Module Design

Most commercial and industrial systems require a large number of collectors to satisfy the heating demand. Connecting the collectors with just one set of manifolds makes it difficult to ensure drainability and low pressure drop. It would also be difficult to balance the flow so as to have the same flow rate through all collectors. A module is a group of collectors that can be grouped into parallel flow and combined series-parallel flow. Parallel flow is more frequently used because it is inherently balanced, has a low pressure drop, and can be drained easily. Figure 5.20 illustrates the two most popular collector header designs: external and internal manifolds.

Generally, flat-plate collectors are made to connect to the main pipes of the installation in one of the two methods shown in Figure 5.20. The external manifold collector has a small-diameter connection because it is used to carry the flow for only one collector. Therefore, each collector is connected individually to the manifold piping, which is not part of the collector panel. The internal manifold collector incorporates several collectors with large headers, which can be placed side by side to form a continuous supply and return manifold, so the manifold piping is integral with each collector. The number of collectors that can be connected depends on the size of the header.

External manifold collectors are generally more suitable for small systems. Internal manifolding is preferred for large systems because it offers a number of advantages. These are cost savings because the system avoids the use of extra pipes (and fittings), which need to be insulated and properly supported, and the elimination of heat losses associated with external manifolding, which increases the thermal performance of the system.

It should be noted that the flow is parallel but the collectors are connected in series. When arrays must be greater than one panel high, a combination of series and parallel flow may be used, as shown in Figure 5.21. This is a more suitable design in cases where collectors are installed on an inclined roof.

The choice of series or parallel arrangement depends on the temperature required from the system. Connecting collectors in parallel means that all collectors have as input the same temperature, whereas when a series connection is used, the outlet temperature from one collector (or row of collectors) is the input to the next collector (or row of collectors). The performance of

(a) External manifolding (b) Internal manifolding

FIGuRE 5.20 collector manifolding arrangements for parallel flow modules.

(a) External manifolding (b) Internal manifolding

FIGuRE 5.20 collector manifolding arrangements for parallel flow modules.

Outlet manifold

Inlet manifold

FIGURE 5.21 collector manifolding arrangement for combined series-parallel flow modules.

Collector rows

Supply manifold

Balancing valves

Return manifold

FIGURE 5.22 Direct-return array piping.

such an arrangement can be obtained from the equations presented in Chapter 4, Section 4.1.2.

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