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 have located the answer to question #3 at the following location (Titus website):

https://www.titus-hvac.com/file/1163/esvperf_singledual_2023_V2.pdf

 

[LittleInch]

Difficult to tell without more data, especially on your coil.

1) Essentially more pressure drop across the coil will or should increase flow rate. What that will do is increase the mean temperature in the coil and hence increase heat outflow or if the outlet temperature is approaching the outlet air temperature from the coil it will raise the outlet temperature which again increase heat output form the coil.

the actual effect will vary by coil so vendor information is required to confirm that. Sometimes it will make no or negligible difference, other times it will increase the heat output.

2) Possibly both - not clear. More pressure drop across a CV open the same amount will flow more water.

3) Good

4) A diagram always helps, but assuming your three ports are inlet hot water, hot water to the coil, hot water bypass to the return line, then it just means there is a constant water circulation. The key is making sure that when you're on full bypass of the coil, the pump doesn't see a lot less flow resistance compared to going through the coil. For me the ideal situation is that the pressure drop when on full bypass should be the same as when the coil is fully open, then the pump flow rate doesn't change hugely depending on what is happening.

But it all depends on the relative pressure drops. If the pressure drop in your flow and return piping is say 80% of the total pressure drop and 20% the coil, then you're prob ok with a straight piped bypass. But if its the other way around then you really need to control the pressure drop / flow through the bypass when fully open.

Deas that help / make sense?

 

[HVAC-Novice]

I assume they talk about Valve authority when they want more pressuredrop (over the valve). this is somewhat true, but also obsolete if you use PICV valves.

You select a coil with the parameters you want to operate it. that gives you a range of coils. You want to choose one with low pressuredrop (both on water and air) and high dT. No one in their right mind would choose a coil with extra pressure drop. Use manufacturer selection software.

if you can't find Titus information, contact them or find a different manufacturer. Besides PDf tables, they all should have selection software. A hydronic coil capacity isn't a single number. it depends on air and water flow and temperatures. A hydronic coil could have 1,000 Btu/h, or 20,000 Btu/h.

If you use 3-way or 2-way depends on how you operate the system. For a condensing boiler, you want high dT, which is not possible with 3-way valves.

If it is a single coil and single pump, I would consider controlling by changing pump speed and no control valves. Feasibility of that depends on many things...

 

[Mark_B]

Difficult to tell without more data, especially on your coil.

1) Essentially more pressure drop across the coil will or should increase flow rate. What that will do is increase the mean temperature in the coil and hence increase heat outflow or if the outlet temperature is approaching the outlet air temperature from the coil it will raise the outlet temperature which again increase heat output form the coil.

the actual effect will vary by coil so vendor information is required to confirm that. Sometimes it will make no or negligible difference, other times it will increase the heat output.

2) Possibly both - not clear. More pressure drop across a CV open the same amount will flow more water.

3) Good

4) A diagram always helps, but assuming your three ports are inlet hot water, hot water to the coil, hot water bypass to the return line, then it just means there is a constant water circulation. The key is making sure that when you're on full bypass of the coil, the pump doesn't see a lot less flow resistance compared to going through the coil. For me the ideal situation is that the pressure drop when on full bypass should be the same as when the coil is fully open, then the pump flow rate doesn't change hugely depending on what is happening.

But it all depends on the relative pressure drops. If the pressure drop in your flow and return piping is say 80% of the total pressure drop and 20% the coil, then you're prob ok with a straight piped bypass. But if its the other way around then you really need to control the pressure drop / flow through the bypass when fully open.

Deas that help / make sense?

1) Still finding this hard to understand. I always thought the pressure drop at the outlet of the coil was related to the resistance against flow inside a given coil. Given inlet pressure remains the same, but outlet pressure is decreased to a lower value, that would indicate lower flow due to increased friction within the coil, similar to the effect of pumping water a longer distance using the same pump and pump speed. I could see how increasing pressure at the coil inlet would increase pressure drop through the same coil, and how that would be indicative of more flow.

2) Got it thank you.

4) You are correct about your piping assumptions. Thank you.

 

[Mark_B]

I assume they talk about Valve authority when they want more pressuredrop (over the valve). this is somewhat true, but also obsolete if you use PICV valves.

You select a coil with the parameters you want to operate it. that gives you a range of coils. You want to choose one with low pressuredrop (both on water and air) and high dT. No one in their right mind would choose a coil with extra pressure drop. Use manufacturer selection software.

if you can't find Titus information, contact them or find a different manufacturer. Besides PDf tables, they all should have selection software. A hydronic coil capacity isn't a single number. it depends on air and water flow and temperatures. A hydronic coil could have 1,000 Btu/h, or 20,000 Btu/h.

If you use 3-way or 2-way depends on how you operate the system. For a condensing boiler, you want high dT, which is not possible with 3-way valves.

If it is a single coil and single pump, I would consider controlling by changing pump speed and no control valves. Feasibility of that depends on many things...

