Problems with Heating Coils

Low-temperature hot water (LTHW) and low-pressure hot water (LPHW) systems operate below 250 °F/121.1 °C (equivalent to 29.8 psia/15.1 psig). This is approximately the same as LP steam systems, which operate below an MAWP of 15 psig/29.7 psia, equivalent to 249.8 °F/ 121 °C. 

Many LTHW and LP steam systems run only on a seasonal basis, providing HW or steam for space heating in winter. 

Both types of boiler systems may incorporate finned copper heating coils, which are located above the furnace and gas-pass tubes (smoke tubes or fire tubes) and provide for indirect heating of domestic HW. Where coils are fitted and the boilers are only fired during winter months, domestic HW heating usually is provided via gas heaters for the summer. 

These coils are subject to a variety of aggravating problems that may develop over time and ultimately lead to coil replacement. The frequency of replacement may be as often as every 2 to 3 years in some cases but perhaps only every 7 to 10 years in others. The timespan depends primarily on the particular source of water used for boiler MU and domestic supply and any pretreatment provided. Usually the first time that developing problems may be seriously investigated is when users complain of an inadequate HW flow, reduced pressure, or low temperatures. 

Problems associated with domestic coil systems include  

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Problems with heating coils Internal coil corrosion Note corrosion debris is green hydrated copper carbonate Cu[11IC03 nH20 red cuprous oxide Cu20 /ntemal coil deposition Acid corrosion from soft water. Pinhole corrosion from 02 and C02. Erosion corrosion over 6 ft/s flow. Hard water scale from hard water. 

S. cerevisiae is produced by fed-batch processes in which molasses supplemented with sources of nitrogen and phosphoms, such as ammonia, ammonium sulfate, ammonium phosphate, and phosphoric acid, are fed incrementally to meet nutritional requirements of the yeast during growth. Large (150 to 300 m ) total volume aerated fermentors provided with internal coils for cooling water are employed in these processes (5). Substrates and nutrients ate sterilized in a heat exchanger and then fed to a cleaned—sanitized fermentor to minimize contamination problems. 

The heaters are fed from hot-water flow and return mains, and to ensure uniform distribution of the heating medium, adequate connections to each row or bank of tubes or sections are necessary. To reduce air-locking problems, venting of the heater flow connections should be arranged. Parallel and counterflow are common arrangements with water coils. Counterflow is preferred, as this gives the highest possible mean temperamre difference. 

Finally, we have been sloppy in associating the flow rate of steam with the heating coil temperature. The proper analysis that includes a heat balance of the heating medium is in the Review Problems. To side step the actual calculations, we have to make a few more assumptions for the valve gain to illustrate what we need to do in reality ... 

The coil heat transfer coefficient had a value of 0.1159 and a x2 statistic of 27.8. This value suggests that a gross error is present. Some clues can be found from examination of the residuals of the balances. They are presented in Table 7. There appears to be a problem with the balances for coils 2 and 3 their residuals are different from the others (especially coil 2). The reconciled value for the coil 2 cracking temperature is also significantly different from the measured value, thus suggesting an abnormal situation associated with both coils 2 and 3. 

As this trend levels off with larger columns, it is recommended that values estimated for a 60 cm column are used. If heat transfer is a problem, then heat transfer coils within the column, or even an external heat exchanger, may become necessary when operating a large, industrial bubble column-type fermentor. Scale-up of an internal loop airlift-type fermentor can be achieved in the same way as for bubble column-type fermentors for external loop airhfts see Section 7.7. 

Effective wastewater management in this phase of veneer and plywood production can be accomplished in part by in-plant operational changes and in part by equipment modification. For plants with hot water vats that are heated indirectly with steam coils, the discharge results from spills in loading and unloading the vat and from periodic cleaning. Plants operating in this manner need only to collect the water in small ponds or tanks and reuse it for vat makeup water. Adjustments in pH by the addition of lime or caustic soda may be required to control acidity and prevent the development of corrosion problems. 

Indirect, coil cooling is not generally applicable because of the danger of overcooling. The problem with most catalytic reactions is the removal of heat at a high temperature and not cooling, as such. Special means must be used, particularly where pressures and temperatures are high. Some of these are ... 

Despite their common usage, direct radiators achieve their modern success through the accumulated improvement of many decades of experience with such diverse problems as voice coil heat buildup, diaphragm nodes, enclosure types, and the compHcations of multiple radiator/crossover networks. 

The primary alcohol sulphate pastes with an active detergent vaiying between 30% and 75% AD are viscous liquids (see 5.6.1. and 6.6.3.)The apparent viscosity is dependant on AD concentration, temperature and the concentration of sodium salts (NaOH, Na2S04). High AD pastes tend not to deaerate and may therefore cause problems in bulk transfer. Pastes are stored in mildly agitated stainless steel vessels. Solidification should be avoided and therefore heating coils with hot water are required. The hot water temperature should be 45 C for Coco-PAS and 60"C for Alfol (C16 - CIS) PAS. 

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