Heating System: Design Considerations
Gravity circulation is created as a result of pressure difference between two columns of water having unequal temperatures. The hotter water, as it expands, has less density than cooler columns of water which displace it and start up a circulation.
This circulation will continue while a temperature difference – and, therefore, a pressure difference – exists between the two columns.
This principle of pressure variation applies to systems relying on a pump to create the circulation. Pressure differences are caused by the pump action, the pushing action on the outlet side creating a positive pressure (P) while a negative pressure suction (S) is present on the inlet side. From this it is evident that at some point (P) must merge with (S). This is called the ‘neutral point’.
Positive and negative
Whether the circuit water works under negative or positive pressure relies on where the pump is positioned in the circuit. If it is fitted in the return pipe, the circuit will be under negative pressure: if in the flow circuit it is under positive pressure. In many cases, it makes little difference which method is chosen. A situation where it is important, however, is where pipework is taken through a roof space.
Should the pump be fitted in the return, and the circuit placed under suction, it may suck air into the circuit through the vent pipe. This is because of the limited static head available under these conditions, such as in bungalows with solid floors, where overhead circuitry may be necessary.
Avoid placing a pump at the lowest part of the system, as this encourages the collection of sediment.
With many modern pumps, the ‘heads’ are adjustable between l.52m and 4.87m, and there is no problem in adjusting the circulator to provide the desired system output and response.
As the pump, or circulator, provides a definite motive force, sizing for any small-pipe or loop system using conventional pipework circuitry presents little problem. For an average three-bedroomed house of about 93m2 area of floor, the following guide can be used to find the correct size of the forced-circulation pipework for a given radiator load.
Up to 5kW (15.000 Btu) 15mm pipe
5kW to 11.5kW (40,000 Btu) 22mm pipe
11.5kW to 22kW (70,000 Btu) 28mm pipe.
It is assumed that the layout is conventional and closer to a square than long and thin and has a ground and a first floor. Actual runs of pipe should not be excessive – a 25m long run of 15mm pipe, for example, should be avoided. Water always takes the easiest path, following through the circuit or radiator offering the least resistance.
The capacity of a heating circuit is based on that part of the circuit which contains the greatest load; this is called the index circuit, and may be the longest pipe circuit or that which carries the greatest heat load. As long as the pump is capable of meeting the demands of this circuit, it follows that it will also be able to satisfy the resistance of all other circuits.
The index circuit may try to rob other circuits of heat. This is adjusted by restricting the flow of water to this circuit to the extent necessary to allow remaining circuit requirements to be met, and is called ‘balancing’ the system.
Both a lockshield and handwheel valve should be fitted to conventional radiators. The handwheel allows radiators to be shut down individually or the temperature of the radiator to be reduced by restricting the flow of water; the lockshield is used to balance radiators.
It is used in conjunction with clip-on thermostats, one of which is attached to the flow pipe and the other to the return pipe. The lockshield is adjusted until the correct mean temperature of the supply pipework on each radiator is achieved.
The mean temperature is that halfway between the flow and the return temperature – 82°C flow, 71°C – mean temperature 77°C.
Static head in inches, feet or metres is the height of a column of water above the point of measurement, and static pressure, psi or bar, is the pressure exerted by a column of water on an area of 645mm2 (1sq.in). There is relationship between static head and static pressure. Because the density of water varies with temperature, all pressure calculations in hot water are based on an arbitrary water temperature of 16.7°C (62°F).
Where the millibar (or lbf/in2) unit of pressure is too large a unit for calculation, the bar (or in.wg) is used. This is the pres- sure exerted at the base of a column of water 25mm (lin.) high when the water temperature is 16.7°C.
A boiler should be slightly higher in output than the capacity required. The heat output for all the radiators should be calculated plus about 3kW, other than for pumped primary circuits, with about 25 per cent allowance for exceptionally cold weather.
In a single-pipe system, the radiators are connected in series, and there is a bypass pipe beneath each radiator. Water flowing from the first radiator is fed into the second, and so on.
By the time the water reaches the last radiator, it may have cooled by about 10.5°C. Heat output from this radiator is, therefore, reduced by about 20 per cent. With two-pipe systems, however, all the radiators receive the water at more or less the same temperature, allowing for some heat loss in transmission.
Though it may appear that twice as much pipe is required with this system, it is not so. There is, in fact, very little more needed than in a single-pipe system, and it certainly is more satisfactory in operation.
Central heating can dry out the atmosphere and cause physical discomfort.
An over-dry atmosphere can damage woodwork, furniture, pianos, pictures and soft furnishings. Natural moisture is drawn from timber and fittings, causing warping and other damage.
Air humidity is measured as relative humidity (rH). This is defined as the ‘percentage of moisture or water vapour in the air by weight at any given temperature to the weight of water vapour required to saturate the air at the same temperature’.
If the rH factor is too low the atmosphere appears stuffy. If it is too high we suffer what might be called the ‘tropical’ effect. While the degree of comfort required by people differs, a level of 50 per cent rH is suggested for rooms of 21°C.
About 430 millitre at 0°C = 100 per cent – rH very high
10°C = 50 per cent – medium correct
21°C = 25 per cent – very low.
Though you can open windows to reduce any imbalance, this is negative and the best solution is a humidifier.
These are basically water containers which allow moisture to evaporate and rectify the moisture loss. The most usual is the radiator-hung version. The best types have an absorbent pad which holds the water and, as heat builds up, allows water to evaporate through the pad. Effectively, an efficient 35 litre humidifier will evaporate its contents in 24 hours.
Electric, fan-assisted humidifiers are also made. Another type converts water into micro-atomised particles which are ejected into the atmosphere by an atomisation humidifier.
10. November 2011 by admin
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