Heating is the step that brings the oil up to the set temperature, and it is also the energy item that is easiest to calculate and easiest to calculate wrongly in the running of the equipment. Getting it wrong is not fatal, but it leads people to completely opposite conclusions about which way of heating is more economical. Split this account into three variables and the source of the difference comes into view.
The two ways of heating transfer the heat differently
The contact heating places a heating band or a coil close to the oil and carries the heat into the oil by conduction and convection. The efficiency of the heat exchange depends on the contact area, on the flow state of the oil and on the temperature difference. Where the oil flows past, the heating is thorough; in a still dead corner the heat can accumulate.
The far infrared carbon fibre heating is radiant heating, in the form of a surface heater it radiates directly to the vessel or to the oil. It responds quickly and it suits the preheating of a large volume vessel as a whole. Whether the heating is even depends on the shape of the vessel, on the distance and on the emissivity of the surface, and with a complicated shape a local overheating is easy to get.
The difference between the two routes is not which of them is more advanced, it is that the conditions of use are different. For the oil in a closed pipework the contact method is more direct, and for the heating of a tank the radiant method is more efficient.
The three variables of the energy account
The first variable is the effective heating power and the duty cycle. The power on the nameplate of the heater is not the effective power that reaches the oil, and the duty cycle decides whether it runs continuously or is switched in intermittently; this part sets the order of magnitude of the electricity cost.
The second variable is the temperature rise, in other words the difference between the target temperature and the starting temperature. The energy for the heat-up is roughly the effective power multiplied by the heat-up time, and the heat-up time follows the temperature rise. With a low starting oil temperature, the same target temperature costs one extra share of the electricity.
The third variable is the running time. After the heat-up the system goes into the holding phase, where the energy is the heat loss power multiplied by the holding time, and the heat loss power depends directly on the temperature rise, on the ambient air movement, on the insulation of the vessel and on the losses at the tank opening. This is the part that is the main source of the electricity cost in the long-term running.
The difference between continuous running and intermittent running
The characteristic of the continuous running is that the heat-up cost is spread out: after one heat-up the temperature is held for a long time, and the treatment energy per unit of oil falls as the batch grows. The intermittent running has to pay the heat-up electricity again every time, and if the intervals between the batches are not long, the share of the heat-up in the total electricity cost is high.
So with the same equipment and the same way of heating, the energy per unit treated can differ considerably between the continuous running and a site that runs for two hours a day. When the working rhythm is chosen, taking the heating cycle into account is more meaningful than watching the meter reading alone.
The really comparable measure is the energy per unit treated, that is the heat-up electricity and the holding electricity both counted in and then divided by the oil that was actually treated. Comparing the absolute values on the meter shows nothing: with a different volume of oil treated, the meter readings are not supposed to be the same.
It should be said that the total power of the ZYD-100 is 72 + 5 kW, of which the heating is only a part; the exact allocation is given by the nameplate and the drawings of the machine. Before the electricity cost is converted, this figure has to be established precisely.
| Item of comparison | Contact heating | Far infrared carbon fibre heating |
|---|---|---|
| Way the heat is transferred | Conduction and convection | Radiation |
| Response speed | Slower | Fast |
| Suitable for | The flowing oil in a pipework | The preheating of a tank or a vessel as a whole |
| Evenness | Depends on the flow velocity, heat can accumulate in the dead corners | Depends on the distance and the shape of the vessel |
| Main cost in the holding phase | Heat loss power × holding time | Heat loss power × holding time |
- Confirm the effective heating power and the duty cycle first, and only then talk about what saves what.
- Record the heat-up electricity and the holding electricity separately, and see whether the running is continuous or intermittent.
- Check the insulation of the vessel and the losses at the tank opening, the payback period on the insulation is usually shorter than expected.
- For the winter work with a low starting oil temperature, put the preheating time into the production plan.
In my account the energy question comes down to three variables: the effective power, the temperature rise and the running time; every other statement can be converted back to these three.