Contributed by Grant Laidlaw
Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten. We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in industry is not in place.
| Robyn asks: Dear Grant, in air-conditioning a lot is said about cooling, but heating is not spoken much about. I ran into open elements which I found strange. Could you please give us some insight on this topic, thanks. |
GRANT LAIDLAW
Hi Robyn, thanks for the question. Heating, even in South Africa, is necessary and can be diverse. I will look at heating mostly as it is applied by our industry and will focus on elements.
In order to overcome heat loss, heating is provided to the space by means of supplying air which is hotter than the required roomair temperature. This air can be heated in various ways and in various positions in the system.
The position where the air is heated depends mainly on the air-conditioning or heating system selected. In the case of a central plant the air is usually heated in the central air handling equipment. In the case of smaller individual units, the heating is also done in these units. Occasionally when the window surface in a space is large down draughts could be experienced at these windows during winter – skirting board heaters are recommended. Gas heating equipment is available which we do not generally deal with.
Robyn, the four main methods used to heat the air in air-conditioning equipment are:
- By means of electric heating elements
- By means of hot water coils
- By means of steam heating coils
- By means of heat pumps
The choice of method depends on the advantages and disadvantages of each, as detailed below.
1. Electric heating elements Advantages:
- Standard equipment being used
- Low capital cost
- Easy to control
- Low maintenance requirements
- Reticulation of electricity is both easy and cheap
Disadvantages:
- Running cost could be high (maximum demand)
- Fire risk
2. Hot water coils
Advantages:
- Extremely fine control is possible
- Simple installation
- Hot water can be produced centrally
- Alternative fuels such as oil and coal can be used
- Water can be heated by reclaimed heat
- Off-peak electricity can be used
- Heat can be stored
Disadvantages:
- Much higher capital cost
- Higher maintenance cost
- Trained supervisory personnel required
- Reticulation pipe work required
- Central plant-room required
3. Steam heating coils
Advantages:
- Smaller pipes required due to high latent heat of condensation being released
- Steam produced centrally
- Alternative fuels can be used
Disadvantages:
- More complex installation
- Higher capital cost/if steam generators are installed for heating purposes only
- Higher maintenance cost
- Trained personnel required
- Reticulation pipe work required – central plant-room required
- Steam heating coils are not in common use except in industrial applications.
4. Heat pumps
Like an air-conditioner, a heat pump uses refrigerant to move heat from one location to another. In fact, the two systems operate identically in the summer, extracting heat from inside your home and exhausting it outside.
Advantages:
- Versatile: Smaller units offer heating and cooling and can heat water and air
- Environmentally friendly: With no open flames or fumes, these risks don’t exist with an electric heat pump, making it one of the safest types of HVAC equipment. The lack of direct fossil fuel emissions also makes heat pumps more eco-friendly
- Energy efficient: Even the most advanced furnace type systems or element-based systems can never be more than 100% efficient, whereas heat pumps boast an excess of 300% efficiency
- Solar power: Heat pumps pair well with solar panels – if you have solar panels or are thinking of installing them, know that they are compatible with electric heat pumps
- Better humidity control and air quality
Disadvantages
- Reduced efficiency in cold weather: Effectively extracting thermal energy from outdoor air becomes increasingly difficult as the temperature drops
- The need to defrost: Sub-freezing temperatures cause ice to form on the outdoor heat pump unit. This forces the system to enter defrost mode, melting the ice so it can continue operating
- More noise: Longer run cycles also mean the fan and compressor are on more often in cold weather, which creates operating noise
- Shorter lifespan
- Higher capital and maintenance costs
Robyn, let us have a closer look at electric heating elements. Heating of air in an air-conditioning system can be achieved by means of electric heating elements because of the advantages mentioned earlier. Electric heating elements basically consist of a length of resistance wire, commonly nickel chromium, to which a
voltage is supplied.
The resistance wire may be as you found, bare or encased. There has been considerable debate on the relative merits of bare and incoloy (or sheathed) elements. The incoloy elements are a bit more expensive, have higher air side pressure drop and require more space. Bare coil elements are cheaper to repair if only the coil is damaged. Labour costs can decrease out this advantage. Bare coil elements are rarely used in South Africa and as such will only be dealt with briefly.
