When upgrading an existing cooling system, a company usually asks multiple suppliers to quote on a specific piece of equipment. A more effective approach would be to analyze the situation by preparing a heat load evaluation and a load profile to determine the actual requirements.
By investigating and compiling all of the various heat load requirements, including entering and leaving water temperatures, as well as approximate time frames of operation, recommendations can be made for load shifting or implementing a free cooling approach, should these opportunities appear viable. Further discussions can also determine applications for heat recapture. Here, examples help show how these techniques work - separately and together -for optimal improvement.
Load ShiftingTransferring a heat load that has previously been on a chiller to a cooling tower or fluid cooler system is called load shifting. As an example, supposed you have a 100 hp air compressor with an air dryer that requires approximately 24 tons of cooling, which would require 28 kW of energy when 100 percent loaded. At a diversity factor of 80 percent, the total kilowatt hour (kWh) would be 22.4 kWh. At a cost of $0.13 per kWh, the cost of running the air compressor is $2.91 per hour, annualizing the operating cost at $25,509.
If the air compressor duty was “load shifted” to a cooling tower - assuming the pumping for each system is equal - the only energy consumed would be for additional fan horsepower in the tower. The net energy savings could be 20.6 kW, resulting in a net hourly saving of $2.67 per hour, and an annualized savings of $23,289.
Free CoolingIn “free cooling” applications that need temperatures cooler than available tower water on the warmest days - for example, a tower that operates above 80°F (26°C) on warmer days - a heat exchanger that will pre-cool the chilled water with cooling tower water can be used for a good portion of the year.
Consider an example: an application in the Boston area with a chilling requirement of 60°F (15°C) to process. The ASHRAE design wet bulb temperature is 75°F (23°C), and a leaving water temperature of 82°F (27°C) will be achieved. With a properly sized cooling tower, a heat exchanger assembly and filtration, it is possible to deliver 57°F (13°C) water to the heat exchanger, resulting in 60°F (15°C) water to the chiller. Because the chilled water flowing through the chiller is at the chiller setpoint, the chiller does not operate. In this situation, the resulting savings would be 4,764 hours of operation.
Additional savings also could be achieved during partial-load operation of the chiller. As an example, the entering water to the heat exchanger may be 62°F (16°C) and the chilled free cooling may remove only 5°F (2.7°C). In this situation, the chiller would only operate at 50 percent of its capacity, providing further energy savings of an additional 798 hours.
Heat RecaptureIn process cooling, emphasis has always been on removing the energy from the process and dissipating it back into the atmosphere. This is done by utilizing air-cooled or water-cooled condensers connected to cooling tower systems, and through the use of cooling towers.
As the price of energy has risen for both electricity and fossil fuels, this heat becomes more valuable. Plants want to be more efficient on utility costs, so it is important to review how this heat can be captured and used in the facility.
For example, in many plants, boilers are used to heat up the process, and then the processes are cooled with cooling tower water or chilled water. Boiler makeup water is required on a constant basis. If a boiler required an average of 20 gal/min of makeup water and entering city water temperature is 50°F (10°C), this water would have to be heated from 50 to 212°F (10 to 100°C) in the boiler.
A more cost effective solution would be to use a cooling tower system delivering 90°F (32°C) water to the tower. Boiler makeup could be preheated to 87°F (30°C) by utilizing the tower water and a heat exchanger. This would result in a savings of 370,000 BTU/hr and reduce the natural gas costs to operate the boiler. In a plant that operates around the clock all year, this would result in savings of 10.27 m3 of natural gas and reduce the cooling tower electrical load by approximately 24.7 tons per hour. (Note that 1 m3 of gas is approximately 36,000 BTU/hr.)
In conclusion, when load shifting, free cooling or heat recapture are implemented, it may also extend the life of the process equipment by reducing the usage of the primary systems. In many states and provinces, there may be power saving programs that will assist in funding the energy savings portion of a project. Funding may be based on peak demand savings, total annualized kilowatt hour savings or a percentage of the total cost of the energy efficient project. Information for these programs is easily available from your local utility supplier.
It is easy to see that the time spent in pre-planning can increase productivity and reduce annual operating costs.