Hi
I'm new on here - so if this topic has been well-covered but not found by my topic search, please send me gentle pointers!
I'm a heating engineer and have a minor problem which potentially affects a large number of high-efficiency condensing boilers (aka furnaces - 2 nations divided by common language!). To work efficiently, these boilers need relatively tight control of the delta-T between the Flow and Return. Too big damages the boiler (thermal stress) but too little - ie. Return temperature 'catching up' to the Flow temperature - means that the boiler won't condense. The Return temperature needs to be below about 56C degrees and the delta-T less than 20 C degrees. (There are situations, eg. when the Flow setpoint for some reason needs to be above 75 degrees, when the ideal is not achievable!)
The boiler pump needs to slow down as the Return temperature rises, so that the maximum delta-T is achieved. This is particularly vital if the boiler is connected to a header: if the 'shunt' pump runs too fast, water whizzes between the boiler-side connections on the header and straight back to the boiler, so the Return temperature is virtually the same as the Flow.
Expensive boilers have modulating pumps which usually have 3 operating speeds. The boiler controller selects the optimum pump speed to match the total heat demand, so that the Flow / Return delta-T is maximised but the demand is still satisfied.
Cheaper boilers have a fixed-speed or manually adjustable pump which matches the average demand. However, there are often situations (especially low demand) when the pump is running too fast and the boiler efficiency suffers.
I'm looking for a way to mate an existing single-phase AC-motor pump (up to 300W - ie. VERY small) to a VFD with servo temperature control, with NTC sensors on the Flow and Return connections of the boiler. The process would be:
- on initial start up, pump runs at full speed.
- as the Return temperature rises, the pump speed decreases progressively until a predetermined temperature difference between Flow and Return temperatures is maintained.
- in the event that the Flow temperature rises, the pump speed increases to maintain the target delta-T.
- in the event that the Flow temperature keeps rising so that the desired delta-T cannot be achieved, the pump should run at full speed.
Obviously (??), a 'large' VFD designed for a motor of 1kW or larger would be too expensive. But really small ones seem not to exist as stand-alone products. I am aware that solar-thermal systems include pump speed control (so that the system flow is matched to the temperature rise in the panels achieved by the current sunshine intensity) but these are integrated into the solar controllers. Also ,they may not in fact use variable frequency...?
The point of all this is to maximise the output efficiency of the boiler. Getting the delta-T right can improve this by several percent, first because the delta-T between the water and the heat-exchanger surfaces will be greater (basic physics = more heat transferred) and secondly because one the boiler is in condensing mode, an extra 2 or so percent of efficiency will result. With the price of fuels going up, we need every percentage point we can get!
I'm new on here - so if this topic has been well-covered but not found by my topic search, please send me gentle pointers!
I'm a heating engineer and have a minor problem which potentially affects a large number of high-efficiency condensing boilers (aka furnaces - 2 nations divided by common language!). To work efficiently, these boilers need relatively tight control of the delta-T between the Flow and Return. Too big damages the boiler (thermal stress) but too little - ie. Return temperature 'catching up' to the Flow temperature - means that the boiler won't condense. The Return temperature needs to be below about 56C degrees and the delta-T less than 20 C degrees. (There are situations, eg. when the Flow setpoint for some reason needs to be above 75 degrees, when the ideal is not achievable!)
The boiler pump needs to slow down as the Return temperature rises, so that the maximum delta-T is achieved. This is particularly vital if the boiler is connected to a header: if the 'shunt' pump runs too fast, water whizzes between the boiler-side connections on the header and straight back to the boiler, so the Return temperature is virtually the same as the Flow.
Expensive boilers have modulating pumps which usually have 3 operating speeds. The boiler controller selects the optimum pump speed to match the total heat demand, so that the Flow / Return delta-T is maximised but the demand is still satisfied.
Cheaper boilers have a fixed-speed or manually adjustable pump which matches the average demand. However, there are often situations (especially low demand) when the pump is running too fast and the boiler efficiency suffers.
I'm looking for a way to mate an existing single-phase AC-motor pump (up to 300W - ie. VERY small) to a VFD with servo temperature control, with NTC sensors on the Flow and Return connections of the boiler. The process would be:
- on initial start up, pump runs at full speed.
- as the Return temperature rises, the pump speed decreases progressively until a predetermined temperature difference between Flow and Return temperatures is maintained.
- in the event that the Flow temperature rises, the pump speed increases to maintain the target delta-T.
- in the event that the Flow temperature keeps rising so that the desired delta-T cannot be achieved, the pump should run at full speed.
Obviously (??), a 'large' VFD designed for a motor of 1kW or larger would be too expensive. But really small ones seem not to exist as stand-alone products. I am aware that solar-thermal systems include pump speed control (so that the system flow is matched to the temperature rise in the panels achieved by the current sunshine intensity) but these are integrated into the solar controllers. Also ,they may not in fact use variable frequency...?
The point of all this is to maximise the output efficiency of the boiler. Getting the delta-T right can improve this by several percent, first because the delta-T between the water and the heat-exchanger surfaces will be greater (basic physics = more heat transferred) and secondly because one the boiler is in condensing mode, an extra 2 or so percent of efficiency will result. With the price of fuels going up, we need every percentage point we can get!