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The significance of range and approach

It’s important to understanding cooling tower principals to achieve greater chiller efficiency, says Dan Mizesko

| | Oct 28, 2020 | 11:15 am
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It’s great to be back in Dubai!

I was in the United States in January, and we were starting a major retro-commissioning project of the 45,000 RT Harvard Medical Center District Energy Plant. In March, when COVID-19 was starting to have a major outbreak in the Boston area, we were forced to stop the project and demobilise. The team of US Chiller Services engineers and I were forced to quarantine in the United States, until we could all get back to Dubai.

I came back on July 30. I was not even back for one week when someone called me requesting assistance, as their chillers were not meeting the Design Chilled Water Set Point. As I knew, it was uncomfortably warm and humid. I immediately asked what his entering condenser water temperature was and was told it was 37 degrees C. I asked a few more questions, such as if he knew the wet bulb design of his cooling towers, and I realised that despite being an experienced engineer, he was unsure of some of what I was asking.

I requested him to send me the documentation, which he did. On analysing, it emerged that the problem was his cooling towers were not designed to operate at capacity with the wet bulb temperatures Dubai was having at the time.

Being back in Dubai, I was preparing to write an article for this column on the topic of Condenser Water Reset and Condenser Relief, when it struck me that a brief explanation of Dry Bulb Temperature, Wet Bulb Temperature, Tower Approach and Range might be a good idea before tackling Tower Reset and Condenser Relief, hence the reason for the following write-up…

If you question most chilled water plant operators and ask them to explain the purpose of a cooling tower, most would say something like, “The tower is supposed to cool the temperature of the water to the design entering water temperature of the chiller.” Further questioning would typically reveal that most are not able to explain how the ambient temperatures and humidity affect the performance of the tower. If the temperature was 30 degrees C outside, the average operator might tell you that they would expect their cooling tower to put out 30 degree C water. However, most times, this is not the case. In addition to that, one must ask, “Is the tower operating efficiently?”

Two important factors that determine cooling tower performance are range and approach. To understand these, we must understand the difference between dry bulb and wet bulb temperatures.

Temperature

A typical thermometer indicates dry bulb temperature. It does not take into account the relative humidity in the air. Relative humidity is an expression of how much moisture is actually in the air, compared to how much there could be at this temperature. If the humidity is 100%, the air is completely saturated with water and no evaporation is possible. This means that the cooling tower cannot benefit from the effects of evaporative cooling. The cooling tower at 30 degrees C with 100% humidity will underperform another similarly sized cooling tower at 30 degrees C with 40% humidity.

To measure the effects of the temperature and humidity together, a psychrometric chart is needed. The chart combines the effects of humidity and temperature to calculate the ‘wet bulb temperature’. There are also many online calculators available that will require you to input the temperature and relative humidity to calculate the wet bulb temperature. These are great tools for any plant to check tower efficiency.

The wet bulb temperature describes the effects of evaporative cooling on cooling towers. This is why all chilled water plants should measure ambient dry bulb temperature and humidity and input these into a wet bulb calculator.

Approach

Cooling tower approach is the difference in temperature of the water entering the basin/sump and the wet bulb temperature. A cooling tower with a smaller approach – that is, small delta between basin/sump water temperature and wet bulb temperature – is considered superior.

Range

Range is the difference between the temperature of water entering the cooling tower and the temperature of the water leaving the cooling tower. It is determined by the heat load on the tower and the water circulation rate. If the pump speed is constant and heat loads are constant, the tower range does not change. This means that for a clean, properly functioning tower, the wet bulb temperature does not affect cooling tower range. Consequently, in practice, for a given water flow-rate and heat load, if the wet bulb temperature increases, the tower inlet and outlet temperatures increase proportionally. The result is an unchanged range.

Typically, cooling towers are designed to cool a specified maximum flow-rate of water from one temperature to another at an exact wet bulb temperature. For example, a designed cooling tower may be guaranteed to cool 10,000 gpm of water from 35 degrees C to 27 degrees C at 23 degrees C wet bulb temperature. In this case, the range is 8 degrees C and the approach is 4 degrees C.

In closing, with an understanding of wet bulb temperature, range and approach, we are able to understand the lowest basin/sump temperature for the current temperature and humidity. Using the cooling tower range, you can estimate how efficiently the condenser loop is functioning in comparison to cooling tower design.

Next month, I would like to explain Condenser Water Reset/Condenser Relief; and with the clear understanding of Wet Bulb, Range and Approach, you will understand how we can efficiently operate condenser pumps and tower fans without trying to overcool the condenser water, which is simply not physically possible.

 

The writer is Dan Mizesko at US Chiller Services. He may be contacted at dmizesko@uscsny.com.

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