Heat pumps, placement

Thus, the appropriate placement of heat pumps is that they should be placed across the pinch14. Note that the principle needs careful interpretation if there are utility pinches. In such circumstances, heat pump placement above the process pinch or below it can be economic, providing... 

A refrigeration system is a heat pump in which heat is absorbed below ambient temperature. Thus the appropriate placement principle for heat pumps applies in exactly the same way as for refrigeration cycles. The appropriate placement for refrigeration cycles is that they also should be across the pinch. As with heat pumps, refrigeration cycles also can be appropriately placed across utility pinches. It is common for refrigeration cycles to be placed across a utility pinch caused by maximizing cooling water duty. 

Townsend, D.W. Linnhoff, B. Heat and power networks in process design—Part I Criteria for placement of heat engines and heat pumps in process networks. AIChE J. 1983, 29 (5), 742-748. 

Optimal placement of heat engines and heat pumps, etc. 

To ensure energy saving, the position of unit operations with respect to Pinch must respect the rules of the Appropriate Placement. For example, a heat engine has to be placed either above or below the Pinch, but not across it. Distillation columns should not be placed across the Pinch. Contrary, a heat pump is optimally placed across the Pinch. This subject will be discussed in more detail in the Chapter 11. 

The improvement of design can be realised by Appropriate Placement and Plus/Minus principle. Appropriate Placement defines the optimal location of individual imits against the Pinch. It applies to heat engines, heat pumps, distillation columns, evaporators, furnaces, and to any other unit operation that can be represented in terms of heat sources and sinks. 

The best placement of heat pumps is across the Pinch (Fig. 11.7). If the heat pump is placed above the Pinch, the amount Qe+W is reintroduced into the process, and the hot utility load decreases by W. This supplementary energy is more expensive than the thermal energy. On the contrary, if the heat pump is placed across the Pinch the device can take an amount Qe from below the Pinch and bring it above the Pinch, such that both utility loads diminish by the same amount Q. ... 

As can be seen, refrigeration is needed to condense the overhead vapor streams at low temperatures, with the lowest temperatures in the condenser of the demethanizer. In synthesizing a profitable design, an important question concerns how to position the heat pumps (refrigeration cycles) to satisfy the heating and cooling demands. This question will be addressed after several principles are established for the proper placement of heat pumps. ... 

Figure 10.50 Alternatives for the placement of heat pumps (a) above (b) below (c) across Tp.

It should be pointed out that appropriate placement of heat pumps and heat engines should also account for the utility pinches, and not just the process pinch as stated above. However,... 

Townsend, D.W., and B. Linnhoff, Heat and Power Networks in Process Design. I. Criteria for Placement of Heat Engines and Heat Pumps in Process Networks, AIChE J., 29,742 (1983a). 

A pump design objective, then, is the interception and removal of the gas while still in a high-mass-flux, directional condition. The technique of cryopumping is ideally suited for this particular situation because of (1) the flexibility in cryopanel placement to fit the jet and thus avoid losses associated with additional piping to pumps, and (2) the ability to make use of the heat sink characteristics of the cryopanels to pump the high-mass-flow, transient rocket nozzle flow. 

Another control is the use of barriers. Placement of wires in conduits will reduce the potential for damage to the wires. Light bulbs in trouble lights have a protective metal cage. Raised concrete islands and concrete-filled steel columns protect gas pumps in service stations. Heat shields in automobile engine compartments protect some components from thermal damage. 

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