Technical / Extra Edition

Evaporative Condensing Energy Efficiency

June 4, 2012
Due to escalating energy costs over the past several years, supermarket chains are exploring new equipment designs to reduce their energy consumption. The greatest amount of energy is required by the refrigeration systems followed by the HVAC systems and then the lighting in a typical supermarket.

The standard design in the supermarket industry for refrigeration is semi-hermetic compressors and air-cooled condensing units.

Over the past decade, the case manufacturers have designed their refrigerated cases to operate at the highest possible evaporator temperature. This has the maximum effect on lowering the compression ratio of the system which increases the energy efficiency of the system. The next best option to increase the energy efficiency is to lower the condensing temperature. One method to accomplish this is by evaporative condensing that reduces the head pressure during high ambient operational conditions.

Air-cooled condensing is based on ambient dry bulb temperature and evaporative condensing is based on web bulb temperature. Historically, the design considerations for selecting one over the other has been based on 105°F dry bulb or higher and 75° wet bulb or lower for the supermarket industry.

Energy costs have expanded the usage for evaporative condensing to designs with lower dry bulb and higher wet bulb conditions. Evaporative condensing will reduce the refrigeration horsepower required compared to air-cooled condensing. The initial equipment costs and ongoing water costs are more than offset by the operational energy savings of the evaporative system.

Let’s look at a detailed comparison of a typical supermarket refrigeration system designed with air-cooled condensers versus one with evaporative condensing.

The comparison for this example will consist of the following specifications:

ABC Supermarket, Miami

Size: 40,000 square feet

Refrigerant: 404A

Voltage: 208-230/60/3

Ambient Design: 100° dry bulb / 80° wet bulb

Evaporator Loads:

Low Temperature -20° suction temperature / 260,000 Btuh

Medium Temperature +20° suction temperature / 470,000 Btuh

Condensing Temperatures:

Air cooled design – low temperature at 110° / medium temperature at 115°

Evaporative condensing – low and medium temperatures at 95°

Compressors: semi-hermetic reciprocating

System design: parallel compressors with two suction groups, low temperature operating at -20° suction and medium temperature operating at +20° suction.

System Selection: Air-Cooled Condensing

Rack A

-20° suction / 110° condensing

Evaporator load – 260,000 Btuh

Compressors – four uneven capacity compressors

Capacity – 304,000 Btuh

THR – 485,573 Btuh

Compressor power required – 53.2 kWh

Condenser – six 1½ hp fans – 9 hp

Condenser power required – 8.7 kWh

Rack B

+20° suction / 115° condensing

Evaporator load – 470,000 Btuh

Compressors – four uneven capacity compressors

Capacity – 551,000 Btuh

THR – 760,729 Btuh

Compressor power required – 61.45 kWh

Condenser – six 1½ hp fans – 9 hp

Condenser power required – 8.7 kWh

Total power – 132.05 kWh

System Selection: Evaporative Condensing

Rack A

-20° suction / 95° condensing

Evaporator load – 260,000 Btuh

Compressors – three uneven capacity compressors

Capacity – 306,000 Btuh

THR – 453,441 Btuh

Compressor power required – 43.2 kWh

Condenser – 10 hp & 1½ hp fan / 2 hp pump (low and medium temperature)

Condenser power required – 7.3 kWh

Rack B

+20° suction / 95° condensing

Evaporator load – 470,000 Btuh

Compressors – three uneven capacity compressors

Capacity – 551,000 Btuh

THR – 703,220 Btuh

Compressor power required – 44.6 kWh

Condenser – 10 hp & 1½ hp fan / 2 hp pump (low and medium temperature)

Condenser power required – 7.3 kWh

Total power – 95.1 kWh

The Advantages of Evaporative Condensing Are:

The compression ratio of the low temperature system is reduced by 18 percent, which reduces the required horsepower by 13.4 hp.

The compression ratio of the medium temperature system is reduced by 23 percent, which reduces the required horsepower by 22.5 hp.

The horsepower reduction is a direct result of the lower compression ratio of the systems. The life cycle of the compressors will be extended due to the lower compression ratio.

The compressor capacities of both designs are within 1 percent of each other. Both low and medium temperature evaporative systems will require one less compressor. Each system will be at least two feet shorter, which will reduce the size of the machine room or machine house. The other advantages of reducing the number of compressors is that other items will be eliminated — high and low pressure controls, oil failure controls, suction line filter and suction and discharge copper piping. This will reduce the possibility of refrigerant leaks in the future as well as maintenance on the controls. The power required for the evaporative systems is lower, which reduces the size of the electrical service required, circuit breaker, contactors, and wire size.

The required power for the two air-cooled condensers necessitates 18 hp of fan power versus a 10 hp fan motor and 2 hp pump for the evaporative condenser. The evaporative condenser has a 1½ hp low-speed fan to operate in lieu of the high-speed 10 hp fan during low load conditions for additional energy savings. The evaporative condenser does not require aluminum fins for heat transfer. The lack of aluminum fins is an advantage in salt air environments. Salt air conditions corrodes the aluminum fins and reduces heat transfer and greatly reduces the life cycle of the condenser. The evaporative condenser maintains superior heat transfer due to the solid copper design and has a longer life cycle.

The initial cost of the equipment needs to be evaluated along with the operational, water, and maintenance costs. The compressor systems will be reduced and the condenser costs will be increased. The overall equipment costs are typically increased by 5 to 10 percent.

The estimated energy costs are significantly reduced due to the lower compression ratio of the evaporative systems. The operational costs at these design conditions will be reduced by an average of 30 percent.

The evaporative systems require water for energy efficient operation. The amount of water usage and sewage needs to be accounted for along with water maintenance. The estimated amount of water required for the evaporative condenser is 957,000 gallons per year. This average estimate is based on three cycles of concentration. Based on local commercial water rates, the estimated cost of water is $7,278 per year. The advantage of operating evaporative condenser systems in the Southeast with the high wet bulb design is that there is a high amount of rainfall per year. The average amount in the Miami area is 58 inches of rainfall per year. The amount of water required for the evaporative condenser can be supplemented by rain water reclamation or HVAC condensate reclaim. Based on this store of 40,000 square feet, the estimated amount of rain water that can be reclaimed would amount to 1,344,000 gallons per year. This would require storage capacity and a water filtration system. The other option is to drain the HVAC condensate lines into the evaporative condenser sump. Both of these options would reduce the operational cost of the evaporative design and add to green initiative goals.

The return on investment for the evaporative condenser designed system is a payback of less than two years. The payback for this design will vary based on total evaporator loads, specific equipment selections, water rates, and electrical rates.

This example of a typical supermarket load profile for a store located in Miami-area design conditions shows the advantages of evaporative condensing. This design reduces the energy consumption of the highest energy load in the supermarket. The factors considered in this study include the equipment costs, operational costs, and maintenance costs. The initial equipment costs are higher but these costs are offset by lower operational costs year after year.

Publication date: 6/4/2012

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