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Are cast iron radiators as efficient as modern radiators?

时间:2017-06-10 15:46来源:未知 作者:Kevin
  
Yes. All the heat you put in comes out as heat in your room. You will be as warm as you 
 
want providing the radiator is sized correctly. 
 
While a cast iron radiator takes longer to heat up than a flat panel radiator, it also takes 
 
longer to cool down. The efficiencies of both types of radiator are approximately 
 
equivalent. A cast iron radiator in general has a larger internal volume than a flat panel 
 
radiator, requiring more work from the boiler but this may be offset somewhat by the 
 
extended heating time given by the thick cast iron. By localising temperature control to the 
 
local environment of each radiator, thermostatic radiator valves help to eliminate wasteful 
 
heating and can help regulate the temperature throughout a house by allowing cool 
 
rooms to heat for longer than a well insulated room or one where there are other heat 
 
sources contributing to the ambient temperature other than the radiator.
 
The best efficiency savings are not to be made through the type of radiator, rather 
 
through a highly efficient boiler, thermostatic radiator valves, clever use of insulation and 
 
by turning off radiators in rooms that are not used.
 
What is efficiency? Well, the Oxford English Dictionary defines efficiency as:
 
The ratio of the useful work performed by a machine or in a process to the total energy 
 
expended or heat taken in.
 
We’re looking at central heating systems, so the ‘machine’ is a cast iron radiator. The 
 
‘useful work performed’ is heat emission – the amount of electromagnetic radiation 
 
that is emitted per unit area per second, or how much heat is given off by the radiator in a 
 
given time.
 
The’ total energy in’ is the thermal energy of the water fed in to the radiator, so the 
 
efficiency of a radiator can be said to be the ratio of energy required to heat the radiator 
 
to the energy emitted by the radiator.
 
Efficiency of a cast iron radiator = Energy emitted by the radiator / Energy required to heat 
 
the radiator
 
It turns out that there are three stages of a cast iron radiator’s heating process; heating 
 
up, normal operation and cooling down.
 
Heating up
Conservation of energy tells us that the total energy in must be equal to the total energy 
 
out. For a central heating radiator we can say:
 
Energy of hot water = Energy emitted by radiator + energy required to heat the radiator
 
From the above statement it is clear that increasing the amount of energy required to heat 
 
the radiator will decrease the energy (heat) emitted from the radiator until the radiator 
 
reaches the same temperature as the water,  at which point the energy required to heat 
 
the radiator will be 0. The question to ask at this point is:
 
How much energy does it take to heat 1kg of cast iron from 16ºC to 60ºC?
 
Specific heat capacity (C) characterises the amount of energy required to change a body’
 
s temperature by one degree (Kelvin or centigrade, either is fine for relative calculations 
 
such as these). A high specific heat capacity means that a lot of energy is required to heat 
 
the material, but also that the material will be slow to cool down.
 
Water has a high specific heat capacity, and that is one of the reasons why island nations 
 
such as Britain enjoy milder winters than continental countries – oceans are better at 
 
holding heat than land.
 
The specific heat capacity of cast iron is 0.46, so 460 Joules are required to heat 1kg of cast 
 
iron by 1ºC. The specific heat capacity of steel is very similar to that of cast iron, therefore 
 
it follows that the energy required to heat 1kg of steel is much the same as for cast iron. 
 
Cast iron radiators are very heavy, so the difference between the two types of radiator is 
 
not in the material, but heating a mass 2, 3 or 4 times that of a panel radiator requires 
 
proportionately more energy.
 
It is fairly evident that in practical terms a cast iron radiator will take longer to heat up 
 
than a thin steel panel one. So in the heating up stage, it’s safe to say that cast iron 
 
radiators are less efficient than thinner, panel ones.
 
The energy required to heat one kg of water by one degree is 4181 Joules; over 9 times the 
 
energy required to heat the same mass of cast iron. Put into context, the difference 
 
between heating a flat panel radiator and heating a cast iron one seems to be somewhat 
 
negligible.
 
An efficient boiler is paramount in any central heating system.
 
Normal operation
 
With the heat of the radiator in equilibrium with the heat of the water, a cast iron radiator 
 
can be approximated by a blackbody – an idealised physical body that absorbs all 
 
electromagnetic energy incident on it and reemits all energy as thermal radiation.
 
