ASHRAE Journal - February 2011 - 19

cility that is rejecting plentiful amounts of heat to atmosphere boilers incurred a significant operating penalty in carbon credat a higher temperature than the air or geothermal source for its. However, the need to install a new plant was viewed as a the heat pump installation. rare opportunity to make a significant improvement in site opHarnessing the heat output from cooling systems can be erating cost. The new system was required to provide efficient done in several ways. The simplest is to run the plant at a high- operation across a wide range of cooling and heating duties er heat rejection temperature so that the heat output is at a use- and to be flexible in operation, allowing for future changes in ful temperature. It is usually necessary to include a means of site utilization. heat rejection to atmosphere to cover for the times when there Four alternatives were analyzed. The first proposal was to is a need for cooling but no demand for heating. The additional install a modern gas-fired boiler plant with packaged ammocomplexity required to harness the heat includes an extra con- nia chillers. The second scheme also used packaged ammodenser, usually heating water or some other heat transfer fluid, nia chillers for cooling, but with a combined heat and power and typically a shell and tube vessel or plate heat exchanger. plant providing the electrical power required to run the chillers If the temperature lift in the cooling system is wider than and delivering the heating requirement. In the third scheme acceptable limits for the compressor, then an additional stage a comprehensive geothermal network provided the heating, of compression is required. This is more complex because again with packaged ammonia chillers. The fourth scheme an extra compressor is in the system, but discharge gas from was the only one to integrate the two requirements thermodythe main plant sometimes can be taken directly to the suc- namically in what was called a “thermally coupled” system. tion of the high-stage This was based on a Energy Life-Cycle Qualitative Technology compressor so the heat central plant cooling Rating Cost Score recovery condenser system, but with adGas Boiler/Ammonia Chillers 5.26 $36.37 Million 15 is the only additional ditional high-pressure heat exchanger. If this compressors raising CHP/Ammonia Chillers 7.87 $34.50 Million 19 is not feasible (because the condensing condiGeothermal/Ammonia Chillers 2.13 $26.73 Million 23 the cooling and heattion sufficiently high ing circuits need to be Thermally Coupled Heating and Cooling enough to meet the 3.93 $31.39 Million 13 kept separate, or the heating requirement. discharge gas from the Table 1: Assessment of alternative schemes. The alternatives were cooling compressors is too hot to be fed directly to the suction rated according to the energy required, the present value of the of the high-stage compressor), an additional heat exchanger, system and qualitative factors (items that could not be assessed either a cascade condenser or a desuperheater, also is required. in terms of energy or value, but still had a bearing on the utilRecent developments in ammonia compressors (first in re- ity of the proposed solution). The results of the analysis for the ciprocating machines designed for 580 psig [40 bar] and then project are shown in Table 1. in single and twin screw compressors) have enabled heat reThe qualitative assessment included consideration of faccovery plants to be designed for all three configurations. Sin- tors such as future flexibility, disruption to site during installagle screw compressors now are available with operating pres- tion, initial capital cost, environmental impact and operational sures sufficient to heat water to 203°F (95°C), so the full range complexity. A low score indicates that the scheme is attractive. of sub-steam temperature requirements can be met. Although the geothermal system was cost effective over the If the required water temperature is above 187°F (75°C) 15-year life of the plant and consumed the least energy, it was then a steel bodied compressor, rated for 1,125 psig (75 bar) not favored in the qualitative analysis for several reasons. The is required, but for lower temperatures a body of spheroidal required size of an underground collector would have covered graphite (SG) iron, rated for 725 psig (50 bar), is sufficient. most of the site, and it was perceived as a significant restricThe steel body is significantly more expensive. In a two-stage tion on future site use. For example, the collector would have heat pump, the segregation of stages should be arranged so that made it difficult to sell a portion of the land for development steel bodies are only required for the high-stage compressors. in the future. Significant operational and environmental benefits have With the geothermal solution ruled out, the thermally coubeen achieved through the installation of a combined heating pled heating and cooling system was the most attractive opand cooling system at a confectionery factory in England that tion. It used the least energy of the three alternatives, it had manufactures candy bars and chocolate. As with many such the lowest life-cycle cost, and it scored best in the qualitative facilities, the heating and cooling requirements were previous- analysis. The system was installed early in 2010, and has been ly handled separately. Cooling was done by a bank of 12 R-22 fully operational since March. The cooling loads are summachillers with a total installed capacity of 1,535 tons (5400 kW) rized in Table 2 (Page 20), and are all serviced by a propylene and heating was provided by coal-fired steam boilers deliver- glycol loop that delivers glycol at 32°F (0°C). ing 60,000 lb/h (7.5 kg/s) of low pressure steam. The heating loads can be split into three main streams: The client was required to replace the equipment because cooking, cleaning and “closed loop” loads. The cooking reR-22 use was becoming severely restricted, and the coal-fired quires steam at 260°F (125°C) and so cannot be fed by the
February 2011 ASHRAE Journal 19

ASHRAE Journal - February 2011

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