Plate heat exchanger or internal coil
- Thread starter Enfield250
- Start date
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Thermal Man has gone quiet suddenly, he's usually very quick with his responses.
Now he's been caught out with his bluffing big-timing BS, I think a re-generation is imminent.
Mechanical engineer, started working for the NHS & doing day-release, then consultants; project engineer on a pharmaceutical site with lots of AHUs, lots of BMS controls, then consultants again, self-employed, more consultants.
And now redundant.
he's been banned again , must be into double figures by now!
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Yes, TM/TM2/TM3/TM4/Water Systems/Gayton Tonner/Drivel, etc., I got that.
The question is, when a HW tap is turned on, the heating flow to the AHU stops; what happens then? Does the AHU blow out cold air until the tap is turned off and, during freezing weather, do you allow the heating coil to freeze solid and split? Or do you stop the air flow completely, which could be very irritating with the fan noise going on & off?
Now, you could say that the fresh air & recirculation dampers go to 100% recirculation during a HW demand, but you'd never move the dampers that fast unless the actuators were pneumatic and you haven't previously mentioned recirculation or the need for pneumatic actuators. So you can forget that escape route.
Similarly with a WC system supplying radiators, if the outside air temperature falls, the boiler flow temperature will increase to compensate for the increased heat loss. With an AHU, if the outside air temperature falls, the supply air temperature, Ts, will also fall and the heating control valve will immediately open to maintain Ts at the set point. Where does WC come into this?
It has been known for frustrated teenagers to make up a fantasy girlfriend or boyfriend. You're the only person I know of who has fantasy heating installations. May I suggest that you should get yourself a life?
Used to fit the indoor Unico air handlers in commercial premises.
We didn't fit any fresh air duct. It was a constant volume air supply 100% recirculated. The fan was very silent.
Its an american system and mainly aimed at the domestic market afaik.
(They recommended a glycol mixture for their outdoor units to prevent freezing)
Anyway these units were supplied from a thermal store.
Obviously domestic is different but could the same not be acheived from a combi boiler with a buffer/thermal store added?
Is the hydronic circuit of fan coils usually variable volume with constant temperature and whats the disadvantage with variable temperature-constant volume? More accurate control with the former I'm guessing.
We didn't fit any fresh air duct. It was a constant volume air supply 100% recirculated. The fan was very silent.
Its an american system and mainly aimed at the domestic market afaik.
(They recommended a glycol mixture for their outdoor units to prevent freezing)
Anyway these units were supplied from a thermal store.
Obviously domestic is different but could the same not be acheived from a combi boiler with a buffer/thermal store added?
Is the hydronic circuit of fan coils usually variable volume with constant temperature and whats the disadvantage with variable temperature-constant volume? More accurate control with the former I'm guessing.
You might, but the store temperature would start to drop when the DHW was on and the cold draughts would be very objectionable once the AHU couldn't maintain the supply air temperature. The whole idea of supplying an AHU from a combi is very dubious and seems to be a fantasy of TM, in which he hadn't thought out such design details. A better approach would be a system boiler with stored DHW; with modulating valves on the AHU and DHW, the system could supply heat to both and give priority to the AHU.
The standard detail used to be a 3-port diverting valve on the return on all the AHU coils (constant temperature, variable volume). The entire system, once commissioned would have a constant flow rate.
This arrangement isn't very energy efficient; some systems now use 2-port valves (no hot flow water diverted into the return) and variable speed pumps to conserve power. This can achieve big energy savings on the pump running costs.
The problem with variable temperature systems is that the temperature control (mixing valve, boiler flow temperature control) is usually in a distant plant room and a change will take some time to affect the AHU plant. Coils (all convection) also have very different heat emission characteristics at varying temperatures compared to radiators. It was usual to supply AHUs and DHW calorifiers from a constant temperature (CT) circuit and radiators from a variable/modulating temperature (VT/MT) WC circuit.
If you had 3-port diverting valves controlled by a BMS system, you would be able to monitor the percentage opening for each valve. You could vary the flow temperature so that the valve with the maximum opening was at around 85 or 90% open. You would minimize the heat losses from the pipework and the amount of flow water being diverted into the return.
TM3!
That was a variable water flow rate through the coil (by means of the 3-port diverting valve) and a constant water flow temperature.
into the coil.
How do they do that, without having a mixing valve and a pump set? It isn't possible. You're suggesting that the cool return water will mix with the hot flow water.
It doesn't work because the return is at a lower pressure, having been through the coil and lost pressure due to the friction. The only way to do it is with a 3-port mixing valve on the inlet to another pump, or an injection mixing pump (2 additional pumps per AHU coil). The 3-port diverting valve arrangement is much simpler.
You haven't got a Scooby, TM.
Thanks for the tip, rang them and they advised we phoned eco-equipped. So we did and we have now ordered.
And no need for 'uni strut', whatever that is!
Variable temperature constant volume the laws of physics are against you when the transfer is to air. Lower temp on the water side and all you'll end up with is a draught (and associated cooling process) rather than just a lower discharge air temp, down to a point. Picture the curves on a graph where the cooling action of the forced air overwhelms the transfer from the h/ex.
With CT-VV unless you are 1) blowing like a fiend or 2) the surface area of the water to air h/ex is too small you have a good base point of the h/ex with a high surface temp. There are all sorts of proprietary controllers to handle this situation, such as the Siemens RLM162 I think it is.
....oh and correct commissioning of course, but that seems to be a thing of the past also!
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