Fluido Secundario - Spm Brasil

8/8/2019 Fluido Secundario - Spm Brasil

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Refrigeration Systems in Supermarkets withUtilization of Intermediary Fluids

Alexandre Presotto Jr.,Consulting Engineer, SPM Engenharia, BrazilCarlos Guilherme Sffert, Consulting Engineer, SPM Engenharia, Brazil

Abstract

The article describes the operation of refrigeration systems in supermarkets that utilize

intermediary fluids, showing its advantages and disadvantages both technical andeconomically , in comparison with traditional installations that use the R-22 on directexpansion cooling systems.

These installations were initially designed with the purpose of reducing the amount offrigorigenous fluid in the system and also of making possible its substitution by eliminarmeans of ammonia or other compatible and environmentally nondangerous fluid. The systemtherefore arises as a technical and economical feasible alternative for the substitution ofhalogenous fluids utilized in refrigeration systems in supermarkets.

Based on the experience of several operating installations in Brazil, this article points outthe evolution of this type of refrigeration system, which makes possible the elimination of thedefrosting routines in medium temperature circuits. The target of this system design is

maintaining the energy performance in low temperature circuits similar to that achieved in thedirect expansion system.

Introduction

By the end of the 70s, many scientists were concerned that halogen gases could benoxious to the troposphere ozone layer within the Earth atmosphere. In 1987, representativesfrom various countries gathered in Canada and signed the Montreal Protocol. Thisinternational agreement set deadlines for the gradual phasing-out of these gases. The initialdeadline was set around the year 2020.Since then, social pressure to reduce the use of fluids considered dangerous has increasedprogressively, expediting the process of eliminating CFCs and HCFCs besides forcing themarket to adapt itself to this new reality as soon as possible.

In Brazil, because of this new challenge, we have developed plants using secondary fluidsas an alternative to the use of CFCs and HCFCs in refrigeration systems in supermarkets,during the past ten years. These plants were initially conceived with the purpose of reducingthe quantity of refrigerants in the plant, since it is limited only to the quantity required by thecentral plant, and the secondary fluid transports the cold from the plant to the consumerspoint of sale display cases and warehouses. Therefore, it was possible to avoid the utilizationof halogen fluids in the system by using ammonia.

Clima 2000/Napoli 2001 World Congress Napoli (I), 15-18 September 2001


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Refrigeration Systems in Supermarkets with Utilization of Intermediary Fluids

1. OPERATION OF THE SYSTEM

The system is provided with a liquid chiller that cools down a water solution with anantifreeze agent, which is able to keep this solution in the liquid state at low temperatures, asdescribed in the figure 1. The solution flows through a distribution pipework, driven by acentrifugal pump, from the machine room to the cabinets and cold rooms. The temperaturerequired to preserve the product is kept by a suitable selection and control of the secondaryfluid temperature and the heat exchange surface temperature of the coils.

Figure 1: System Operation

LEGEND1 - Liquid chiller2 - Cooling tower3 - Secondary fluid pump4 - Condenser pump5 - Expansion vessel6 - Display cases and warehouses7 - Defrosting valves (just for low temperature system)

2. MEDIUM TEMPERATURE SYSTEMS

2.1. Description of the System

The medium temperature cooling systems for supermarkets usually employ R-22 directexpansion with an evaporating temperature of 10C approx. The heat exchange surfaces andthe other components were designed bearing in mind this condition. The great differencebetween the evaporation temperature and the product preserving temperature requires the useof a control system which operates temperature control under partial load conditions andmanages defrosting routines due to the ice formed on the coil.

Installations using intermediate fluid normally operate with a solution at 7C. Thus, it ispossible to use coils with almost the same heat exchange area. However, there is still the needfor temperature control and defrosting routine operations on the coils, since this last operationcauses a significant reduction of energy efficiency.



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The solution we designed intends to operate the installation with a smaller differencebetween the intermediate fluid temperature, approximately 2C, and the product temperature.Through an adequate trade-off between the selection of the solution flow rate and the heattransfer surface area of the coils it was possible to keep the temperature required for a goodproduct preserving. This design alternative eliminates the temperature control and defrostingroutines in cabinets and cold rooms and allows achieving a better global energy efficiency.

