amino acids, dextrins, mercaptans) and non-organic (chromates, phosphates) substances can be corrosion inhibitors.

The metal protection by changing its steady potential is called an electrochemical protection. It is the most effective and applied when other methods do not provide the required life duration of the object to be protected. The electrochemical protection is realized by the polarization of current external supply source or by means of connecting with metal (protector) that has more negative potential or more positive potential than the metal to be protected. Polarization is a change of potentials of metal and solution (the cathode – to more negative value and the anode – to more positive value) observed by the electrical current passing through the electrochemical system. Displacing the metal potential from the equilibrium state in the right direction, i.e. reducing the potentials difference formed between the metal and the solution, it is possible to reduce the corrosion rate.

The cathodic protection with the help of a sacrificial anode is mostly used for refrigeration units. The aluminum (grade AP1, AP3) and zinc (grade TsP1, TsP3) anodes are used to protect the objects made of carbon and low-alloy steel. And the protectors from steel (grade St0 St3) are used for the protection of objects made from copper-based alloys and nickel.

Sacrificial anodes have a limited radius of protective action, for example, for a tube straight section it does not exceed 2 m, and for a bend section – twice as little. That’s why several anodes are fixed on the object to be protected.

The sacrificial anode protective action gets better when electrical resistance in the place of contact reduces (not more than 0.02 Ohm). So the place for fastening should be dressed and degreased. The sacrificial anode is destructed in the place of contact, that’s why a periodical check of fastening strength and destruction degree of the sacrificial anode is needed. The sacrificial anode destructed more than 40% from the initial weight should be replaced.

Cold-storage building

Cold-storage compartment in the operation process is under environmental effect, solar radiation, variable air temperature, atmospheric precipitation and internal factors connected with functioning (low temperature and high air humidity, static and dynamic load). That’s why the aging of guard and load-bearing elements of cold-storage warehouse occurs revealing itself as the deformation of structural elements, the damage of structural vapour- and waterproofing materials and heat-insulating materials, the moistening of heat-insulating materials.

As a result of reduction of heat-protective properties, leakproofness and strength of load-bearing and heat-insulating constructions, the costs for heat removal from the compartments to be cooled are increased, products losses connected with the disturbance of technological mode and sanitary condition of compartments are grown.

So it is necessary to check the technical state of cold-storage building, its compartments to be cooled and to carry out respective works for avoiding the worsening of enclosure heat-protective properties below the admissible limit values.

Maintenance of cold-storage building provides for the seasonal inspections of the following: main structural and guard elements every three months, and all elements twice a year – in spring for definition of works scope on current repair of heat-insulating enclosures performed in summer, and in autumn for preparation to the operation in winter conditions.

During the inspection determine the state of:

  • coating (breakage, blistering, breakdown of roof covering);
  • fire-proof belts;
  • walls (cracks, protrusion, local moistening, ice crust);
  • floor structures (horizontal position of floors, dints, moistening, ice crust);
  • load-bearing elements of the framework (cracks, vertical and horizontal position);
  • heat-insulating doors (adjoining);
  • ground heat system.

Besides the visual inspection perform the instrumental monitoring of:

  • heat-protective properties of enclosure structures;
  • hardness and deformation of load-bearing elements of the framework;
  • ground heat system.

Local moistening of inner and outer wall points at the presence of ruptures in vapour-and heat-insulating layers. Moistening of the outer wall in places of seams location between panels of the framework load-bearing elements (a column, a wall beam of floor structures) that often appears in winter when it thaws and indicates the worsening of the construction heat-insulating properties. Frost or ice formation on the wall surface from the passages and halls side points at the entry of warm outdoor air.

Frost appearance on chambers ceilings with higher temperature than of above located chambers indicates that heat resistance of floor structures reduces.

Sagging and lifting of the floor placed on the ground points at the worsening of floor heat-insulating properties and ground freezing.

The causes of surface moistening, cracks in structures, deformation of structures and floor should be found and eliminated.

Heat-insulating properties of enclosure structures are determined by destructive and nondestructive inspection methods. The destructive inspection method assumes the sampling from enclosure structures, for example, with the help of drift, the investigation of their properties (heat conduction, humidity, hardness) and the extension of these properties to the whole construction. The nondestructive inspection method is based on the measurement of heat flux passing through the enclosure structure using a heat flowmeter and on the estimation of structure heat resistance. When using this method the structural integrity is not disturbed but it gives only a local value of the measured heat flux.

