As energy costs remain volatile and environmental regulations become increasingly stringent, more and more plants are looking for cooling equipment that will provide both high efficiency and low emissions over the lifetime of the application. Low flow, low temperature, high efficiency (LLH) cooling system designs offer a possible solution. These systems can provide optimal process cooling performance while reducing operational costs, raising energy efficiency and minimizing environmental impact.
LLH systems drive supply temperatures down and temperature differentials up. As a result, they use lower flow rates and smaller fans, ductwork, pumps and piping compared to conventional systems. LLH designs can be used in many applications, but they lend themselves particularly well to variable air volume (VAV) systems, which meet changing load requirements by adjusting the amount, rather than the temperature, of cool air that flows to the refrigeration zones.
Rockwell Collins, a manufacturer of communication and aviation electronics, had a 410-ton chiller installed in 1979 that was starting to require maintenance. The base load of the plant is 600 tons, and the owner wanted to find a way to add one 600-ton chiller to handle the load efficiently without making a lot of infrastructure modifications.
The original 410-ton chiller was designed for a 58°F (14°C) supply air temperature (sat) and a 45°F (7°C) leaving water temperature (lwt) on the evaporator with 3 gal/min/ton of 85°F (29°C) condenser water. The new 600-ton LLH chiller was built to operate with a 60°F (16°C) sat and a 40°F (4°C) lwt on the evaporator with 2 gal/min/ton of 85°F condenser water. The lower leaving water temperature enabled the existing air units to handle the added load with colder air, and the added capacity was provided without requiring any changes to air units, fans, ductwork, chilled water piping or condenser water piping. An additional benefit was that the new chiller is more efficient at a 40°F lwt at 2 gal/min/ton (0.63 kW/ton) than the old chiller was at 45°F lwt at 3 gal/min/ton (0.66 kW/ton).
Life-cycle cost-analysis tools can help system designers and plant managers determine the operating cost savings potential of an LLH system by calculating the optimum supply air temperature and leaving water temperature. These design tools also can evaluate a job from a life-cycle cost and “carbon footprint” perspective, providing detailed information on levels of utility-generated CO 2, SO2and NOX.
Figure 1 shows an example of how such tools can determine energy savings. The base “conventional VAV” system was operated on six different sites, and the energy consumption was set at 100 percent at each site. (By definition, a conventional VAV system is designed for a 55°F [13°C] supply air temperature [sat], a 44°F [7°C] lwt and a 10°F [5.6°C] differential for both chilled water and condenser water.) By comparison, the right side of the graph shows the energy consumption and energy savings achieved by using LLH concepts.
The key LLH system concepts used in this cost-analysis model originate in every part of the cooling system, including the cooling towers, cooling coils, pumps, chillers, air handlers, VAV boxes and controls.
Cooling TowersAn LLH design can provide benefits to cooling towers in any size range, from 50 to 50,000 tons or more. For example, an 800-ton, two-chiller application with a conventional VAV system would require 2,400 gal/min at a 10°F (5.6°C) differential or 3 gal/min/ton. A typical cooling tower for this application would require about a 40 BHP motor.
In contrast, an LLH design with a 41°F (5°C) chilled lwt would require only 1,600 gal/min at a 15°F (-9°C) differential or 2 gal/min/ton. A typical tower for this application would require a 30 BHP motor. In addition to using less BHP, this tower would be smaller and weigh less.
Of course, the higher temperature differential would cause the chiller to work at a higher head and consume more energy. Running a life-cycle cost-analysis program could help engineers strike the optimum balance between the tower and the chiller.
Cooling Coilsis on the chilled water side of a system. Consider a conventional system in which the typical design differential is 10°F (5.6°C). If the temperature entered at 44°F (6.7°C), it would leave at 54°F (12.2°C), and the system would require 101 gal/min of water to produce 504,000 BTUs/hr (41 tons). For a new LLH design using a 16°F (9°C) differential, the same coil, now with a 41°F (5°C) entering water temperature, could produce the same 504,000 BTUs/hr with only 63 gal/min, or 37.5 percent less water.
If an existing system has a cooling coil with a two-way valve and a chiller water pump with a variable-frequency drive (VFD), the same benefits could be achieved by implementing an LLH design and turning the chiller water temperature down from 44°F to 41°F.