2.2. ResultsThe results achieved as far as they belong to product conservation in these plants, by using

a secondary fluid, are equivalent to the results obtained in a cooling plant with a R-22 directexpansion conventional system. However, as there are no more interruptions in fluid supply tothe coil, these conditions do not change throughout the day, assuring the perfect preservationof the products according to the quality standards for merchandise, as shown in the figure 2.

Figure 2: Results Achieved In Cooled Products Systems

Temperature Meat DairyProducts

Cold Cuts Fruits &Vegetables

oC oC oC oC

1 Chiller evaporation -6 -6 -6 -6

2 Fluid intake at the coil -2 -2 -2 -2

3 Fluid exit from the coil -0,5 +2 +2 +4

4 Air blowing 0 +3 +3 +5

5 Products +1 +4 +4 +8

6 Air return +5 +8 +8 +10

Note: Results achieved during normal operation of the plant in supermarkets.

2.3. Performance of the SystemConsidering that it is possible to operate with a higher evaporating temperature, there is an

increase in the COP of the plant that enables saving of energy required to operate theintermediate fluid pump, as shown in the table 1.



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Table 1: Total Load Performance For Medium Temperature System

System Refrigerant Condenser RangeoC

C.O.P.compressors

C.O.P.comp.+ pump

Direct evaporation R22 air -10/+45 2,99 -Direct evaporation R22 water -10/+37,5 3,57 -

Intermediate Fluid R22 air -6/+45 3,41 2,91

Intermediate Fluid R22 water -6/+37,5 4,11 3,61

Intermediate Fluid R404a air -6/+45 2,96 2,63

Intermediate Fluid R404a water -6/+37,5 3,75 3,20

Intermediate Fluid R717 water -6/+37,5 4,08 3,58

However, comparing the performance of the system under partial load conditions, using allthe annual average weather conditions, and not only the summer average, a larger increase inthe system COP is noticed. This happens because in direct expansion plants, the thermostaticvalve requires a certain pressure difference to assure the required yield. Considering that in

indirect systems chillers and electronic expansion valves are used, there is no need for apressure differential. Therefore, in addition to increasing the evaporation temperature, in thesecases, we also operate with a lower condenser temperature, to achieve a major gain in theplant yield, as shown in the table 2.

Table 2: Partial Load Performance For Medium Temperature System


C.O.P.compressors

C.O.P.comp.+ pump

Intermediate Fluid R22 water -6/+30 5,01 4,51


Note: Operating conditions estimated for all the annual average weather conditions in the city of So Paulo -Brazil (dry-bulb temperature of the air 21C and wet-bulb temperature 17,5oC).

2.4. Fluid UsedA secondary fluid, widely used in medium temperature systems, is the propylene glycol

water solution. Considering that the content of propylene glycol in the solution is very little(e.g.: less than 20%), the physical and thermodynamic properties of the solution are roughlysimilar to those of water, as shown at table 3.

Regarding corrosion, a propylene glycol water solution has an extremely low corrosionpower when in contact with copper or brass. However when using inhibited propylene glycolthese rates are also low for carbon steel.

The table 4 shows comparative data on the effects of corrosion taken from product catalogscomplying with ASTM D1384 standards test:

Table 3: Physical And Thermodynamic Properties

Product Temp.oC

DensityKg/m3

Specific HeatKcal/kg.oC

Heat ConductionKcal/h.m.oC

Viscosity Kin.m2/s

Water +5 999,8 1,005 0,485 1,55 x 10-6

Propylene Glycol Sol. 20% -2 1027,6 0,938 0,397 4,26 x 10-6



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Table 4: Astm D1384 Standards Test

Material Water Propylene GlycolSol. 30%

Inhibited P.G.Sol. 30%

Copper 2 4 3Brass 5 5 4

Carbon steel 212 214 1

Inhibited propylene glycol aqueous solutions are also compatible with almost all materialsused in common cooling plants, both equipment and for sealing joints and connections. Themajor concern is to avoid contact with:

Zinc. Galvanized steel. Gray cast iron. Water with excess chlorine. Water with excess sulfates.Regarding toxicity, propylene glycol USP grade is used mainly in the food, cosmetics and

pharmaceutical industries. There are versions of inhibited propylene glycol totally nontoxic(used in animal food). It complies with all specifications of the Brazilian and AmericanPharmacopoeia, and it can also be used as a direct or indirect additive to food.

