098

20%

Table 36-2 Comparison of Heat Recovery Design Set Points.

achieve full capacity and improved efficiency. However, efficiency will always remain lower than a comparable chiller system designed for a lower lift requirement.

A variation of this applied chiller heat-recovery technology involves the use of two interrelated systems. One system functions at typical temperatures and efficiencies with, for example, a condenser water rise of 85 to 95°F (29 to 35°C). In such a system, the 95°F (35°C) exiting condenser water bypasses the cooling tower and enters the second system where it is cooled by refrigerant and returned at 85°F (29°C). To accomplish this, the compressor work is relatively low. The refrigerant in the second system is then condensed, giving off its heat to the heating loop. Thus, 12,000 Btu/ton-h (1 kWhh/kWhr) plus the heat of compression from both systems' compressors are passed to the heating loop. When heating requirements are lower than the heat energy made available by the cooling process, it is bypassed to the tower.

Absorption Cycle Heat Recovery

Though not commonly used in heat recovery applications, absorption cycle systems provide abundant quantities of low-temperature heat. While a single-stage LiBr absorption chiller with a COP of 0.66 is considered a very inefficient refrigeration machine, it is an extremely efficient heat-producing machine. While 1 Btu (or kWh) of heat input produces only 0.66 Btu (or kWh) of refrigeration effect, it produces 1.66 Btu (or kWh) of rejected heat (this should not be confused with the absorption heating cycle). While more efficient in providing cooling than a single-effect absorption chiller, a double-effect LiBr absorption machine with a COP of 1.0 is also a good heat producer, as it rejects about 2.0 Btu (or kWh) of heat per Btu (or kWh) energy input. Hence, by combining the beneficial use of the cooling effect and the recovered heat, the absorption machines can operate at an extremely high performance rate.

However, as with vapor compressions systems, the heat product is at a relatively low temperature, which requires large pipe and pump sizes, and there is also an efficiency penalty for elevating the temperature of the recovered heat. Generally, the limit on using full heat rejection of absorption chilling machines is about 100°F (38°C). This is usually the maximum temperature of the combined condenser and absorber cooling water circuits. By targeting only the absorber cooling water circuit, heat can be recovered at substantially higher temperatures in LiBr machines. The amount of recoverable heat will be proportional to the driving energy input to the generator.

With ammonia-based machines, heat recovery potential is greatly increased due to increased absorber operating temperatures. Due to the large temperature glide, absorber temperatures can exceed 180°F (82°C), allowing for relatively high temperature heat recovery. This opens up a wider range of useful applications and reduces pump and pipe size requirements.

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