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Capex-opex conundrum

This article is to demonstrate the direct higher operating costs linked to a customer making a decision based on the first cost of evaporative cooling equipment; The importance and fast payback that certified evaporative cooling equipment brings to the total system cost is very evident, say Rafael Van Eijcken, General Manager, BAC Middle East, and Emmanuel Timothy, Senior Project Engineer, BAC Middle East

| | Dec 12, 2021 | 7:49 pm
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By only looking into initial cost as the principle for the value for money, a customer ignores the impact the equipment causes when it comes to operating performance of a cooling tower in an HVAC plant. A certified unit may typically be more expensive compared to non-certified equipment, which measures differently in size, weight and power. However, if customers focus on the importance of certification and understand how easy it actually is to verify on this prior to selecting a model or manufacturer, it helps avoid the situation of choosing the wrong cooling tower, which can cause serious consequences due to performance issues in the installed site.

Before we address the problem of performance verification, let us consider a typical example in the Middle East region, where an HVAC application is operating year around with a load variation from 100% in summer to 75% in winter. The summer condition is to cool 100 LPS from 40 degrees C to 35 degrees C at an ambient wet bulb temperature (WBT) of 32 degrees C. In this case, the cooling capacity to be rejected would be 2,093 kW.

1) Let us refer the selected cooling tower with 100 LPS @ 45 degrees C – 35 degrees C – 32 degrees C as CT-A. The units will have a length of 2,585 mm, a width of 5,500 mm and a height of 3,653 mm, with 29 kW absorbed fan power and 30 kW installed fan motor. The overall sound power level of CT-A would be 104 dB(A).

2) Now, let us consider an undersized model CT-B, which would only deliver 85%. Typically, this unit would be 15% smaller in physical size or, alternatively, have the same size as CT-A but with an absorbed fan power of only 20.1 kW with a 22 kW installed fan motor. The overall sound power level of CT-B would be 102 dB(A). CT-B is lower in cost compared to CT-A.

3) For 2,093 KW, which has to be dissipated at WBT 32 degrees C, CT-B will supply 1.35 degrees C warmer outlet than that designed. It will take a WBT of 30.79 degrees C to supply the required 40 degrees C / 35 degrees C water temperatures.

4) Many a times, when customers contact a cooling tower manufacturer on the problems faced on site, the cooling tower manufacturer’s response would be that the supplied cooling tower is correctly sized, with safety in mind, although they are aware that they are taking a risk, considering that the customer will not opt for verifying the documents, since the first cost of CT-B was the main benefit in front of the customer.

5) It is hard to find out on site the problems caused by an undersized cooling tower, until the problems are really felt by the customer to an extent, when the performance issues create serious problems to the site, several years later.

6) Although there is power saving on the fan motor kW for CT-B compared to CT-A, the excessive pressure caused by the 1.35 degrees C warmer water supplied to the chiller will, in turn, increase the chiller power consumption during a year-round operation by six per cent, which economically becomes a worry, as customer will start to feel it when the bills paid are becoming higher.

Let us now look at the economic annual impact the customer will face by considering the above example by using CT-A and CT-B. Both cooling towers will use variable-frequency drives and run with a concentration factor of five.

Rafael Van Eijcken

The fan kWh requirement-based year-round operation for CT-A will be 55,540 kWh; for CT-B, it will be only 50,800 kWh. However, it is a different story if we look at the electrical energy needed for the chiller. For CT-A, we need 1,114360 kWh, but for CT-B we would need 1,178,700 kWh, which is six per cent more. Therefore, by adding the chiller and fan kWh, CT-B still needs five per cent more electrical energy on an annual basis. At a typical cost of AED 0.38 fils/kWh, this will represent an annual operating cost of AED 22,648.

In addition, CT-B will have more water consumption, as the chiller will need to work harder, which means more waste energy is dissipated, which will result in more water that is evaporated. Considering CT-B, the water consumption will be 300 m3 [65,991 Imperial Gallons] more.

If we consider the water rate at AED 4.6 fils/IG (plus the sewerage charges), this will add another AED 4,500 per year, approximately. Refer to the below chart as an illustration between additional cost versus energy and water costs with a three per cent year on inflation rate price increase.

Emmanuel Timothy

The total operating cost difference for water and electricity for the system with CT-B will be AED 27,148. This will result is almost half of the first cost of CT-A. From this, it is clear that a small price advantage for CT-B, which may exist, melts away the benefits, like rainfall while the sun is shining.

Conclusion

  • Value for money does not just come by looking at first cost and blindly accepting a manufacturer’s document, which makes a self-declared thermal performance.
  • A customer must be able to challenge such a declaration of a manufacturer by questioning whether the thermal performance has been tested previously, or insist for an acceptance test according to a recognised standard as the minimum needed to take out the guesswork in believing the declared thermal performance.
  • By insisting on a performance test, the customer will be able to find out at the time testing if the cooling tower fails in the test. But a failed test will not end the misery the customer is going through but increases further, wherein many unpleasant discussions with the manufacturer carry on with no final resolution.
  • The smart way to handle this problem is to select a certified cooling tower via either third party controlled outside or internal lab testing. Only then the owner is sure prior to purchase that they will not have higher operation costs due to underperformance. Certified thermal performance testing removes risk towards system economics and removes guess work; it also removes the problems of litigation, penalties and compensation, should an already purchased product be found to underperform, because by then it is too late!

 

 


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