Regarding its flammability, propylene glycol in solutions with concentrations above 80%has a flashpoint of 102oC. The product is not flammable under this concentration.

3. LOW TEMPERATURE SYSTEMS

3.1. Description of the SystemConventional low temperature plants with R-22 direct expansion usually operate with

evaporation set at 30oC approx. Therefore, in low temperature systems it was established asa design concept to operate with the same evaporating temperature in the liquid chiller and asupply temperature of the intermediate fluid of 27C. Under these conditions, and using thesame coils as in the conventional direct expansion plants with a proper circuit, it was possibleto assure the temperature required in cold rooms and cabinets of frozen products (-20oC).

The gain obtained in approximating the temperature differentials is due to the following

factors: Greater inside heat transfer coefficient, which results in a greater overall coefficient; Utilization of heat exchanger with countercurrent flow; Uniform heat transfer throughout the coil; Minimization of superheat.



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Other factors should be highlighted in the defrosting procedures, since they also result inan improvement in the quality of the plants:

Electrical defrosting acts more effectively in coils with intermediate fluid than in dryexpansion due to the heat diffusion caused by the intermediate fluid throughout the coiland closings, that also provides shorter periods of defrosting.

Speed of temperature recovery in display cases and warehouses after defrosting is fasterthan in direct expansion system, since there is no limitation on the coil capacity throughthe expansion valve.

3.2. ResultsThe results achieved regarding product preserving in plants operating as described above

assure the conditions required for the storage and display of frozen products in supermarkets,as shown in the figure 3. Although defrosting routines are still required, it was noticed that theoscillations in the solution temperature after defrosting are much lower than the variations in

temperature of evaporation during the same periods of time in direct evaporation systems.

Figure 3: Results Achieved In Frozen Products Systems

Temperature Island FreezeroC

1 Chiller evaporation -30

2 Fluid intake at the coil -27

3 Fluid exit from the coil -25

4 Air blowing -24

5 Products -20

6 Air return -18

Note: Results achieved during normal operation of the plant in supermarkets.

3.3. Performance of the SystemIn the same way as in medium temperature plants, in low temperature plants the

performance of the system at full load undergoes a small decrease due to the addition of therequired power to pump the solution. As shown in the table 5.



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Table 5: Total Load Performance For Low Temperature System


C.O.P.compressors

C.O.P.comp.+ pump

Direct evaporation R22 air -30/+45 1,71 -Direct evaporation R22 water -30/+35 1,98 -

Intermediate Fluid R22 air -30/+45 1,82 1,52


Intermediate Fluid R404a air -30/+45 1,35 1,21

Intermediate Fluid R404a water -30/+35 1,78 1,54


However, when analyzing the plant under partial load condition, again using under all theannual average weather conditions instead of the summer average only, there is a real gain inthe COP plant. As shown in the table 6.

Table 6: Partial Load Performance For Low Temperature System


C.O.P.compressors

C.O.P.comp.+ pump

Intermediate Fluid R22 Water -30/+29 2,22 1,92

Intermediate Fluid R717 Water -30/+29 2,37 2,07

Note: Operating conditions estimated under all the annual average weather conditions in the city of So Paulo -Brazil (dry-bulb temperature of the air 21C and wet-bulb temperature 17,5oC).