The method, assuming a contactless measurement of the enclosure surface temperature with the help of device called a thermographic camera, does not have the above disadvantage. There can be a large area of enclosure surface (for example, the whole wall of multistorey cold-storage warehouse) in the visibility range of the sensing element – the infrared receiver. The area temperature field uniformity is judged by a colour or tinge on the display screen. Detected areas with lower temperature are examined in detail.

Industrial-condenser-Bitzer

Ground heat system

Maintenance of ground heat system consists of the monitoring of ground temperature conditions under the cold-storage building, of the technical state of heat system elements, for example, of transformer at electric heating; heat exchanger-heater and pump at liquid heating as well as of works check.

Sanitation and disinfection of compartments and their equipment to be cooled

Industrial compartments (chambers, accumulators, passages, load platforms and others) including the equipment located in them as well as transport and lifting facilities must correspond to technical requirements and also to industrial sanitary requirements. The observance of sanitary and hygiene norms and requirements is provided by sanitary inspection during manufacturing, storing and transporting of food products and also by sanitation and disinfection of compartments and equipment. A chemical and bacteriological analysis performed in a laboratory is a means of sanitary inspection.

The current sanitation is carried out after finishing of each personnel shift, when the technological process is stopped and in case of idle time exceeding 1 h. During sanitation perform the cleaning of equipment and enclosures surface from products residues by a mechanical method, washing with cool water first (20…25 °С) and then with hot water (70…90 °С) and rinsing with cool water.

During disinfection usually carried out once a week, the surface is mechanically cleaned first, rinsed with cool water and washed with a hot solution (70…90°С), then a disinfectant composition is applied and after 15-20 minutes it is washed off with hot water and rinsed with cool water. The bacteriological laboratory checks the disinfection quality. The microbiological check of surfaces sanitary condition is regulated by the instruction. Thus the monitoring of chambers with air temperature of -12 °С is performed every three months, and with the temperature higher than the above mentioned – twice as many. The sanitary requirements to compartments and equipment are given in sanitary rules.

For keeping the proper sanitary condition the chamber wall and ceiling should have a smooth surface without slots and be painted and covered with washable panels. Floors should be waterproof, without slots and dints. Walls, floors and doors surfaces dirtied during the operation should be cleaned not less than once a shift. Grease and dirty floors and doors in chambers and passages at air temperature above zero should be washed with a hot washing solution and wiped dry.

Cold-producing devices of chambers should be periodically defrosted. In chambers equipped with cooling batteries, the products located under the batteries should be previously covered with a tarpaulin or polymer film to prevent them from falling snow and melt water and then to remove from the chamber. The cooling chambers after unloading and before loading a new batch should be disinfected.

Air ducts are treated with solutions for 40 minutes and then washed with water checking the hydrogen-ion concentration of washing water with the help of phenolphthalein. The instance of such solution is a water solution containing 6 % of liquid glass, 4 % of soda ash, 2…3 % of sodium tripolyphosphate and 0.6 % of synthamid-5.

The quick-freezing machines are washed and disinfected during defrosting. Washing of the machine comprises the following operations:

  • mechanical cleaning;
  • washing with warm water;
  • degreasing;
  • rinsing with warm water;
  • wiping dry;
  • lubricating with edible fat.

Water alkali solutions, for example, 1-2 % soda ash solution or 0.1-0.2 % caustic soda solution are mainly used for washing and degreasing.

The machine disinfection assumes the carrying out of the following operations:

  • surface treating with a disinfectant solution (irrigation, wiping);
  • holding for 30-40 min.;
  • rinsing with water;
  • wiping dry;
  • lubricating with edible fat.

Chlorine substances (chloride lime, chloramine, potassium and sodium hypochlorites, dichlorodimethylhydantoin), quaternary ammonium salts and others are widely used for disinfection. Chloride lime is applied dry (the ratio – 1 kg per 1 m2 surface), in the form of a solution containing 0.5-1 % of active chlorine (the ratio – 1-0.1 dm3 per 1 m2); the chloramine is used in the form of 0.2-1 % solution (the ratio – 1 dm3 per 1 m2).

The soda ash (0.5…1 % solution), caustic soda (0.5 % solution for equipment treating and 10 % solution for compartments treating) are used as washing agents.

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