Selecting a Process Cooling System
Do You Need A Mechanical Chiller?For cooling temperature requirements above 80oF (26oC), an evaporative cooling system such as a cooling tower or a radiator-type fluid cooler usually is sufficient. Processors can save up to 98.5 percent of process water costs by using cooling towers to remove process heat. These systems provide an economical source of water that can be used with heat exchange equipment such as hydraulic coolers, air compressor intercoolers and aftercoolers, and refrigeration condensers.
Alternative methods of evaporative cooling such as wells, ponds and city water can be considered, but each has drawbacks. City water tends to be prohibitively expensive. Wells, particularly deep ones, can be effective cooling water sources, but they are limited in availability, and their maintenance requirements usually are more expensive. Ponds also can be used as long they are big enough to provide enough water and to sustain a uniform temperature; however, water from ponds may not be sufficiently clean for use in a process cooling system. Heat exchangers normally are added to evaporative cooling water sources to create a closed-loop system.
Mechanical refrigeration equipment is required to produce chilled water in the lower temperature ranges. Chillers can furnish water in the leaving water temperature (LWT) range from 70 to 20oF (21 to -6oC). Temperatures as low as -30oF (-34oC) can be obtained with specialized equipment. If the LWT will be less than 45oF (7oC), an antifreeze solution such as ethylene or propylene glycol is needed to prevent freezeup.
The refrigeration circuit of a mechanical chiller consists of three basic components:
In the evaporator, the refrigerant absorbs heat from the process water as it moves through the chiller. The boiling refrigerant turns into a hot vapor, and the compressor pressurizes the vapor so it can be moved to the condenser, where it is cooled with either air or water. The cooled refrigerant returns to liquid form and is cycled back into the evaporator (figure 1).
Sizing the ChillerChoosing the correct chiller depends largely on the application size. For every 12,000 BTU/hr of heat removed, one ton of chilling is needed; however, this figure is based on a LWT of 50oF (10oC). Liquids are more difficult to condense at lower temperatures, so if the LWT needs to be less than 50oF, the chiller will be derated two percent for each degree below 50oF. Factors such as condenser water or air temperature and flow also affect the efficiency of the chiller, so it is important to consult with an experienced equipment supplier or installer when sizing a chiller.
Portable or Central? Mechanical chillers primarily are classified as portable or central. Portable chillers are self-contained and can be moved from one point in a plant to another. Generally, portable chillers are used only where capacity requirements are 20 tons or less, but many models are available up to 40 tons. Because they can easily be moved and interchanged, portable chillers provide flexibility and can minimize the effects of downtime.
Portable chillers most commonly are used where additions are made to an existing plant or where an existing central chiller installation needs supplementation. The fact that they are mounted on wheels makes it simple to roll them to the point of use. When purchasing a new central chiller with higher capacity is too costly, existing equipment can be supplemented with a portable chiller to increase production. Portable chillers also are used when different temperatures are required at different locations. They are especially advantageous for low temperature cooling applications (below 50oF) where operating a central chiller below 100 percent efficiency may be too expensive. Sometimes a set of independent portable chillers can be tied together with a central tank system, giving the system redundancy and improved efficiency.
Central water chillers are used as a common chilled water source for a number of machines or an entire plant when only one temperature is required. Sizes for central chillers range from 30 tons to 300 tons or larger. This type of chiller generally has multiple compressors, more efficient unloading and more sophisticated controls. One central chiller running at full capacity is more efficient and less expensive to run than several portable chillers with smaller, partially loaded compressors.
CompressorsOne area where chillers technology has improved in recent years is in the types of compressors available. The chiller's compressor is responsible for removing suction vapor from the evaporator circuit, raising its pressure enough so that heat transfer takes place in the condenser. The three most common compressor types are reciprocating, scroll and screw.
Reciprocating compressors use a piston design to increase the pressure of the refrigerant gas. Various combinations of piston sizes (bore), length of piston travel (stroke), number of cylinders and shaft speed determine the compressor's power. The piston creates pulsating bursts of compressed air, making reciprocating compressors relatively noisy. The friction created by the piston can reduce efficiency and cause high vibration levels. Reciprocating compressors have become obsolete because of energy efficiency concerns and the availability of rotary compressors, including scroll and screw designs. The smooth compression used by rotary compressors reduces vibration and noise.
Scroll compressor technology is based on two scrolls: the first is fixed while the second scroll orbits around the first. The interlocking scrolls trap the gas between them, and as the mobile scroll moves around the fixed, the gas is compressed. The mobile scroll makes three orbits around the fixed. During the first orbit, gas is trapped and enclosed in two pockets at the opening of each scroll. The second orbit moves the gas closer to the center of the scroll, where its volume is reduced. The third orbit compresses the gas further, and the gas is then discharged through a port in the center of the fixed scroll (figure 2). Scroll compressors require less electricity but are only available up to 25 hp. Multiple scroll compressors often are used together to increase the capacity of the chiller or to create a central chiller.
Screw compressors work by trapping gas between the threads of one (single-rotor) or two (twin-rotor) screws. In a twin-rotor screw compressor, the most common type, the two screws rotate against each other, trapping and condensing the gas between them. In a single-screw design, the screw rotates against two stars, trapping gas in the threads of the single screw. Screw compressors generally are used from 30 hp.
CondensersA chiller's condensers remove heat from the refrigerant, converting the hot gas from the compressors into a liquid refrigerant that can absorb heat from the process water. Because the condensers must absorb heat from the refrigerant, they must be continuously cooled with either air or water.
Water-cooled equipment is more efficient than air-cooled equipment. The lower head pressures made possible by water-cooled condensers increase equipment capacity and reduce the required compressor horsepower per ton of cooling. Water-cooled condensers tend to be more efficient, smaller and quieter than air-cooled types and should be used whenever cooling water is available. Water-cooled chillers are usually available with or without an integral pump tank.
If a source of cooling water is not available, air-cooled condensers are required. Because air-cooled condensers give off a considerable amount of heat, much thought must be put into where to locate the condenser. Outdoor air-cooled chillers are located outside to keep hot air away from temperature-controlled areas inside the plant. Split system chillers are available with remote condensers located outside, connected to the evaporator and compressor inside the plant. Split systems are sometimes thought to be advantageous because of the cooling properties of outside air, but fluctuations in ambient temperature may interfere with condensing efficiency. These systems require more components and specialized personnel for installation. Heat-reclaim chillers are located inside the plant. Hot air from this type of condenser can supplement plant space heating needs during cold weather. When space heating is not required, the hot air can be vented outside (figure 3).
RefrigerantR22, an HCFC-based refrigerant, has been a popular choice for most chillers. However, the 1990 Clean Air Act Amendments call for an end to R22 production by 2030. R22 has been targeted by the Montreal Protocol to be phased out in new equipment by 2010 and eliminated by 2020. Additionally, the European Union is enforcing an accelerated phase-out, banning HCFC-based refrigerants for new installations starting in 2004, with a goal to eliminate service for existing installations by 2010. Europe's accelerated phase-out timeline may push North American processors to accelerate phase-out as equipment manufacturers meet the earlier deadlines to remain competitive in Europe.
Because of these restrictions, R134A and R410A refrigerants are increasingly available, especially for large, screw-compressor chillers. R410A is a blend of HFCs, which do not deplete the ozone but may contribute to global warming. R134A refrigerant is environmentally safe and operates under lower pressures, reducing the amount of energy consumed.