With the two-way valve in place, the leaving water temperature does not go down; instead, it actually goes up, typically increasing 0.5 to 1°F (0.3 to 0.6°C) for every 1°F reduction in the entering water temperature, depending on the type and circuitry of the coil. It also is important to note that the capacity delivered is the same, so the leaving air temperature does not change. This means that the same capacity (in BTU/hr) can be delivered at substantially reduced water flows (in gal/min), or that additional capacity could be obtained if the water flows were maintained and the entering water temperature was reduced. For the right applications, these alternatives can provide significant opportunities either to reduce pumping BHP or to increase cooling capacity.
PumpsMany plants would like to lower the water temperature to reduce chilled water pumping energy consumption. In a conventional 800-ton VAV system that requires 1,920 gal/min (2.4 gal/min/ton), the energy pulled by the motor would be 52 kW, assuming a total head requirement of 110' with pump and motor efficiencies of 80 and 95 percent, respectively. On this same system, if a VFD were applied to the pump and the chiller could produce the low temperature, the required flow would be reduced from 1,920 to 1,200 gal/min (1.5 gal/min/ton) if the supply water temperature setpoint were reduced from 44°F to 41°F. In this example, the total head would be reduced to 49' by following the pump law of head, which states that friction loss is reduced by the square of the gal/min flow reduction.
The pump and motors might need to be resized to get back to their original efficiencies, but the energy requirement would be reduced to 16 kW - a 36 kW savings, which represents nearly a 70 percent reduction in pump energy consumption.
The reduction in pump energy illustrates the pump law, which states that power varies by the cube of flow reduction. This is especially true if a critical zone reset strategy is used that allows the pressure in the piping to be driven only by the most pressure-demanding valve.
In either new construction or existing plants, the pump energy can be cut by more than 66 percent while the pipe sizes remain the same. On a new construction job, the pipe size might be reduced with the goal of optimizing the first cost, operating costs or both.
ChillersWhile turning down the leaving water temperature reduces energy use in the pumps and other areas of the cooling system, it increases the energy consumption of the chillers. The 800-ton refrigeration system example cited earlier uses two 400-ton screw chillers. Those two systems consumed 464 kW when they were producing 44°F water. When the leaving water temperature was reduced to 41°F, the energy consumption jumped to 490 kW.
It is important to note, however, that the meter is not hooked to the chiller alone, but to the entire plant. While the chiller consumed an additional 26 kW, the chilled water pumps saved more than 36 kW for a net 10 kW savings. Plants that use highly efficient chillers would see an even greater savings.
One way to recover the lost 26 kW of energy would be to use chillers piped in a series, rather than the conventional method of piping chillers in parallel. Series chillers take advantage of cascade cooling - i.e., the first chiller produces a higher leaving water temperature and works with a higher suction temperature - and therefore cost less to install and operate. Even when combined with the downstream machine, which produces colder water than a conventional system, most series chillers will show a savings in first cost (because the upstream chiller can produce more capacity) from 2 to 4 percent while also showing energy savings of 4 to 10 percent. And even with the extra pressure drop of placing the two evaporators in series, the overall system efficiency typically will be improved.
Another important reason to use series chillers is to facilitate the use of variable primary flow systems. In a common application with two series flow chillers, no change in flow will occur in the first chiller when the second chiller begins operation. Compare this situation to a parallel flow application on a common pump set. When the second chiller is activated, valves on the second chiller are opened, and the flow through the first chiller is reduced. It is a race to make sure the second chiller can be brought online and both chillers stabilized before the safety shutoffs are activated or, worse, a chiller barrel freezes. While variable primary flow systems can be implemented on parallel chiller installations, series chillers are much easier to control in this application.
Low-Air-Temperature ApplicationsAt least half of the energy consumption of a process cooling system typically comes not from the refrigeration side, but from the air side of the system. For this reason, LLH designs frequently are extended to the air side using supply air temperatures in the 45 to 48°F (7 to 9°C) range for chillers and 50 to 52°F (10 to 11°C) for packaged equipment.
The goal is to optimize the energy consumption of both the air and refrigeration sides of the system. In plants with an efficient centrifugal chiller, the right “balance” sat typically is around 45°F (7°C). With less efficient screw chillers, a 48°F (9°C) sat offers the best balance. If scroll compressors are used, especially in air-cooled applications such as rooftop VAV, the right balance might be closer to 52°F (11°C).
Energy is the single largest operating expense in most manufacturing plants, and these costs are rising rapidly. LLH cooling system designs can help maximize energy efficiency while minimizing emissions. Such systems yield sustainable environmental and financial results over the lifetime of the plant.