BARE COIL ELEMENTS
Bare coil element. Images supplied by Grant Laidlaw
These heating elements consist of a coil of nickel-chromium resistance wire held in place by a metal frame and porcelain spacers. When the resistance wire breaks, only the wire has to be replaced. The other advantage of the bare coil element is its low residual heat. This is an advantage when the fan is switched off while the heater is on and involves material that can melt such as plastics, or have been used for the construction of parts close to the heater bank. Examples of this are in diffuser in inducting grills.
INCOLOY ELEMENTS
Incoloy element structure.
Robyn, Incoloy is an alloy made up primarily of nickel, iron and chromium, with smaller amounts of aluminium and titanium added in certain cases. It’s commonly used as a heating element due to its high melting point (1 350°C), excellent corrosion resistance, and good thermal stability even at extreme temperatures. Incoloy elements are the most popular heating elements in South Africa.
They are mechanically stronger and if the sheath makes contact with the body or casing of the heater bank there is no danger of short circuits. Sheathed (or incoloy elements as they are commonly called) consist of a thin coil of resistance wire (diameter 2–3mm) which is installed centrally in an incoloy tube (incoloy is a metal similar to stainless steel). The space in between the coil and the incoloy sheath is filled with a magnesium powder which after heat treatment of the element changes into magnesium oxide.
At both extremes of the element there is a ‘cold end’ which is where the spiral is replaced with a thick piece of metal wire to which the terminal pin is welded. Incoloy elements are made in three different surface area-to-output ratios. This is the capacity in watts per square centimetre of outside sheath area. These three types of elements with their common name and output per square centimetre are tabulated below:
The total black heat has the lowest surface loading and therefore the lowest surface temperature. This element will not glow at all when energised. The normal black heat element will glow a dull red when energised in still air. The red heat element will glow red in still air. The stove element rating is given for comparative reasons. There is no restriction on the velocity over these elements. All can be installed in still air where normal convection currents can take place.
Black heat elements are often specified because their surface temperature and thus their radiant heat is so much lower. Due to their lower temperature these elements seem to last longer. Because the lower surface loading elements are longer, they are also more costly. The approximate length of the various elements per kW capacity is given. This table is for 8mm diameter elements which is the most common. The 6.6mm and 11.2mm diameter elements are available as specials.
Incoloy elements are available in the following standard ratings, 500W, 750W, 1 000W, 1 500W, 2 000W, 3 000W and 4 000W. Elements up to 2 000W are available in total black, normal black and red heat elements; 3 000W elements are available in normal black and red heat whilst the 4 000W elements are only available in red heat.
The reason for this is that the elements would otherwise become too long. All elements can be bent by the manufacturer in any shape desired. When purchasing an incoloy element the voltage, the wattage and the type of element – i.e. total black heat, normal black heat or red heat as well as the shape of the element – must be specified. Standard elements have a 12.5mm clinched bush, which means they are not suitable for immersion in water or for use in especially high humidity and require a 12.5mm hole for mounting.
The terminals are right angle tab terminals. Any special size and shape is available from the manufacturers. Before deciding on any special size or shape it is best to discuss requirements with the manufacturer.
- Finned elements: These elements are sometimes used because their surface-to-capacity ratio is much higher than incoloy elements. These are made in mild steel or stainless steel. Steel can be prone to rusting in high temperatures and humidity. These elements are also more expensive.
- Heater banks: From the range of elements available any capacity heater bank can be made up. The total capacity required is obtained from the heat loss calculations. This load includes the heat loss from the space as well as the heat required to heat the ventilation and/or the infiltration of air. A safety factor can be applied but is often not desirable because of the extra load on the electrical system and the extra kVA requirements (maximum demand). In particularly cold periods it is usually sufficient to switch the heaters on a bit earlier.
Robyn, I hope that this increases your understanding of heating, in the next issue I will conclude with heating using hot water coils. Thanks to everybody for the overwhelming response. I receive on average over 60 questions per month and cannot publish all of them. But keep them coming, as I may answer you directly. Looking forward to hearing from you.
REFERENCES
- SETA training
- ASHRAE
- ACRA
