Controls/hydronics/Air handling project for deeper learning - input appreciated

[Mark_B]

I am in the process of building a miniature air handler/VAV/Boiler system at home. The purpose of this system is for more deeply learning Hydronics, controls, fan/VAV controller interactions, PI(D) loops etc and it is not designed to be efficient, simple, or reasonable for real life application.

Here is my current parts list:

Controls
- Will provide upon request. JACE-based controls with extra sensors to collect information during test runs.

Equipment

- Fantech FG8 EC exhaust fan, 0-10VDC speed controllable, 428CFM @ 0 static
- Titus DESV 06, equipped with single row water coil "B016 coil, vav06, 1R/1C, ASC"
- B&G Ecocirc+ 20-18 stainless pump, 0-10VDC variable speed control
- 4-gallon 1440W Bosch ES4 water heater
- Inox Pro stainless 0.53 gallon expansion tank

I understand/believe the water heater is vastly under powered. I'm not concerned with being able to run this for a long time and am okay with 10-20 minute sessions while observing/recording behavior.


I have read basics about using Cv, pressure drop and line vs valve size for sizing of control valves. Our engineer provided me a spreadsheet to simplify selection of control valves but I still do not understand the 'why.' I have done a fair amount of equipment procurement based on engineer-furnished schedules and would often hear something from vendors like "If you want that capacity we need to increase the (water) pressure drop on the coil." The implication seemed to be that increased water pressure drop was required for increased Btu output. I didn't think much at the time and just assumed they were suggesting to furnish coils with more rows.

1) Was this intuition correct?


2) Is the water pressure drop they were speaking of about the coil itself, or the equipped control valve, or both? Can a control valve with larger pressure drop deliver higher capacity (I would guess it would have the opposite effect by reducing flow)?

3) I have been unsuccessful finding pressure drop/technical specifications about my Titus VAV coil. Is there some resource or do you need to be in Titus sales system?

4) Is the use of a three-way valve in a single coil system going to have some effect I am not aware of? I assume in my closed-loop system it would just route some of that unusable pump power back to my water heater and have no ill effect.

 

[Mark_B]

I am currently reading Chapter 6, Fluid Flow in Piping, of "Modern Hydronic Heating and Cooling" by John Siegenthaler, P.E.

I noticed the Titus coil performance table provides a loss of head per GPM through the coil instead of loss of pressure. This is interesting as the book explains viscous friction removes head from a fluid passing through a pipe, and part of that energy transfer is head being converted to thermal energy via friction. Another way increased velocity increases heat transfer. It may be a very small quantity but interesting nonetheless.

The head choice is also a bit difficult as it includes the density of the fluid which is dependent on temperature. The table gives an air/water temperature difference of 125* with 180* entering water as the basis for calculations but that makes my calculations a bit more difficult. To me this suggests the density of the fluid flowing through the coil will affect the pressure drop per GPM through my coil. I suppose I can use the entering water pipe thermistor I intended for use during Btu calculation to also feed data into a head formula within my controller.

 

[Snickster]

1) Yes. There are heat transfer calculations where you can determine the heat transfer coefficient. Other fluid properties contribute to heat transfer such as thermal conductivity of fluid, viscosity, etc. but without velocity they don't matter much. It has been a while since I looked at in detail but I believe high Reynolds number is major parameter which predicts turbulent flow.

2) I guess the more mass flow (or volumetric flow) the greater the average temperature of the fluid in the coil for a given specific heat (for a greater specific heat of fluid in coil less temperature drop will occur for given amount of heat transfer) and therefore greater heat transfer.

There are a lot of factors in heat transfer such as all the properties of the heat transfer fluids, the flow regime (laminar or turbulent), and the configuration of the heat exchanger (counterflow, parallel flow, single pass, double pass, etc., etc.). But in your case you are dealing with water on the tube side and air on the other side in basically a HVAC application. There is not much variations possible such as variation of fluid properties or heat exchanger configuration, so you can only varry the flowrates to vary the Reynolds number and flow regime. Their is also other parameters that go into calculation of the heat transfer coefficient such as the Froude number and others but it has been a while since I did a detailed heat exchanger sizing and design project so I can't list all of the top of my head.

 

[Snickster]

I noticed the Titus coil performance table provides a loss of head per GPM through the coil instead of loss of pressure. This is interesting as the book explains viscous friction removes head from a fluid passing through a pipe, and part of that energy transfer is head being converted to thermal energy via friction. Another way increased velocity increases heat transfer. It may be a very small quantity but interesting nonetheless.

In engineering head and pressure are used interchangeably. In engineering equations head is easier to work with so it is used, and relevant fluids calculations are based on energy terms so it must be used. Expressed mathematically:

Head= Pressure(psi)*144/Density of Fluid (lb/ft3). The units of head are ft-lb/lb which is energy per pound weight. Where the units of pressure are PSI or PSF, etc.

Pressure drop calculations for incompressible flow are based on the Bernoulli equation which terms are all of energy content (energy due to pressure, velocity kinetic energy, and elevation difference potential energy) so that the total energy although remains constant is converted from one form into the other. Frictional energy loss subtracts from the total available energy. You need to study basic engineering "Fluids" first couple of chapters to understand.

The head choice is also a bit difficult as it includes the density of the fluid which is dependent on temperature. The table gives an air/water temperature difference of 125* with 180* entering water as the basis for calculations but that makes my calculations a bit more difficult. To me this suggests the density of the fluid flowing through the coil will affect the pressure drop per GPM through my coil. I suppose I can use the entering water pipe thermistor I intended for use during Btu calculation to also feed data into a head formula within my controller.

For incompressible fluid like water you can assume a constant density (62.4 lbs/ft3 for water) without significant error (fraction of a decimal point) and use the above equation to convert to pressure drop.

For instance 5 feet of head loss of flowing water per the above equation:

5 = P(144)/62.4

P = 5(62.4)/144= 2.166 psi

 

[Snickster]

Seems like you are really interested in heat transfer. Here is a good reference that goes over the basics of heat exchangers in an understandable way.

 

[Snickster]

Here is an old Trane manual on VAV systems that will be helpful to you.

 

[Snickster]

Here is basic reference materials for hydronic systems from Bell and Gossett.