Thank you for the references to Valve authority and PICV. Terms I am not familiar with that I will go and research.

The vendor was just trying to provide a solution that matches the specifications in the provided schedules. All parties did want to offer the solution resulting in the lowest possible pressure drop. My question is simply by what mechanism increased pressure drop and increased capacity are related, given that the system pressure remained the same.

I was able to locate the Titus PDF and linked it in post #2 on this thread.

Good point on the high dT, I only chose the three-way valve as I was afraid the pump would not be able top ramp down to a low enough value for my expiriments. Also I am interested in at what flow rate the pump is most efficent.

 

[Snickster]

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.

I believe what the vendor is saying is that you need to increase the water flow pressure drop through the coil because the higher the pressure drop the higher the velocity of flow through the coil. The flow velocity over the coil internal heat transfer surface determines the heat transfer coefficient. The higher the velocity the higher the flow coefficient. If velocity is not high enough there will be very low heat transfer coefficient and very low heat transfer. So in your case if you are flowing a maximum of 4 gpm through the coil, the velocity may be so low through the coil that you don't get much heat transfer and your coil becomes sort of useless. You need to check the specs of the coil to see the recommended minimum velocity/flowrate.

Likewise on the air side there is a minimum face velocity of air across the coil in order for proper heat transfer. This is also related to the velocity required to produce the required heat transfer coefficient of the air to coil surface. If air velocity is too low then proper heat transfer will not take place. Your fan performance table is attached. It produces 428 CFM at 0 in. wg. static pressure and 121 CFM at 1.5 in wg. static pressure. The static pressure is the pressure drop across all system components (duct system, VAV and coil), the largest drop will be across the coil. So if the total pressure drop of your system is 1.5 inches water gauge then you will only get 121 CFM flow which may be too low for the coil required face velocity. The face velocity is simply the velocity of the air divided by the total projected area of the coil face in CFM per square feet.

 

[Mark_B]

Awesome, thank you. So the total pressure drop will be pressure drop will be the drop across a 8" - 6" reducer and short pieces of round duct, plus (If I read the Titus document correctly) somewhere between 0.07 - 0.13" WC across the box with damper full open, plus between 0.04 - 0.15" WC through the coil depending on CFM at coil.

It seems the variable I did not account for was the reduction to 6" just before the VAV inlet, although I assume the 8" fan reduced to 6" will still produce more power than the 6" version which was rated about 90CFM less at 0 static? (Assuming similar behavior as static increases). I am not sure where to look to determine the loss of reducing to 6". Regardless I will soon find out...

 

[Snickster]

Awesome, thank you. So the total pressure drop will be pressure drop will be the drop across a 8" - 6" reducer and short pieces of round duct, plus (If I read the Titus document correctly) somewhere between 0.07 - 0.13" WC across the box with damper full open, plus between 0.04 - 0.15" WC through the coil depending on CFM at coil.

You appear to be talking about the total pressure drop across the VAV unit including inlet connection reduction. The 8" x 6" reducer produces a minimal pressure drop so you can ignore that. So it appears at maximum flow you get 0.13 + 0.15 = .28" wc which appears OK for the fan you selected as you will get close to 400 CFM out of the fan.

It seems the variable I did not account for was the reduction to 6" just before the VAV inlet, although I assume the 8" fan reduced to 6" will still produce more power than the 6" version which was rated about 90CFM less at 0 static? (Assuming similar behavior as static increases). I am not sure where to look to determine the loss of reducing to 6". Regardless I will soon find out...

A reduction of 8" to 6" is normally expressed in equivalent length of straight duct. So a 8"x6" reducer will be equivalent to about say 4 feet of 8" duct or so. Add all of the 8" duct length including duct straight length plus equivalent length of all duct fittings such as elbows, tees, duct inlet loss, to get total duct length. Then multiply by pressure drop per unit length of duct at given flow and add to the VAV losses indicated above to get total losses. The pressure drop per unit length for duct sizes at given flowrates can be found in references and internet searches. Duct losses should not be that much at 0.1" to 0.2" wg or so for relatively short overall length.

I believe that you should get 300 to 400 CFM from your fan at the pressure drop calculated.

 

[Snickster]

I think your air side will be ok as the pressure drops across components in the air system don't appear to be that high to significantly restrict the air flow from your fan.. I think that you are going to have issues with your water side as 4 gpm for that size VAV seem excessively low and the velocity internal to the coil may be too low to get any significant convective heat transfer coefficient between the heating water and the inside surface of the coil.

 

[Snickster]

Here is some information on duct design. You can find more on the internet.

 

[Snickster]

I just realized your heater is 4 gallon, not that your flow is 4 gpm so your actual flow must be much less than 4 gpm which appears very low. I would check with Titus literature to see if they have a minimum recommended water flow rated for the coil.

