High-temperature hot water systems

A high-temperature hot water system is usually defined as a system operating above 149 °C (300 °F). The corrosion problems associated with such systems are summarized below  

In a properly designed system there is little opportunity for scale formation, because there is no evaporation within the system and thus little makeup water is needed. Therefore, sohds in makeup water do not concentrate, and saturation values are not exceeded. However, when designing such a system, it is a good practice to include the use of a pretreatment, such as zeolite softening. 

Deminerahzed water also may be used sometimes as makeup. Characteristics of makeup water are important with respect to corrosion in high-temperature hot-water systems. If the circulating water pH is properly adjusted, much of the corrosion potential can be minimized. In all-steel systems, the pH can be adjusted to 11.0 to minimize corrosion. However, in bimetallic systems, pH values should not be allowed to reach this level because of possible alkaline reaction with brass, bronze, copper, and/or aluminum. 

Before a new hot water system is put into operation, it should be cleaned of all pipe dope, grease or cutting oils, dirt, sand, and soldering flux. If these substances are not removed, they may result in the formation of concentration cells and greatly increase the corrosion load. Phosphates are 

Chromates, nitrates, nitrites, borates, and sdicates have been employed as corrosion inhibitors in hot water circulating systems. However, their use must be carefully controlled because they can cause problems in mechanical or patent circulating pump seals. Evaporation can occur, resulting in crystal-hzation of the inhibitor, with resultant wear on moving parts. Buffered chromates at 150 to 250 ppm concentration have been employed successfully. 

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Warm water or low-, medium- or high-temperature hot water systems are categorized in Table 27.8. Warm water systems may use heat pumps, fully condensing boilers or similar generators, or reclaimed heat. In many cases the system design may incorporate an alternative heat generator for standby purposes or for extreme whether operation. Under such circumstances the system may continue to function at warm water temperatures or could operate at more conventional LTUW ones. 

WATERSIDE PROBLEMS IN MEDIUM-TEMPERATURE HOT WATER AND HIGH-TEMPERATURE HOT WATER SYSTEMS... 

System type Low and medium temperature hot water systems up to 120°C High temperature hot water systems above 120°C ... 

Pressurization of medium- and high-temperature hot water sealed heating systems referred to above may take the following forms. 

MTHW/MPHW) systems typically operate above 250 °F and below 350 °F/176.7 °C, whereas high-temperature hot water and high-presssure hot water (HTHW/HPHW) systems operate above 350 °F... 

Performance and Reliability is why major sporting arenas, hospitals, military installations and other critical use facilities depend on our high performance, High Temperature Hot Water boiler systems. 

Temperature runaways [temperature hot spots] /[reactor instability]. Pressure and bed temperature and reactor unsteady water in feed/[maldistributionj. Local high temperature/hot spot with T > 100°C above normal [maldistribution of gas low[ /instrument error/extraneous feed component that reacts exothermically. Local low temperature within the bed [maldistribution of gas flow[ /instru-ment error/extraneous feed component that reacts endothermically. Exit gas temperature too high instrument error/control system malfunction. Temperature varies axially across bed [maldistribution[. ... 

Active systems that focused sunhght to produce high temperatures were developed in the nineteenth century. Domestic hot water systems were first buUt and marketed in the early twentieth century. By midcentury, active systems using air to heat homes appeared, but their acceptance was limited because of their high costs. 

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