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Most common errors in refrigeration (I)

First part of this special about the most common errors in design, installation, operation and maintenance in refrigeration systems in supermarkets and large surfaces.

by Giovanni Barletta *

Mechanical refrigeration systems are a determining factor in the conservation of the cold chain in storage, distribution and marketing of retail products, which is why their proper use is essential in each facility of this type of establishment.

- Advertising -

The process of "cold food" includes all those systems and subsystems involved in the preservation of the cold chain, whether in supermarkets, large areas and even convenience store of refrigerated or frozen products for retail sale; from the refrigeration system, to the exhibition in the sales area through the conservation chambers and the process areas. Regularly, the display coolers and cold rooms are connected to a cold room (Rack). This cooling plant consists of a group of compressors that discharge in parallel to a common condenser through a "manifold" of high pressure and dissipate all the heat to a condenser located outdoors which is regularly condensed by air.
This refrigerant, already in liquid phase and after being stored in a tank either horizontally or vertically, is distributed in a network of copper pipes throughout the business to each evaporator or final display.

Normally there are only two temperatures in the medium temperature (MT) and low temperature cold central, where the first is typically evaporating between 12 and 16 F and Low Temperature (LT) between -23 and -25 F.

The problem of these facilities is that they are highly demanding both in their reliability and availability as well as in the stability of the temperature and humidity conditions of the refrigerated furniture, as it compromises the quality of the product either displayed or stored and that in many cases is very sensitive to changes in these variables and if not properly maintained, it ends up affecting not only the commercial management of the business and the prestige of the business, but also putting the life and health of people at risk from the high associated maintenance costs to each intervention.

The many years of work within the commercial sector either as responsible for maintenance, or its operation or having intervened in their designs allows me to conceptualize them and classify this problem under several sections that are sometimes interrelated:
1 Considerations from the design.
2 Limitations and challenges in the installation processes.
3 Disadvantages generated from the operation.
to. From the cold power station.
b. From the exhibitor (User).
4 Maintenance challenges.

Design 
The design is the genesis of everything, from here begin the speculations on what is the most appropriate architecture and in this each continent (Europe and North America) has its preferences, as well as each manufacturer his. From all this analysis the advantages between simpler architectures at the cost of operation with conventional efficiency or networks and more complex controls in search of gaining some points in efficiency and security are debated. There is no universal criterion on which is ideal; The user and client must understand the pros and cons of each one and where to focus their investment. Some criteria go through aesthetic issues at the point of sale, ease or complexity to do the maintenance, either from a central or from the end points, likewise in the will or not to monitor and centralized records.

- System architecture: On the one hand are the multi-circuits centralized systems where all the mechanical and electronic elements and the control are located directly in the rack, including liquid line solenoid valves and defrosting (If applicable), evaporation pressure regulating valves, control and temperature record all installed, monitored and operated from the cold central; On the other hand, we have distributed systems or loop type where all the aforementioned elements are scattered throughout the store and the controls are handled locally in the exhibitor or cold room as the case may be.
  
- Defrosting: Another important design factor to consider is the fact of defrosting by electrical resistance or using the heat of the refrigerant as a source of energy for defrosting (Hot Gas or Cool gas) poses a dilemma between what is more favorable, the simplicity of the design of the Pipe networks in the shop that thaw with the resistances at the expense of the additional electrical power needed causing a higher power consumption or the gain in defrosting efficiency that is obtained when defrosting with hot refrigerant either taken in the discharge line ( hot gas) or already cooled in the condenser to the height of the liquid receiver (Cool gas) and take advantage of the latent heat of the phase change, making the defrost cycles shorter and aiming at the efficiency of the system, but at a cost higher in the installation. More complexity in valve systems, controls and final fine adjustment.

- Dimensioning of pipes: Additionally, special care should be taken in the correct dimensioning of the liquid supply pipe networks to avoid flashing of the refrigerant at the inlet of the expansion valve and especially those of suction gases where the speeds are critical for an appropriate oil return .

The lack of design or a poor development of this, will be reflected either in excessive speeds with a penalty in the pressure drop or in the opposite end where large diameters prevent oil returns.

There are two variables that determine the good sizing of a pipe and that become opposite each other: the speed that is high enough to guarantee an adequate oil return but that is not so high as to penalize excess pressure drop.

It is regularly measured in association with a pressure drop equivalent to a change in temperature in the line. Good practices recommend the following conditions:
- Liquid Line, Speed ​​300 fpm, Pressure drop 1 ºF
- Horizontal suction lines, Speed ​​700 fpm, Pressure drop 2 ºF
- Ascending vertical suction lines, Speed ​​1500 fpm, Pressure drop 2 ºF
- Then comes the theme of elevators or "raisier" that guarantee to go up to partial loads. In figure 1 we find an example of the elevators in an installation.

