Energy recovery is done most effectively by countercurrent heat exchange. The greater the heat transfer surface and the lower the mass flow throughput, the more energy can be recovered. In addition, the longer the countercurrent contact distance (and therefore the dwell time or contact time) and the better the heat exchange per unit area, the more energy will be recovered.
Important Parameters and Relationships for Comparison of Suppliers
Transformation of one litre of water at 100 °C from the liquid phase to the vapour phase requires a heat input of approx. 700 Watt (as dictated by the laws of physics).
As the inlet water is usually lower in temperature than 100 °C, additional energy is required for evaporation. In order to achieve maximum energy efficiency, it is important to recover as much of this energy as possible for reuse in the process.
Accordingly LOFT evaporators incorporate generously dimensioned heat exchangers which maximize heat recover as much heat as possible from the effluent distillate, therefore ensuring maximum energy efficiency.
LOFT vacuum evaporators provide particularly large surfaces and distances for countercurrent heat exchange relative to their design throughput rates to ensure outstanding heat transfer. For example our LE 200 evaporator unit has 5 x the heat exchange surface than comparable evaporators from other manufacturers.
Our unique design features and the low temperature gradients resulting from the large surfaces reduce fouling and deposits on the heat exchange surfaces. Low surface deposits ensure a constant k coefficient (heat transfer coefficient) over long operating time periods. In this context please refer to the diagramme showing the k coefficient as a function of surface deposits.
When the wastewaters processed have high contents of pollutants or solids, the k coefficient normally drops as operation goes on. LOFT’s generously dimensioned heat transfer surfaces compensate for this effect and ensure ongoing high performance levels in conformance with requirements.
The generously dimensioned heat transfer surface can also effect a higher thickening effect in the concentrate without requiring reduced throughputs. The result: less residual concentrate and reduced disposal costs for you!
An additional benefit is provided by the fact that, on the larger heat transfer surface, longer cleaning intervals are permissible as highest k coefficients are not required to achieve the desired throughput rates.
For this reason we normally dimension an evaporator requiring a nominal design throughput of 100 l/h with a heat transfer surface of 11.3 m² and a surface safety reserve of at least 30 %. This ensures – from a physical standpoint – that at least 130 % of the energy can be recovered. When using the unit to treat unpolluted water, its throughput capacity would therefore be 130 l/h. This reserve is required to offset the surface fouling effects described above without sacrificing capacity.
Dimensioning this evaporator with smaller heat transfer surfaces, say 8.7 m2, would result in a recovery rate of only 77 %. This would mean that operation under idealized conditions, e.g. with unpolluted water, would be possible at the rated throughput, but adverse conditions (high salt content, oil content, higher concentration or surface fouling in the unit) would lad to a reduction in capacity.
An Alternative to Large Heat Transfer Surfaces:
Another way of increasing energy efficiency is increasing the k coefficient. However evaporators with high k coefficients by their design tend to be susceptible to higher surface fouling. As the fouling is compounded by the high temperature gradient, the k coefficient is observed to drop off faster in operation. These effects are particularly predominant when evaporation involves a high degree of concentration or when the wastewater processed has high amounts of solids or other pollutants. We therefore do not pursue this type of efficiency enhancement in the evaporators we offer.
Appendix:
The formula for calculation of heat transfer is as follows ´Q = k * A * Δt
´Q = heat transfer rate
k = heat transfer coefficient
A = heat transfer surface
Δt = temperature gradient across heat exchanger wall, i.e. between vapour and polluted wastewater
