VAV Box Electric Reheat V Heating Coil

[remp]

Any body got and suggesting regarding the use of hot water heating coils in the VAV boxes as opposed to electric heaters. Do you get problems with small valves failing, blocking with dirt from the piping system, poor maintenance,...
Which is regarded as Cheaper, more energy efficient?

 

[SAK9]

Billy,

1.Electric heaters with SCR control will help you control temperature in a tight band whereas hot water coils in VAV boxes with their very low flow rates use only on/off valve control
2. The installation cost for hot water coils is more as compared to electric heaters.But their operation cost may be less depending on the fuel used for the boiler/hot water generator.
3.Electrical heaters pose a fire risk in the building.

 

[quark]

I generally prefer hotwater coils instead of electric heaters. Electricity directly used for heating (with resistance heaters) is thermodynamically inefficient. 1 kW electricity would produce maximum energy at the rate of 1 kW.

But Q = (COP_ref+1)W, which is the COP equation of a heat pump. i.e you can produce more heat than 1 kW when you use the electricity to run a heat pump. Lord Kelvin addressed these issues of inefficiency long back.

I found difficulties even with thyristor controls and other proportionate controls because of high thermal inertia of the heating elements. (note that, generally, electric resistors are good heat resistors)

Secondly, as the surface temperature of an electric heater is very higher than what is required, it cannot be used where you have solvent usage in your production area.

High switching frequency of electronic controls will lead to noise in your other equipment control systems.

Regards,

 

[lilliput1]

Hot water reheat is the way to go. Design for 180°F EWT & 150°F LWT. Include the space heat loss to the reheat duty in selecting the coils. For VAV, make sure you have enough CFM to do the heating duty. Otherwize consider fan powered terminal boxes (available in series or parallel) that can draw in plenum air & mixed it with the primary air (at the minimum CFM if VAV) then heat through the reheat coil. Note however plenum return ceiling arrangement requires wiring to be plenum rated so this may not be feasible for retrofits. HW valves can be electric or pneumatic. With DDC controls valves are typically electric. The DDC control manufacturer ships the DDC controls to the VAV box manufacturer for installation in the box.

 

[ChasBean1]

billyg, most reheat HW systems use analog control - with the loop tuned right, should throttle at the right point. On/off control exists but analog is more desirable and from what I've seen, more popular.