Case Study: Cooling Towers at Five Large Retail Branches — Condenser Approach Held Below 4°F for 24 Months

What is Condenser Approach Temperature?

Condenser Approach Temperatureis the difference between the saturation temperature of the refrigerant in the condenser and the temperature of the cooling water leaving the condenser. It can be defined by the temperature difference as follows:

Condenser Approach = Condenser Saturation Temperature − Condenser Water Temperature Out

This value directly reflects the “resistance to heat transfer” at the condenser tube surface. The cleaner the tubes, the lower the value. In engineering terms, its relationship to efficiency is as follows:

• For every 1°F increase in the approach value, chiller kW/Ton increases by approximately 1.5% (efficiency falls by 1.5%).
• For the best heat exchange, the value should be maintained in the range of 0–2°F, and generally should not exceed 10°F.
• In energy terms, allowing an approach to climb from a clean condition to around 10°F before cleaning is equivalent to kW/Ton moving from 100% up to approximately 115%.

The value functions like a “vital sign” for system efficiency that can be read every day, without waiting for the electricity bill. Engineering guidance generally recommends keeping the approach value as low as reasonably achievable, with the appropriate range usually around 0–2°F, and the value should not be allowed to rise close to 10°F. Because the higher the approach value climbs, the more chiller efficiency declines.

This principle is consistent with chiller manufacturers’ technical documentation, which states that every 1°F (0.56°C) increase in approach raises electricity cost by an average of approximately 1.5% [2]. Experimental research has found that every 1 K (≈1.8°F) increase in Condenser Approach reduces chiller COP by approximately 3.3–3.6% and increases power draw by approximately 3% [1] (equivalent to approximately 1.5% per °F). Both sources confirm the same quantitative relationship.

The limitations of chemical water treatment in cooling towers

With chemical treatment, what is clearly visible is field data showing a repeating pattern: the approach value climbs continuously as scale and fouling accumulate, until mechanical cleaning of the condenser tubes is required every 5 months to reset the value back down. The consequences are:

• The approach value fluctuates in a sawtooth pattern, with periods where it reaches 10°F before cleaning, during which the chiller consumes more energy than necessary.
• Hidden costs arise from chemical costs, cleaning labour, downtime, and the risk of tube surface corrosion.
• Comparative photographs of condenser tube condition confirm that after 6 months on the chemical system, the tube surface shows clear corrosion and blockage, compared with tubes on the ozone system for 12 months which remained in good condition.
• The effect of fouling is more severe than expected, a film or scale layer just 0.6 mm thicker can reduce chiller efficiency by as much as 34% and increase energy consumption by as much as 21%. Research also indicates that condenser fouling is the most likely cause of chiller damage, with cases found of condenser tubes cracking under heavy scale accumulation.

This pattern of operation causes chiller efficiency to fluctuate cyclically in a sawtooth shape: after the tubes are freshly cleaned, the approach value falls and the chiller performs better, but as time passes fouling accumulates, the approach value rises, and the chiller consumes more energy. Until the machine must be stopped for cleaning once again.

The limitations of a conventional chemical system are therefore not only the cost of the chemicals, but also the tube cleaning labour, the downtime, the risk associated with acid cleaning, and the long-term risk of corrosion to the condenser tube surface.

The ozone water treatment system and its installation

Ozone water treatment is the use of ozone gas, or O₃, which acts as an oxidant, to help control microorganisms, biofilm and fouling in the cooling water system without reliance on conventional chemicals.

In this case study, the ozone system was installed in the cooling tower’s cooling water circuit. The ozone generator produces ozone gas from oxygen, which is then mixed into the water via a mixing pump before being returned to the cooling water system, so that ozone circulates with the water throughout the system.

The installation configuration in this case study (example from one branch) is: cooling towers 3 × 500 tons, chillers 3 × 400 tons, circulation pumps 3 × 1,200 GPM, with ozone dosed into the system at a rate of approximately 100 L/min, covering a total pipe run of approximately 50 metres.

The distinguishing feature of this system is that it controls the problem at source, particularly by inhibiting biofilm, a key factor that allows scale and sediment to adhere readily to tube surfaces. When biofilm is controlled, condenser tube surfaces stay cleaner, heat transfer becomes more efficient, and the approach value can be held at a low level on a continuous basis.