3.4. Fluid UsedRegarding the antifreeze agent, the situation for low temperature systems is different from

medium temperature systems because there is not an established fluid the physical propertiesof which are adequate . On the other hand, new options are necessary, particularly in thenorthern European countries, where the deadlines for complete phasing-out of halogens arebeing expedited. Therefore, we have compared the properties of several options forintermediate fluids operating at 30oC.

The characteristics required to work with a good intermediate fluid are:

High thermal conductivity; High density; High specific heat; Low viscosity; Low toxicity; Low corrosion levels; Solubility in water.Evaluating the investigated fluids according to the parameters described above, the results

at the table 7 are observed:



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Table 7: Physical And Thermodynamic Properties

Product Temp.oC

DensityKg/m3

Specific HeatKcal/kg.oC

Heat ConductionKcal/h.m.oC

Viscosity Kin.m2/s

Potassium acetate sol. 80% -30 1.220 0,710 0,374 2,39 x 10-5

Propylene glycol sol. 60% -30 1.073 0,745 0,258 2,80 x 10-4

Ethylene glycol sol. 60% -30 1.105 0,686 0,268 5,90 x 10-5

Silicone -30 908 0,398 0,104 4,38 x 10-6

"thermal oil J" -30 900 0,404 0,120 2,50 x 10-6

"thermal oil Q" -30 1.010 0,357 0,110 3,49 x 10-5

CaCl2 sol. 30% -30 1.280 0,673 0,421 2,09 x 10-5

Regarding corrosion, the table 8 shows comparative data on the effects of corrosion on the

products according to ASTM D1384 standard tests:

Table 8: Astm D1384 Standards Test

Material Water CaCl2sol. 30%

K-Ac.sol. 80%

Ethylene Glycolsol. 60%

Copper 2 30 2 3

Brass 5 93 1 3

Carbon steel 212 245 2 1

The results lead us to the conclusion that potassium acetate, an inhibited organic saltsolution, has more appropriate physical characteristics to operate with intermediate fluid inlow temperature plants.These solutions are also compatible with almost all materials used in ordinary cooling plants,

both in equipment and sealing joints and connections. The main precautions to be taken arethat they should not be used with:

Polytetrafluoroethylene (PTFE); Silicone mixtures; Residues of glycol solution; Water with chlorine; Galvanized steel.Regarding toxicity, Potassium acetate is not a dangerous product. Rapid exposures do not

cause any effect on health. Anyhow, it is recommended to use rubber gloves during theoperations according to the general standards for handling chemical substances. The table 9

shows some situations of contact with the product, their consequences and treatment:

Table 9: Procedures Due Contact With Potassium Acetate Solution

Contact Consequence TreatmentSkin Exposure for long periods of time may cause

light irritationRinse the affected areas with water.

Eye Irritation and possibility of temporary burns Rinse eyes with plenty of water.

Inhalation Possibility of irritation Expose patient to fresh air ventilation.

Ingestion Possibility of irritation Wash mouth with water and do not inducevomiting.

Potassium acetate water solution is a non-flammable product.



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4. CONTROL SYSTEM

Control system for indirect plants is very simple, particularly when compared to thetraditional refrigeration systems by direct expansion. In plants using intermediate fluid,controls are restricted to the machine room, being:

Control loop of the secondary fluid temperature; Control loop of the expansion valve.Both control loops control the system in partial load operation. They are restricted to the

liquid chiller, and are monitored by the chiller microprocessor controller. Thus, it is possibleto maintain the plant stable with fluid temperature variations ranging between +/-1 oC. It isalso possible to optimize the cooling cycle range, assuring a superheating up to 4oC.

When completing the control system, there is the automation of the defrosting routines forfrozen product cabinets and cold rooms. Such routines are monitored by dedicated controllers

that act in synchronicity to avoid both equipment from starting their cycle simultaneously. Italso has a temperature sensor that cuts off the cycle when the coil is completely unobstructed,even though the total operation time has not been reached. Such device prevents theequipment from superheating and makes it easier to recover the normal operation conditionafter defrosting is completed.

