HVAC Delivery System: Water and Steam Movement

Although air might seem to be the natural vehicle to carry heal or coolness heated or cooled air is the desired end product after all water can also be used to move heat or coolness, and steam can be used to transport heat.

Using the latter media may seem wasteful in that when water is used, the heat or coolness must eventually be transferred to air at the end of the line. Furthermore, the ventilation (fresh air) system will probably require air ducts in any case-why then not always use air ducts for the entire energy transportation job?

The answer is mat pushing high volumes of heated or cooled air for long distances in large build­ings requires large amounts of space and energy. A cubic meter of water can hold much more heat energy than a cubic meter of air: steam of course, can do even better.

Although steam is the most efficient carrier of heat energy, it cannot of course, carry coolness so it is generally used in facilities where cooling is not important. Steam, moreover, changes state from a vapour to a liquid (water) when the heal is given off, and this can cause difficulties.

Water-delivered heating/cooling systems use a pathway of pipes to carry hot or cool water to the area to be heated or cooled and to return the “used” water back to the boiler or Air Conditioner. These pipes are analogous to the ducts in an air delivery system.

A simple system features a single pipe in a long closed loop: hot water (in the case of heating) gives off heat, becoming cooler as it goes, until it returns to the furnace. Those near the end of loop may regard such a system as inefficient since, by the time the water gets to them, most of its lost.

This can be remedied to some extent by having the water travel through several loops: then area served by each loop is called a zone.

Temperatures for different heating or cooling zones can usually be set independently, which is useful in a facility where various types of activities are conducted in different areas, or where varying solar heat loads on outside walls create a need for different temperature levels.

For superior levels of service, the supply (heated or cooled) water must be kept separate from the return water. Such a system supplies the water at as hot (or cold) at a temperature as possible. To ensure even water flow, the return pipes should be as long as the supply pipe.

As with the air ducts, if it is necessary to have the furnace supply coolness to another at the same time, a dual set of pipes required. Clearly, such a system is expensive both to install and to operate, but it is useful when the healing and cooling seasons overlap-on hot December or cold May days, for example.

“Without a dual system, occupants will be uncom­fortable on those rare days because the system will already have been reset, and the coolness (in December) or heat (in May) will not be available.

The pipes in a water heat (coolness) energy delivery system are usually made of copper. Steel is preferred for steam systems although wrought iron is also used. These pipes are frequently joined together with threaded ends, although larger pipes may be joined by flanges or welding.

Pipe sizes arc determined using calculations similar to those for air ducts. Water flow is measured in litre per minute rather than the cubic m/per minute used for air. Like air ducts the pipe pathway must occasionally turn or split. Which is accomplished by fittings-short T-or L-shaped pieces of pipe similar to those used in ducts?

Water, when heated, expands. An expansion tank allows this expansion to take place without raising pressures dangerously high. It may also be called a compression tank and is usually located above the boiler. Certain valves are used to keep the system in balance and safe.

The filler or check valves work automati­cally to keep the right amount of water in the system. The former adding more and the latter preventing further additions. The boiler relief valve allows some water to bleed out of the system when pressure is too high, and the flow control valve stops the flow of water when the pump stops.

This flow could continue because of gravity, and if it did, the room temperature could continue to rise past the point. The occupants have selected.

A circulating pump may be used to dis­tribute the water quickly through the pipes to improve the response time. As with the pipes (and the fans in an air distribution system), the appropriate size pump must be selected: size is given in lit/minute.

Once the hot water or steam arrives at the area to be heated, the heat energy must be transferred to the air in the room(s). This is done either:

(a) By convectors:

The hot water or steam flows through thin tubes or past metal fins so that the heat is transferred to these surfaces (tubes, fins, or both): air is blown over these surfaces, picking up the heal and carrying it into the room. Since the heat is being transferred via forced air, the unit may be located on the ceiling.

(b) By radiators:

Are similar to convectors except there is no fan (forced air). The heat transfer surfaces are exposed to the room air, which is heated mostly by radiation although some natural convection may also help distributing the heal.

Radiant heat may also be distributed through “panels.” Which are grids of pipes embedded in the concrete or plaster of floors or ceilings?

Although the heat supply is uniform, the heat distribution may not be consistent: temperature differences-especially near windows or at floor level-may cause discomfort. Coolness is also distributed using convectors: in this situation air is blown over cool pipes or tubes removing heat from the room.