Method and data collection for the ozone water treatment system

This case study collected data from five large retail branches, comparing performance during the period of chemical water treatment with the period after changing to ozone water treatment in the cooling tower’s cooling water circuit. The data collection method was as follows:

• Condenser Approach Temperature was measured continuously on all three chillers at each branch throughout the service period.
• Data was collected both before the system change (chemical) and after the change to ozone, in order to compare under real operating conditions.
• The continuous ozone data collection period was approximately 24 months, covering both the hot season and the rainy season.

The key criterion used to assess results in this case study was holding Condenser Approach below 4°F, a level that reflects that the heat exchange surface remains clean and that the system is able to maintain chiller efficiency on a continuous basis.

Results after installing the ozone water treatment system

Monitoring after the change to the ozone system found that all five branches were able to hold Condenser Approach below 4°F continuously throughout the 24-month study period, with most approach values in the range of approximately 2–3°F — clearly different from the period of conventional chemical treatment. Comparing the two systems in overview shows the following differences:

At some branches there were periods where the approach value rose above the criterion, but the cause could be clearly identified — for example, the ozone system being shut down for cooling tower maintenance for approximately 1 month, or an abnormality in the chiller itself. Once the system returned to normal operation, the approach value returned below 4°F again.

This data helps confirm the relationship between continuous operation of the ozone system and holding the approach value at a low level, a key factor in maintaining chiller efficiency over the long term.

Effect on chiller efficiency and energy saving

Reducing Condenser Approach is not merely an engineering figure. It relates directly to reducing the electricity cost of the air conditioning system, because when the condenser transfers heat more effectively, the chiller does not need to use excessive energy to produce chilled water.

Based on the principle that every 1°F increase in the approach value raises kW/Ton by approximately 1.5%: comparing the chemical treatment period, with an average approach of approximately 6°F, against the ozone period, which held approach at approximately 2.5°F, gives a difference of approximately 3.5°F.

Assessed in engineering terms, this difference translates into a chiller efficiency improvement of approximately 5% on a continuous basis, for as long as the system is able to hold the approach value at a low level.

The actual saving in baht per year, however, depends on chiller load, operating hours and the electricity tariff at each branch, and should be calculated from actual bills on a site-by-site basis. What this case study clearly demonstrates is that holding the approach value low on a continuous basis is one of the methods that concretely reduces energy losses in a chiller system.

Water quality and environmental benefits

Beyond the effects on chiller efficiency and energy saving, the ozone water treatment system also clearly affects water quality in the cooling tower. The case study data found that water in the system after ozone use was clearer and cleaner than during the period of conventional chemical treatment.

The relevant water quality analysis results, such as pH, conductivity, hardness and alkalinity, remained within standard ranges, reflecting that the system is able to control water quality without reliance on conventional chemical dosing.

Environmentally, a chemical-free water treatment system is consistent with the sustainability goals of modern buildings, helping to reduce chemical use, reduce the risk of chemical residue in discharged water, reduce the use of acid for condenser tube cleaning, and support green building approaches as well as sustainable energy management.

Case study summary: installing an ozone water treatment system

The case study of ozone water treatment across five large retail branches demonstrates that holding Condenser Approach Temperature below 4°F on a continuous basis can significantly help maintain chiller efficiency. After changing from a chemical system to an ozone system, the approach value stayed at a low level throughout 24 months, with no need to stop the machines to clean the condenser tubes with brushes or acid at any point.

Compared with conventional chemical water treatment, where the approach value tends to climb until tubes must be cleaned on a cycle, the ozone system makes cooling water management in the cooling tower more stable, reduces fouling, reduces the maintenance burden, reduces chemical use, and allows engineers to verify the results from the actual approach values occurring in the system.

For businesses seeking to manage energy costs efficiently, Econowatt provides energy management and water treatment solutions for air conditioning systems, delivered by a team of specialist engineers, together with a monitoring system to help control water quality, reduce microbiological risk, and support the continuous and sustainable performance of cooling tower and chiller systems.

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References:

• N. Suamir, I N. G. Baliarta, M. E. Arsana และ I P. S. Negara, “Condenser–Evaporator Approach Temperatures and their Influences on Energy Performance of Water Cooled Chillers,” Proc. 14th Int. Conf. on QIR (Quality in Research), Lombok, Indonesia, 10–13 ส.ค. 2015, ISSN 1411-1284, หน้า 428–433.
• [2] Trane (Thailand), a business of Ingersoll Rand, “Exchanger Cleaning Program (โปรแกรมทําความสะอาดอุปกรณ์แลกเปลี่ยนความร้อน),” เอกสารเลขที่ MUL-SLB021-TH, พ.ค. 2011.

Frequently asked questions about Condenser Approach (FAQ)