Heating Systems with Primary-Secondary Rings

Heating Systems with Primary-Secondary Rings


Usually there is no need to arrange a combined system with primary-secondary rings for small houses. Simple but quite reliable schemes are enough (see Figure 49, a). In this scheme the primary ring penetrates both boiler and hot water heater and other consumers are connected in accordance with a usual scheme as a secondary ring. Although the scheme here is «there is no place cheaper», in real life, in most cases, the installed scheme is a little more complicated (see Figure 49, b). The second solution allows us to select the high temperature ring with the boiler for providing hot water from the row of the other consumers and provide priority for this ring over the other consumers. With the help of a three-way valve, the ring can change its shape. The flow of warmth carrier in the ring first passes through the boiler and then it goes through the hot water heater (if there is a need for hot water) or through a bypass to which other consumers are connected.  If you install servodrives which are managed by a controller on a three-way valve, then the ring flow can be divided into two paths in proportion depending on the actual consumption of hot water. In these schemes the circulational pumps of the primary ring are installed on the return pipeline; therefore air purgers must be installed on the ring. Usually they are installed on the collectors of the primary and secondary rings. If you move the pump to the supply pipeline, then you can include the air separator into the system.

Heating systems with primary-secondary rings for small buildings
Figure 49. Heating systems with primary-secondary rings for small buildings

Hydraulic independence of individual contours (secondary rings) not only greatly simplifies the design calculations, reducing them to simple practical recommendations, but also allows us to choose variants of electronic controls: from simple and inexpensive thermostats to complicated weather-dependent controllers.

To design a heating system with different temperature regimes using the scheme of primary-secondary rings is quite simple. We have already examined high temperature and low temperature systems separately. Now we need to integrate them as secondary rings into the primary ring (see Figure 50).

An example of a heating system with primary-secondary rings
Figure 50. An example of a heating system with primary-secondary rings

However, while connecting the secondary heating rings to the hydrocollector, it is necessary to follow a certain sequence, to attach the heating rings to which you want to give priority closer to the boiler — for example, high-temperature heating systems and low-temperature heating systems can be moved to the primary ring. Still, the primary ring, generally speaking, is usually a one-piped system in which each consumer (secondary ring) gives the cooled water back to the system and the the further the consumer is from the beginning of the ring, the cooler the water will be that it receives. Let’s put the low temperature rings there.

Designing the combined system will not be too difficult. The main thing in this work is not how to draw pipelines in the scheme, but how to put them in a real house without getting confused by the pipes. To facilitate the task, some manufacturers of heating equipment make ready-made hydrocollector units after the purchasing of which you should simply connect them to the boiler and the consumers. Usually, the regulating three- and four-way valves with automatic equipment come together with the hydrocollectors (see Figure 51). All the equipment is placed in the boiler room and only pipes go to the consumers of warmth. The work is extremely simplified and it is very difficult to be confused by the pipelines if you know the theory of primary-secondary rings.

Hydrocollectors from the company HydroLogo
Figure 51. Hydrocollectors from the company «HydroLogo»

It is better to install hydrocollectors in the vertical position. Then you have the ability to remove the slime which settles at the lower part of the collector pipes from the heating system. And for the top part you should use automatic air purgers and you do not need to install expensive air separators. With this air removal the circulational pump of the primary ring can be put on the return pipeline where, in the zone of lower temperatures, it will work for a longer time.

To organize the motion of the flow of the warmth carrier inside the collector, its profile is not important. This allows natural constructions of rectangular cross-section to be made from a channel bar without using special equipment and machine tools.

The simplest flow distribution devices for zoning the internal volume can be used in hydrocollectors. This zoning can be done with the help of a waved or flat wall (see Figure 52) which is fixed inside the collector without hermetic welding of the perimeter with saving of end openings between the internal and external walls with the area of not less than one-fourth of the living cross-section of the collector.

The scheme of hydrocollectors welded from two channel bars
Figure 52. The scheme of hydrocollectors welded from two channel bars

The main condition of hydraulic stability of a heating scheme with hydrocollectors is to provide low velocities of motion of the warmth carrier (0,2 < v ≤ 0,4 m/sec) in the collector, thereby achieving small pressure changes in it, and the regime of its work, for each of the contours, becomes close to the conditions of work of the expansion tank. The living cross-section flcs (m²) of the collector for passing the warmth carrier with a chosen value of velocity (V) is calculated as follows:

flcs = Q/(3600×V)

where Q (m³/hr) is the total maximum supply of warmth carrier through the collector. Thus the living cross-section of the collector (the opened area inside) is not difficult to calculate, knowing that the supply of warmth carrier through the boiler is approximately equal to its power. For example, the boiler with a power output of 30 KW has a supply of 30 litres/min (1.8 m³/hour); and with a power of 120 KW, 125 litres/min. If you purchased a collector or made it with living cross-section area more than needed, then do not worry. It is important that the velocity of motion of the warmth carrier in it is between 0,2 and 0,4 m/sec.

The distance between the centres of the return and supply pipelines to the secondary rings is made 40, 90, 125, 145, 160 and, less often, 250 mm, and is made according to the diameters of connecting pipelines and the sizes of heating valves — three- and four-way mixers (see Figure 53). Mixers have union nuts with a thread of 1.5 inches on one side — for example, for connecting a circulational pump; and on the other side, an external thread of 1.5 inches for a union nut. When using mixers of other companies, the distance between centres of outlet of the collector can be changed for these mixers.

Mixing valves
Figure 53. Mixing valves (example)

While assembling heating systems on the hydrocollectors, one should follow some simple rules: the heating system must be equipped with an expansion tank of the corresponding calculated volume of warmth carrier; the feeding pipeline should not go right to the boiler — it must be mixed with the return pipeline of the heating system away from the boiler; and on the return pipelines of the secondary rings, strainers (sumps) must be installed.