A radiator painted white will reflect most of the visible light incident on it (so can’t be 
 
approximated as a blackbody), whereas a radiator painted black will absorb most of the 
 
visible light incident upon it and reemit this as thermal energy.
 
For this example we will presume the radiator is painted black.
 
The Stefan-Boltzmann law describes the total energy emitted per surface unit area per unit 
 
time, and for a body at 60ºC (333K – we need to convert to Kelvin here because it’s an 
 
absolute calculation. Kelvin = ºC + 273) .
 
The energy emitted by a cast iron radiator at 60ºC as described by the Stefan-Boltzmann 
 
law is 697J per second per square metre.
 
How does this compare to a steel panel radiator?
A steel panel radiator painted the same shade of black will emit 697J, exactly the same 
 
amount of energy at equivalent temperatures per square metre per second as a cast iron 
 
radiator. A radiator is essentially just a container for hot water.
 
The heat output of a radiator is a function of the temperature of the water inside it and its 
 
surface area.
In other words, a cast iron radiator operating at the same temperature as the water inside 
 
it is just as efficient as a steel panel alternative.
 
Cooling down
As with heating up, the specific heat capacity is key here. Specific heat is the energy 
 
required to change the temperature of 1kg of a material by 1º. We discovered earlier in 
 
this post that more energy is required to heat a cast iron radiator than a steel panel 
 
radiator simply because there’s more iron in the former than in the latter. We also learnt 
 
about conservation of energy – energy can neither be created nor destroyed but must 
 
remain constant in a system.
 
In a cast iron radiator with thick, heavy iron walls between the hot water and cooler 
 
ambient air, there are billions of iron atoms with kinetic energy, which is emitted as 
 
thermal energy and heats the air around the radiator. The heated air is less dense than the 
 
rest of the air, so it rises up like a hot air balloon. More cool air moves into the void which 
 
is in turn heated, and again rises to create a convection current within the room.
 
The kinetic energy from the excited iron atoms in the radiator are transferred to the 
 
surrounding air, as described above, at a rate proportional to the temperature gradient (a 
 
big temperature difference makes things cool faster) and surface area (a larger surface 
 
area means more contact the cool air and the hot radiator surface).
 
At the same time the specific heat capacity of iron dictates that to cool iron by one degree 
 
requires -460 Joules (just as 460J are needed to raise the temperature, we need to lose 
 
energy to cool the temperature).
 
That energy is lost by radiative transfer to the air around the radiator. At the beginning of 
 
the heating process, a certain amount of energy was spent heating the radiator to make it 
 
as hot as the water at 60ºC, and we found out that this required less energy and therefore 
 
less time for a flat panel radiator than for a cast iron radiator.
 
There is more mass in a cast iron radiator than flat panel one as we’ve already 
 
established, so the heavy radiator at 60ºC has more energy than the thin radiator at the 
 
same temperature. This means that the energy that needs to be ‘lost’ for the thin 
 
radiator to cool to ambient temperatures is significantly less than the energy that needs to 
 
be lost to cool the thick, heavy cast iron radiator.
 
All the energy spent heating the cast iron radiator in the first place is now dissipated slowly 
 
as the radiator cools down; no energy is destroyed, it’s just dissipated slowly with a 
 
radiator that continues to heat the room well after the boiler has been turned off.
 
The verdict
While a cast iron radiator takes longer to heat up than a flat panel radiator, it also takes 
 
longer to cool down. The efficiencies of both types of radiator are approximately 
 
equivalent. A cast iron radiator in general has a larger internal volume than a flat panel 
 
radiator, requiring more work from the boiler but this may be offset somewhat by the 
 
extended heating time given by the thick cast iron. By localising temperature control to the 
 
local environment of each radiator, thermostatic radiator valves help to eliminate wasteful 
 
heating and can help regulate the temperature throughout a house by allowing cool 
 
rooms to heat for longer than a well insulated room or one where there are other heat 
 
sources contributing to the ambient temperature other than the radiator.
 
The best efficiency savings are not to be made through the type of radiator, rather 
 
through a highly efficient boiler, thermostatic radiator valves, clever use of insulation and 
 
by turning off radiators in rooms that are not used.

BY KEVIN LIU
Email:beizhu07@beizhugroup.com
Ph:86-15386899894
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