This article on maintenance in refrigeration systems complements the first part of the note published in the previous edition called "Most common errors in refrigeration".
by Giovanni Barletta *
The maintenance of commercial refrigeration systems is exposed to the stereotype of all maintenance programs, in different areas of the industry in general and is very well defined by these famous phrases:
"When everything goes well, nobody remembers that it exists,
When it goes wrong they say that it does not exist,
When it is to spend it is said that it is not necessary,
But when it does not really exist ... all agree that it should exist ... "
.TO. Suter - Westinghouse. "
Maintenance is always seen as an expense and not as an investment, does not have adequate resources and is not in the priorities of senior management, and what is even more unfortunate, is that its impact on operating costs is not measured when this is not done properly, either due to loss or loss of product, energy inefficiency in the refrigeration cycle, which implies cost overruns in the energy bill, premature aging of the equipment, high costs due to frequency in replacement of parts, labor cost overruns in addition to the impact on the production chain or the image of the retail business, when the buyer does not find the products kept within an adequate cold chain.
The maintenance, rather than a prevention of the causes mentioned above, which in itself returns the investment in a very short time, it has to be understood as a tool to prevent a failure, and also allows to analyze parameters aimed at the efficient management of energy .
1 The first thing that a service provider should do and - is almost never done - is to know thoroughly the system that is going to intervene, because as we showed in the previous article, there are different architectures, different criteria in control systems and automation, as well as simple details such as knowing the defrosting parameters, drip times, valve and regulator adjustments; it would be endless to enunciate the parameters that make characteristic and particular to each system.
2 Having instruments and appropriate tools, without them it is very difficult to get to the detail, either of a diagnosis, or the fulfillment of a routine that actually adds value to the service. Within this requirement, it must also be understood that they are properly calibrated so that they are not objects of doubt or require double verification because they are inconsistent.
3 Make a list of basic readings, and it is understood that they should be taken in the key points of the system, some of them and minimum would be:
- Compressor Discharge Temperature
- Temperature & Pressure at the Condenser Output
- Temperature & Pressure at the Evaporator Outlet
- Ambient Temperature (Ext. & Int.)
- Temperature in Compressor Suction
- Supply voltage to the compressor per phase
- Electrical consumption in Amperes per phase
- Compressor Discharge Pressure
- Compressor Suction Pressure
4 Perform basic calculations
- Overheating at the evaporator outlet
- Total overheating (in the suction of the compressor)
- Discharge gas temperature
- Subcooling of liquid at the condenser outlet and before the expansion valve
- Temperature difference between air and condenser in the condenser
- Temperature difference between air and refrigerant in the evaporator
- Calculate the voltage and current imbalances using averages and deviation as shown in the 1 chart example
- Compare all these values against the original design conditions or against the values in the start-up of the system
- Keep a historical record of all these data, they will always be useful
1 chart. Example of calculation of voltage unbalance.
Routines required to avoid premature failures
1 Prevent overheating of the refrigerant in the discharge and with it of the compressor avoiding to exceed the maximum temperatures that would generate premature failures of lubrication and generation of contaminants to the system, for this:
- Verify that the selection of the compressor is in function of the specific application (in its operating range) and with its recommended cooling system
- Check the refrigerant gas charge
- Clean the condenser and check the operation of the fans
- Verify the operation of the liquid injection system
- Isolate the suction line correctly
- Correct adjustment of the overheating of the expansion element
- Check the high and low pressure switch setting
- In the 2 chart are the recommended discharge temperature values as well as the maximum admissible values.
2 chart. Recommended values of discharge temperature
2 Guarantee the proper return of oil to the compressor and avoid lubrication failures due to the absence of this, for which it is essential:
- Verify that the pipe diameters are properly selected
- Avoid possible obstruction in the hole of the suction accumulator
- Check the direction of rotation of the Scroll compressor
- Check slopes on the suction line to the compressor (1 "every 20 ', approximately)
- Arrange traps in suction lift sections of more than 1,50 mts
- Install oil separator when the condition requires it
- Search and repair possible leaks
- Adjust the pressure and temperature controls correctly to avoid compressor operation in short cycles
3 Prevent the return of liquid to the compressor, either in operation or in a prolonged shutdown and for this:
- Properly select the unit, the evaporator and the expansion valve
- Avoid keeping cold room doors open for too long
- Check and correctly apply the complete pump cycle (Pump Down)
- Review and schedule defrosts properly
- Apply Suction Accumulator
4 Avoid flooded starts that can cause a blow of liquid and for this:
- Do not exceed the refrigerant gas charge
- Install suction accumulator when the application requires it
- Apply or check complete pumping system
- Review and schedule defrosts properly
- Check the operation of the crankcase heater
- Keep the crankcase heater on during gas charging and before start-up
- Avoid long periods of stoppage and when this occurs, heat the oil at least 12 hours
5 Keep the system always within the highest degree of cleanliness thus avoiding the formation of acids and with it the corrosion of the internal components of the system. The appearance of a system with this problem is very easy to identify after the failure, as shown in figure 3.
Figure 3. System with evidence of humidity and acidity
Always use high vacuum pumps to ensure that you can reach 500 microns, do not leave this in the hands of a set of pressure gauges. Observe in Figure 4 the important differences between the mercury inches of the manometers and a true vacuum in microns
Figure 4. Micron comparative scale
6 Important observations in any maintenance routine
Bubbles in the peephole or liquid viewer, which can mean:
- Low charge of refrigerant gas or insufficient subcooling of the liquid at the condenser outlet
- Low condensation pressures
- Restrictions on the line of liquid or filter drier
Temperature differences in the dryer filter before and after it or find it snowed in this implies pressure drops due to clogging and should be replaced.
Observe the oil window of the compressors when starting up. It should not be sparkling or of a milky color, it implies presence of diluted liquid in the carter.
Observe in detail the levels of the liquid bottle (or liquid receiver), understand that this level depends on the demand of refrigerant in the evaporators, the number of them that is in thawing, among other variables, but unjustified decrease implies almost always leaks and forces a very exhaustive search for it.
* Giovanni Barletta is the Technical Manager of the Andean Area, Caribbean and Central America of Emerson, and current president of the Colombian Association of Air Conditioning and Refrigeration (ACAIRE). You can be contacted at the e-mail: firstname.lastname@example.org