Definition: Plant: meaning a facilities / operation where manufacturing is occurring. Cooling: 1) free of heat 2) capable of lowering temperature. System: an assemblage or combination of things or parts forming a complex or unitary whole: a cooling system
In the previous 2 installments of this subject an overview of terms and the calculation of needs were discussed. So once one has established the needs of the cooling system, it is now a choice of how to supply those needs. This is through a tower system, a chiller system, or a combination of the two. Please note that in some areas plants are still using water from a city or municipality direct, meaning used once and dumped into sewer.
Before starting the discussion the following statement is made, "control of water temperature to a mold, bath is critical for the cooling rate of the plastic. The tighter one controls this, the better the operation, and more consistent the operation and process is. In temperature range an inlet temperature and outlet temperature of less than 2 degrees F is good for critical moldings whiles general molding may be able to get away with 5 degrees." Both towers and chillers are based from the manufactures with a 10 degree delta, meaning a 10 degree differential in temperature to and from the cooling object.
With a tower the cooling rate is based on 15,000 Btu's /hr. per ton. The temperature supplied to a plant is approximately 7 to 8°F above wet bulb temperature (std. is 78F with 95F inlet and 85F supply to process). Usually one is looking at 80 to 90F water temperature but design of system may provide lower temperature. Since ambient air temperature effects wet bulb temperature variation can occur in temperature and tonnage of unit. An example is that a 10 ton tower, at wet bulb of 70°F is rated at 14 ton and at 80°F wet bulb is rated at 8 ton. Again the delta for design is 10 degrees.
Concerns or things to consider with a tower are water supply, water usage, chemicals and system maintenance.
Since a tower works on an evaporation process to take away the heat, there is a constant supply of water to the tower to maintain levels. Thus one has to consider how much water to get going, where is water coming from, (hard/soft, mineral content). Further in operation one now has to consider usage of water to maintain all the levels. Since in fact this is an evaporation process when the water evaporates it leaves behind anything that was carried in the water such as Calcium, and other minerals which shall tend to plate out onto the surfaces of the tower, and or throughout the plant if a direct system** . To counteract this and to maintain the water supply many add chemicals which can precipitate and or reduce buildup and what is referred to a fouling of the water. (Fouling can be biological, mineral and contamination or other in this definition). Finally we have the maintenance of the system, which does include chemical additions and performance of the tower and its parts.
The tower is also open to atmosphere, by some it is referred to as an open system, on the pumping side to the plant, various methods are used to filter and or eliminate contact with this fluid. In one system that takes this fluid and pumps directly to the plant it filters the fluid to take out the impurities and then pumps it to the plant system before returning to the tower. In this system further maintenance and supplies are necessary to replace filters and change filters. In another system, which is referred to a plate exchange or heat exchange, the tower fluid is not in contact with the plant water but is used in combination with a heat exchanger that now eliminates the possible fouling of the molds, pipes etc. within the plant by the tower as it now only cools the heat exchanger which in turn extracts the heat from the fluid serving the plant. Fouling can occur in this area, but now not the molds and plant itself.
What is it that you wish to cool with the tower? Machine hydraulics, feed throats, thermulator, molds, extrusions tanks, all units where a temperature of 80 to 95 is sufficient. Understand that a tower, is a lower cost purchase, and the author has seen these used for entire plant cooling needs and also in combination with chillers to supply a plants cooling needs.
Chiller, with a chiller the cooling rate is based on 12,000 Btu's /ton and can supply fluid temperature in a range of 65 to 20°F. Again the size of the chiller is based on a 10 degree delta at a temperature of 55°F at 2.4 gpm. At temperatures below 50F there is a 2% decrease in rated capacity of the chiller, meaning that at 40°F a chiller rated at 10 ton is actually only capable of 8 tons of cooling. Chillers cost more than towers, but have a lower temperature range. They are also used for beside the press operations in plants that only occasionally need chilled water for one or two applications.
Chiller come in two types, air cooled and water cooled. As the names imply one uses air to cool the coils and drive away heat and the one uses water to perform the same job. If one uses air it should be in an environment that does not exceed 95°F in temperature. Location of the chiller should provide at minimum 800 cfm of air exchange per ton, if not there may be excessive trips of the safeties on the chiller. Caution** a chiller should not be up sized unless growth is emanate. As the condenser life can be shorten rapidly by sizing too big of a chiller, this is due to the on and off cycling of the condenser. By too big it is meant that for example our cooling needs are 5 tons, and we place a 10 or 15 ton chiller in the mix, chillers are more efficient when operation is continuous. Yes this is done all the time, but if one is experiencing down time with chillers due to compressor replacement this is something to look at.
If a water cooled chiller is used than that tonnage need be added to into the tower calculation as the demands of the chiller which is 1:1 meaning each chiller ton requires 1 tower ton of capacity, (if a tower is used). Water supply temperature to the chiller needs to be 65 to 85°F (16 to 29°C) and a rule of thumb states minimally 3 gpm.
If use temperature is lower than 50°F on the chiller than one may consider the addition of an industrial grade of ethylene or propylene glycol (anti-freeze) to prevent freeze up. The amount can be calculated by estimating the needs for the fluid to be 20 to 25°F below whatever is the coldest the chiller shall be set, this is due to the coils which run colder than the set temperature, an example being that a 45°F set temperature may show coils being at 25 to 20°F and thus must be protected.
If the chiller is set up inline to supply direct to the process than filtration should be installed (return side), to prevent clogging of internal flow paths. A basic Y filter shall do with a 40 mesh screen. This should be checked daily initially and adjusted to finding, meaning is clean check every other, but do put on a schedule. In many cases filters and additional chemicals to the fluid are used but in most cases it typically is antifreeze, but do not use auto antifreeze, as some of the ingredients stop leaks and may plug up small passages. Clean the cooling coils at least weekly, and if a portable unit it is highly suggest dot have a shutdown procedure, and startup procedure posted on the unit.
Whichever system or combination of system one uses, understanding the loads, usage and demands placed on them is critical. The location of said units and the ambient temperatures is also of vast importance. If temperature variation due to seasonal changes is not acceptable within the manufacturing process than use of a chiller should be considered, with a tower system to garner optimum control.
NOTE:
There are currently a few closed loop tower type systems which use air over a thermally conductive heat exchanger to cool the fluid. They also use minimal water to help cool when temperatures elevate, but use much less water than an evaporative system. These types of systems are gaining even through at present they are more expensive initially than a standard evaporative system.
Next: Pump Tanks
To be continued
06082012.01
Cooling part III TC
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