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Condenser water reset made easy

Any chilled water plant – from the largest district cooling plant to a standalone system in a building – should take advantage of Condenser Relief strategy

| | Nov 19, 2020 | 6:46 am
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In the October 2020 issue of Climate Control Middle East, I wrote about Wet Bulb Temperatures, Dry Bulb Temperatures, Approach and Range of cooling towers. Now that those terms are fully understood, it’s time to implement a powerful energy saving solution – Condenser Water Reset, also known as Condenser Relief. The fact is that any chilled water plant – from the largest district cooling plant to an office or residential building with one or two water-cooled chillers – can and should take advantage of this strategy. The savings achieved by lowering condenser entering water temperatures are impressive. Typically, compressor power savings are usually 1.5% per degree Fahrenheit or 3% per degree Celsius.

Considering that the title of this article contains the words, “made easy”, I’m not going to get into technical explanations of Lift and Surge; I will just say that you need to keep a centrifugal chiller out of Surge at any cost. Condenser Water Reset/Relief will also protect your chillers, as it will keep a Centrifugal Chiller Compressor within its Lift Range and keep the compressor out of a Surge or Stall condition. In other words, it will protect your compressor from a catastrophic failure.

Chillers nowadays should be selected and operated with ARI 550/590 part load condenser relief calculations in mind. This rating, termed IPLV/NPLV for Integrated Part Load Value/Non-standard Part Load Value, is part of the ARI Standard 550/590-98. The standard, which became effective in December 1998, also includes a revision of the evaporator fouling factor from 0.00025 to 0.0001. All chiller manufacturers must certify chiller performance to the standard.

Chillers rarely operate at design conditions, because design conditions of both design load and design entering water temperatures occur during less than one per cent of chiller operating hours. That means that over 99% of potential chiller operating hours are spent at off-design conditions: Reduced loads, reduced ECWT/EDB, or both. This fact allows the implementation of condenser water reset to not only save energy but to also protect your chiller.

Centrifugal chillers – all of them, no matter the OEM – are designed to use less power by taking advantage of lower-than-design water temperatures that are naturally produced by cooling towers throughout the operating year.

Whether you have a constant speed or VFD chiller, you can take advantage of Condenser Water Reset.

Now that we understand that Condenser Water Reset saves large amounts of energy and protects the chiller, as well, how do we implement it? Well, you could run your cooling towers at full fan speed all the time to try and bring the condenser water temperature as low as you can year round. In the GCC region, this actually would work and save substantial chiller power; however, cooling tower fans can consume from five per cent to 20% of total system power. At some point, attempting to lower condenser water any further will cost more fan energy needlessly. So, we must find a more efficient way.

Remember, you should look at total system energy, not just the chillers. If you have a BMS or CMS, you can have it programmed to perform condenser relief; however, you know the old saying, “Garbage in, Garbage out”. In other words, the person programming it must understand how cooling towers and centrifugal chillers work. Having said this, if you are an owner of a building or are responsible for its operation, you might not have a highly skilled programmer at your disposal who understands all that’s required, so I will provide simple guidelines that will protect your chillers while saving you substantial energy. Granted a more robust program can be developed with tables, graphs of compressor and fan power, chiller characteristics for each combination of compressor load and outside air temperatures – dry and wet bulb – to provide the lowest cost entering condenser temperature and power savings. The problem with this is that most standalone buildings will not invest in a full CPECS (Chiller Plant Energy Control System). They should, but most won’t. The following is for those who do not have a CPECS but want to take advantage of Condenser Water Reset at a low investment:

1) Have your BMS programmed to provide condenser water temperatures as per the chiller’s data information sheets, and operate the condenser water at IPLV/NPLV rating at 100%, 75%, 50% and 25% loads. In other words, if the chiller is operating at 75%, the data sheets will state the required entering condenser water temperature.

2) Program your BMS to control the condenser water leaving tower temperature to 3.8 degrees C above the existing Wet Bulb temperature. In other words, if the Wet Bulb temperature outside is 28 degrees C, have the cooling tower operate to provide 31.8 degrees C water temperature. Running the tower fans any faster or longer will not get the water temperature below 31.8 degrees C. Generally, a cooling tower will provide condenser water at 7 degrees F/3.8 degrees C above the Wet Bulb temperature. In other words, the condenser water entering the chiller condenser will be 7 degrees F/3.8 degrees C above the Wet Bulb temperature. For example: If the Wet Bulb temperature is 28 degrees C, the entering condenser water temperature is 31.8 degrees C. If the Wet Bulb temp is 22 degrees C, the entering condenser water temperature is 25.8 degrees C.

3) Install an automatic cooling tower controller, set it up, as explained above, and you will save substantial power and money and also protect your chillers.

These automatic controllers are relatively low cost, a few thousand dollars and will work well.4) Manually control the condenser water temperature. Get your operator a sling psychrometer. A psychrometer places a thin film of water on the bulb of a thermometer that is twirled in the air. After about a minute, the thermometer will show a reduced temperature. The low point, when there is no additional twirling, is the Wet Bulb temperature. After the operator has the Wet Bulb temperature, have him or her set the condenser water temperature to 3.8 degrees C above the Wet Bulb reading. Have him or her do this every hour, and you will be pleasantly surprised by the amount of savings achieved.

Automatic Condenser Water Reset, when set up properly, will just cost a few thousand dollars and save between 10% and 15% annual chiller energy. Condenser Water Reset performed manually will just cost you the price of a sling psychrometer, maybe USD 100, and save between three per cent and eight per cent annual chiller energy. Either way you get it done, I suggest you get it done, as the savings are there for the taking with minimal investment.

 

Dan Mizesko is with U.S. Chiller Services. He may be contacted at dmizesko@uscsny.com.


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