All these data are gathered by the communications network and sent to a computer so thata supervision software may follow up on all operations. This supervision software does notcontrol or operates the plant, it only transforms the data collected into information that theoperator may easily understand. These data are shown in friendly screens as graphs andreports or are transmitted to the other remote station enabling a distance monitoring.

5. COSTS

The tables 10 and 11 compare the costs of implementation and operation of a coolingsystem in a supermarket in the city of Porto Alegre Brazil, which sales area totalsapproximately 5.000 m2.

Table 10: Implementation Costs

Item System with intermediate fluidR-717US$

System with direct expansionR-22US$

Thermal central plant 130.000,00 90.000,00

Condensers 22.000,00 22.000,00

Refrigerant fluid 450,00 4.000,00

Intermediate fluid 10.000,00 0,00

Controls 13.000,00 35.000,00

Cold rooms and air blowers 185.000,00 175.000,00

Cabinet counters 370.000,00 355.000,00

Materials 90.000,00 105.000,00

Labor 70.000,00 90.000,00

TOTAL 890.450,00 876.000,00

Note: The amounts at table 10 are estimated and were supplied directly by equipment and fixtures manufacturers

and do not include transportation, neither overhead costs.



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Table 10: Operation Costs

Item System with intermediate fluidR-717

US$/year

System with direct expansionR-22

US$/yearPower 67.000,00 76.000,00

Refrigerant fluid 650,00 5.200,00

Intermediate fluid 0,00 0,00

TOTAL 67.650,00 81.200,00

Note: The operating costs correspond to a plant with the following features:

Heat load of cooled products: 250.000 kcal/h; Heat load of frozen products: 60.000 kcal/h; Both plants operating at the average capacity of 70%; KWh cost equals US$ 0,10 (sum of the demand, kWh at the peak hour, and out of the peak hour); The COP values used are the ones for plants with water condenser; Costs with replacement of refrigerant correspond to the amounts used by ASHRAE. For dry-expansion

plants, it indicates that leakage corresponds to 25% the total refrigerant load in the system per year (800kg);

The operating costs did not include the reduction in the heat load in the cooled products systems.

6. ADVANTAGES AND DISADVANTAGES

Systems with intermediate fluid present the following advantages and disadvantages inrelation to the traditional plants operating with R-22 direct expansion.

6.1. Advantages

Reduced power consumption; Lower thermal load for medium temperature systems; Lack of defrosting routine for medium temperature systems; Lack of temperature control for cabinet counters and cold rooms; More effective heat exchangers; Less quantity of refrigerant fluid on the system and much less possibility of leakage; Simple installation and consequent lower cost of preventive or corrective maintenance; More operating reliability (less maintenance occurrences); Simplified control system; Feasibility using non-condemned refrigerants required by the Montreal Protocol.6.2. Disadvantages

Larger physical space at the machine room required for equipment installation; Larger areas for heat exchange required at the cooling coils of cold rooms and cabinet

counters in medium temperature systems;

When using R717 as a refrigerant fluid, a special study of the machine room exhaustsystem is needed.



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7. CONCLUSION

The major advantages of the cooling system using secondary fluid are the feasibility ofusing an environmentally nondangerous refrigerant fluid, simple operation, a simpler designof the control loop, required saving of energy, and stability of the plants operation. Thesefeatures have been be tested in several plants in operation in Brazil for more than ten years,especially medium temperature systems where defrosting routines were eliminated. Thesystem remained permanently in a stable operating regimen, varying only according to thethermal load needs of the store.

For low temperature systems, it was verified that the use of intermediate fluid results in abetter performance of the cooling coils, leading to better results concerning the temperaturemaintenance for cold rooms and cabinet counters.

For more than ten years, it has been proved that use of intermediate fluid in refrigerationplants in supermarkets provide major reliability and stability of operation and represents an

economical feasible alternative to avoid the use of CFCs and HCFCs.

8. EXISTING PLANTS

The table 11 presents a list with supermarkets installations in Brazil, that use scondaryfluid in its refrigeration systems.



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Table 11: Existing Plants in Brazil

Supermarket City / Brazil Year Aream2

ChillerRefrig.

