Plate Heat Exchangers
Polysun makes available some types of systems with internal heat exchangers and others with external heat exchangers. The transfer rate of the exchanger is higher when the surface area of transfer is larger (that means the overall surface area of the heat exchanger), the thinner the walls of the tubes and the greater the conductivity of the material of which the heat exchanger tube is made of. Finally the fluid velocity in the tube is important, which depends on the flow rate. The number of “parallel heat exchanger coils” indicates how many exchangers are used in parallel (for example arranged one in another).
In an external heat exchanger a “fixed entry height” or a “variable entry height” (of a stratifier lance) may be chosen. It is possible to define the lowest position or the highest position of the stratifier lance. Curiously the optimum height of the stratifier lance is not the highest layer, but at about ¾ of the tank height. It results that in the upper part of the tank there is no “perfect” stratification, but heat losses are not as high at the top cover. The stratifier lance should reach at least the level for daily consumption requirements.
The product of the thermal transmission coefficient k [W/m2/K] multiplied by the transfer surface A [m2] gives the thermal transfer rate of the heat exchanger, which results in the order of thousands of W/K. The parameter Δt indicates that which is (or should be) the difference in hot water temperature between the collector and the tank, in order to transfer to the tank by means of the exchanger the power density of 500 W/m2. This parameter depends on the transfer rate and the surface area of the collector.
The volumetric flow rate on the collector side can be determined by means of the flow rate. The volumetric flow rate on the tank side is determined by the fact that the capacity flow rate (= flow rate multiplied by the heat capacity of the fluid) is the same on both sides of the heat exchanger. If a glycol mix is being used in the collector loop, but only water in the tank, the two volumetric flow rates are not the same.
Heat Exchanger in the Consumption Loop
The use of the heat exchanger in the consumption loop doesn’t only regard the fact that heat needs to be transferred in the most efficient way possible, but also that the desired water temperature needs to be reached. For this reason pump regulation in respect to the tank of the heat exchanger is provided for, in such a way that the user can directly obtain water at the desired temperature.
The temperature that needs to be found in the upper part of the tank in order to obtain the desired water temperature is indicated in the lowest line of the dialogue under “minimum tank temperature required”. It depends on the temperature of hot-cold water, transfer rate and nominal flow rate. The nominal flow rate indicates the maximum quantity of hot water received per unit of time. The corresponding “withdrawal power” is indicated, too.