License to Chill - October 2020
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.