Water-quality sampling is a critical step in many manufacturing processes. The usual way to determine water quality is through wet chemistry methodology, or better yet, to provide a slip-stream to an online continuous analyzer. However, because all lab chemistry and most online instruments cannot tolerate high temperatures and pressures, it is necessary to pre-condition many samples, which typically means first passing them through a heat exchanger -- in this case, a sample cooler -- and then through some form of pressure reduction. After conditioning, the sample stream can be collected as a grab sample for use by a wet chemical testing method or a portable instrument. For samples and processes that are more critical, online instruments are a better alternative to grab sampling, as they can quickly detect upsets and trends that indicate the water chemistry might be drifting out of spec.
Maintaining a sample temperature suitable for analysis is critical because the accuracy of most online instruments will drift as the temperature of the sample increases from 77oF (25oC). Even though many instruments have automatic temperature and solution compensation, it is difficult for most to maintain maximum accuracy when both variables are fluctuating simultaneously. By fixing the sample temperature as close to 77oF as possible, solution measurements can be made more accurately.
In a typical sample panel, the primary sample coolers each will use 7 to 12 gal/min of coolant flow for maximum heat exchange to occur. A bank of coolers for up to 15 sample lines (with a sample cooler per line) will require a flow rate of approximately 120 to 140 gal/min. At some plants, this much flow may be hard to find from a source that is cool enough to use. And, while abundant, use of some closed coolant water sources are counterproductive to the sampling process in that they are too warm to ensure accurate readings from the grab sample or online instrumentation. Furthermore, the heat that is rejected to the coolant system (63,000+ BTU per line, or a million BTUs per panel) can be problematic for the plant coolant system. There is no alternative, however. Without an ample and reasonably cool source of coolant, the sample process cannot proceed, and without proper sampling, the care and surveillance of valuable equipment is limited.
Cooling OptionsBased on the typical heat load from a 15-line sample panel, the primary sample coolers could require 80 to 100 tons worth of coolant an hour. Without access to a suitable closed or auxiliary cooling water source, another source would have to be made available. Possible options include an air-condensing chiller or a small forced-air cooling tower. However, both of these options can be expensive and may consume large amounts of space and energy. While a chiller can control water temperature effectively, the process outlet temperature that most chillers are designed to produce often is too low for primary cooling. (Smaller chillers commonly are used for secondary temperature control of the sample line.) Forced-air cooling towers tend to be large, energy-intensive units that can be costly to purchase and install. Like air-condensing chillers, forced-air cooling towers lose efficiency as the temperature of the surrounding air increases.
Many plants will try to use untreated groundwater or surface water supplies. Such water can be laden with solids or minerals. When the water passes through the shell side of the cooler, solids and minerals can plate out on the interior surfaces if the coolant temperature rises severely (greater than 30oF), thereby insulating the tubing and reducing efficiency.
In many cases, use of an air-cooled heat exchanger or untreated water source may seem to be the only alternative when sufficient plant coolant is not available. However, there is another alternative.
Where abundant, cool surface water sources exist (i.e., river, lake or dam), cooling water isolation skids may be a practical alternative to existing plant streams. These skids are designed to allow the use of high-solids or hard water supplies for cooling a process line or battery of sample coolers without concern that the solids in the raw water supply will precipitate out in the process or shell of the coolers. This type of system isolates the raw water from the process or coolers through the use of an intermediate plate-and-frame heat exchanger. The skid can maintain a relative constant temperature in the closed loop back to the process or coolers despite seasonal raw water temperature changes. In industrial process applications, this may allow for better product consistency. In sampling, the online instrumentation may maintain better accuracy, thereby providing better control of the chemical feed process employed to combat corrosion and steam carryover.
In a skid system, the raw water circuit flow is pumped from the source through the shell side of the exchanger and back to the source. (The raw cooling water to the exchanger must be pressurized, either by gravity or a separate pump.) A manual throttling valve is provided on the inlet side of the raw water circuit to the exchanger for isolation during maintenance and as a means of coarse temperature control.
The closed-loop circuit (process side) routes the flow of clean coolant among the exchanger and the process or coolers. A control valve is placed on the exchanger's outlet so cooled water from the exchanger can be blended with returning flow from the process or coolers to maintain a constant temperature in the process or cooler return line. A pump provides circulation within the loop. PH
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