Cooled Products Frozen Products

Bourbon Assis Brasil Porto Alegre 1990 11.000 R22 Propilene glycol Dry ExpansionMaster Sonda Erechim Erechim 1993 4.500 R22 Propilene glycol Dry Expansion

Zaffari Anita Garibaldi Porto Alegre 1993 1.500 R22 Propilene glycol Dry Expansion

Zaffari Marechal Floriano Porto Alegre 1994 2.500 R22 Propilene glycol Dry Expansion

Zaffari Fernandes Vieira Porto Alegre 1995 1.000 R22 Propilene glycol Dry Expansion

Zaffari Fernando Machado Porto Alegre 1996 2.000 R22 Propilene glycol Dry Expansion

Zaffari Higienpolis Porto Alegre 1996 7.000 R717 Propilene glycol Dry Exp. R22

Real Menino Deus Porto Alegre 1996 4.000 R22 Propilene glycol Dry Expansion

Real Rio Grande Rio Grande 1996 4.000 R22 Propilene glycol Dry Expansion

Real Novo Hamburgo Novo Hamburgo 1996 3.000 R22 Propilene glycol Dry Expansion

Big Cricima Cricima 1996 5.000 R22 Propilene glycol Dry Expansion

Big Porto Alegre Porto Alegre 1996 16.000 R22 Propilene glycol Dry Expansion

Zaffari Ipiranga Porto Alegre 1997 7.000 R22 Propilene glycol Dry ExpansionBourbon Canoas Canoas 1997 10.000 R717 Propilene glycol Dry Exp. R22

Zaffari Lima e Silva Porto Alegre 1997 3.500 R22 Propilene glycol Dry Expansion

Master Sonda Centro Erechim 1997 2.000 R22 Propilene glycol Dry Expansion

Big Florianpolis Florianpolis 1997 10.000 R22 Propilene glycol Dry Expansion

Big Joinville Joinville 1997 10.000 R22 Propilene glycol Dry Expansion

Real Iguatemi Porto Alegre 1997 4.500 R22 Propilene glycol Dry Expansion

Real So Leopoldo So Leopoldo 1997 3.000 R22 Propilene glycol Dry Expansion

Real Curitiba Curitiba 1997 1.500 R22 Propilene glycol Dry Expansion

Real Pelotas Pelotas 1997 1.500 R22 Propilene glycol Dry Expansion

Sonda Jaan So Paulo 1997 3.000 R22 Propilene glycol Dry Expansion

Zaffari Carazinho Porto Alegre 1998 1.200 R22 Propilene glycol K-Ac.

Sonda Cidade Dutra So Paulo 1998 4.000 R22 Propilene glycol Dry ExpansionSonda Maria Amlia So Paulo 1998 2.500 R22 Propilene glycol Dry Expansion

Bourbon Ipiranga Porto Alegre 1998 10.000 R717 Propilene glycol K-Ac.Carrefour Pres., Prudente Pres., Prudente 1998 6.000 R22 Propilene glycol Dry Expansion

Carrefour Campo Grande Campo Grande 1998 8.000 R22 Propilene glycol Dry Expansion

Zaffari Cavalhada Porto Alegre 1999 5.500 R22 Propilene glycol K-Ac.

Bourbon Passo Fundo Passo Fundo 1999 9.500 R717 Propilene glycol K-Ac.

Zaffari Cristvo Colombo Porto Alegre 1999 3.500 R22 Propilene glycol K-Ac.

Zaffari Bordini Porto Alegre 2000 1.600 R22 Propilene glycol K-Ac.

Zaffari Menino Deus Porto Alegre 2000 1.900 R22 Propilene glycol K-Ac.

Carrefour Diadema So Paulo 2000 7.600 R22 Propilene glycol Dry Expansion

Bourbon Joo Wallig (*) Porto Alegre 2001 10.000 R717 Propilene glycol K-Ac.

Master Sonda F. Caneca So Paulo 2001 2.000 R22 Propilene glycol K-Ac.

(*) Store under construction


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