Figure 1. Single and double riser

- Refrigeration loads, equipment selection: The bad practice of not getting to the detail of the actual refrigeration loads; the market usually mentions Btu / Hr per linear meter of exhibitor, not understanding that each manufacturer has its specific curve as well as the developments of the manufacturers of new products lead to every time they design with smaller DT in the evaporators and thus achieve evaporation temperatures (TSS) higher with higher energy efficiency. This is so important not only in that the equipment is able to achieve the final temperatures but in having savings of the order of 2 to 3% for each pound that is greater the pressure of suction and this is in ALL the group of compressors that work for that condition.

Just to cite a comparison: Two types of furniture that are connected to the same suction group that evaporates to 15F in MT

  • While a dairy refrigerator demands 1851 BTUHR / FT linear
  • One of the meats served may be in the order of 300 BTUHR / FT LienalLineal

And this changes from manufacturer to manufacturer with each specific development of surface area of ​​evaporators, combined with air movement, etc.

- Selection of equipment: these are not designed for a nominal capacity given that their final performance will depend on the proper combination between each of them. Note in figure 2, that the real operating point of the "system" is the intersection of variables such as TD, (temperature difference between air and refrigerant) Evaporation temperature, and condensation temperature.

Figure 2. Operating point of the system

Installation
Since in many cases there are no designs prepared by specialized third parties, the errors here can be of a different nature:

1 Bad installation practices: Appropriate welding and a permanent stream of nitrogen inside the networks while welding avoids the formation of copper slag and scale as illustrated in Figure 3 where you can see the difference between a facility that uses nitrogen and that I do not use it.

This fact is not taken into account for the cost of nitrogen, but from the new refrigerants and new mixtures that require the use of POE synthetic oil, this is highly solvent and removes the particles that are deposited there, they are suspended in the oil and they go to deposit and obstruct holes of expansion valves, of capillaries or even the same lubrication ducts of the compressor with the risks of damage that this implies.

Figure 3. Difference of internal finishing of copper pipes. difference between welded tubes with and without nitrogen flow.

Other usual bad practices of the installation go through very common issues: As the bad support and inappropriate fixation to avoid causing vibration fractures, the omission of slopes in the horizontal pipe sections in the direction of the compressor, poor installation of thermal insulation causing infiltrations of moisture and condensation of the pipes.

A determining factor in the success of an installation and that is generally not taken with importance are the tests of starting and tuning of the system among the most important are:

  • Evacuation and dehydration. These should be done to achieve and sustain both the positive pressures at 1.5 times the working pressures for 24 hours and the vacuum at 250 microns.
  • Stepped system start. Establish a step-by-step decrease in the temperatures of the rooms and refrigerators to make the entry into operation of the system more homogeneous.
  • Tuning the system overheating setting of the expansion valves. This is one of the tests that are omitted at the start of a system and that can avoid later problems such as the return of liquid to the compressors and instability in the temperatures of the products in the refrigerators of exhibition or the refrigerated chambers.

From the operation
It is not enough with a good design and a correct installation of a refrigeration system for cold food; The operation that the end user gives to the system is as or more relevant and are against which good installation practices can combat, and which negatively impact the reliability, stability and efficiency of the system. Some of those bad practices of operation are the following ones:

1 Bad or inappropriate loads in the levels of the refrigerators or the chambers, preventing an adequate circulation and / or air resounding which in the end results in freezing, risk of damage to the compressors, low evaporation temperatures and penalty in the efficiency and loss of the cold chain by heating the product.

Figures 4 and 5 show very common cases in supermarkets and large areas where the capacity of refrigerators and cold rooms is exceeded.

Figure 4. About capacity in a display fridge.
                                          

Figure 5. About capacity in a cold room.

Figure 6 shows problems caused by poor air circulation in a display fridge.

Figure 6. Generation of ice in an evaporator.

2 Waterproofing and good use of camera doors
The excess of opening of the doors as well as their lack of closure are the culprits of ice formation, infiltration to the evaporators with the risks similar to those of the refrigerated furniture when they make ice, return of liquid and risk in the compressors. Figure 7 shows the consequences of misuse of the doors of a cold room, causing ice formation.

Figure 7. Ice formation in the entrance of the cold room.

3 Closing of the curtains of the display refrigerators
Failure to close the curtains of the display refrigerators is detrimental to the efficiency of the system since at night, in many cases not being counted with the operation of the air conditioning system or in the end due to the high humidity of the area, it intensifies the Operation of the cooling unit increased the hours of operation of the compressors.

* 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 by email: presidenteacaire@acaire.org - Giovanni.Barletta@emerson.com

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