 

[Mark_B]

I just realized your heater is 4 gallon, not that your flow is 4 gpm so your actual flow must be much less than 4 gpm which appears very low. I would check with Titus literature to see if they have a minimum recommended water flow rated for the coil.

According to the spec sheet for the coil:

2gpm, 200cfm and water/air temperature difference of 70* should be about 4,760Btu/hr.

Given that the water heater is 1440W at a claimed efficiency of 98%, I am expecting it to be able to handle about 4800Btu. I have no idea if this will turn out to be true. I am not trying to produce a useful heater, more just trying to build a system that will stabilize with the control system I am trying to learn at work and am okay with dialing down the CFM to a point where water temperature stabilizes, as long as my VAV flow sensor functions at given CFM.

Keep in mind I also plan to turn off the heater and play with airside control programming, turn off air and play with water etc.

Location of spec sheet:

https://www.titus-hvac.com/file/1163/esvperf_singledual_2023_V2.pdf

(This VAV is equipped with the single row coil and is the 6" model)

 

[Mark_B]

Here is some information on duct design. You can find more on the internet.

Thank you. I am going to spend some time reading.

 

[Snickster]

Looks like minimum water flow is 1 gpm tabulated for the coil so if you maintain at least this you should be ok on the water side. The air side should be ok too as the tables go down to 100 CFM. I think with your air system pressure drops you will be at least in the 300 CFM range output from your fan.

 

[LittleInch]

) Still finding this hard to understand. I always thought the pressure drop at the outlet of the coil was related to the resistance against flow inside a given coil. Given inlet pressure remains the same, but outlet pressure is decreased to a lower value, that would indicate lower flow due to increased friction within the coil, similar to the effect of pumping water a longer distance using the same pump and pump speed. I could see how increasing pressure at the coil inlet would increase pressure drop through the same coil, and how that would be indicative of more flow.

Hmmm. What pressure drop at the outlet?. But don't change too many parameters at a time. If your coil length is fixed, the increased pressure drop across the coil will result in increased flow assuming nothing else changes. More water flow increases heat transfer and provides more water / energy to convert into warm air which results in higher exit temperatures or higher average water temperature in the coil.

 

[Mark_B]

Hmmm. What pressure drop at the outlet?. But don't change too many parameters at a time. If your coil length is fixed, the increased pressure drop across the coil will result in increased flow assuming nothing else changes. More water flow increases heat transfer and provides more water / energy to convert into warm air which results in higher exit temperatures or higher average water temperature in the coil.

I just meant pressure drop across coil, measured via inlet and outlet of coil.

What could you change to increase pressure drop besides flow rate? Meaning, if I double my pump speed I assume that would increase pressure drop at coil (and of course would also increase flow). What about the coil could I change to increase pressure drop that would also increase flow?

 

[LittleInch]

Not much. You could add more coils to increase flow without changing pressure drop, but just change one thing at a time.

 

[Snickster]

What could you change to increase pressure drop besides flow rate? Meaning, if I double my pump speed I assume that would increase pressure drop at coil (and of course would also increase flow). What about the coil could I change to increase pressure drop that would also increase flow?

If you double your pump speed you will increase pump flow and pressure output so therefore the pressure drop across the coil. Note though that pressure drop across coil does not improve heat transfer directly. Velocity of flow inside the coil tubes is what really improves the heat transfer coefficient as convection heat transfer is dependent on the velocity of flow over the heat transfer surface. But the higher velocity the higher pressure drop so some people (like vendors) refer to pressure drop erroneously as the cause of high heat transfer. If you had a ideal coil that had very little pressure drop at a very high flow velocity you will still have a high heat transfer coefficient. So if you increase pump speed you will increase flow and hence velocity through the coil and hence heat transfer capacity of coil.

 

[Mark_B]

What could you change to increase pressure drop besides flow rate? Meaning, if I double my pump speed I assume that would increase pressure drop at coil (and of course would also increase flow). What about the coil could I change to increase pressure drop that would also increase flow?

If you double your pump speed you will increase pump flow and pressure output so therefore the pressure drop across the coil. Note though that pressure drop across coil does not improve heat transfer directly. Velocity of flow inside the coil tubes is what really improves the heat transfer coefficient as convection heat transfer is dependent on the velocity of flow over the heat transfer surface. But the higher velocity the higher pressure drop so some people (like vendors) refer to pressure drop erroneously as the cause of high heat transfer. If you had a ideal coil that had very little pressure drop at a very high flow velocity you will still have a high heat transfer coefficient. So if you increase pump speed you will increase flow and hence velocity through the coil and hence heat transfer capacity of coil.

Is the increased heat transfer of higher velocity flow the result of:

1) Increased turbulence and the resulting improved heat exchanger surface contact of a given mass of water as it travels through the heat exchanger

2) Is there a higher average fluid temperature within the coil which is a result of any given mass of fluid spending less time in the coil losing heat to the passing air?

Is there some other effect I am missing related to fluid velocity?

Appreciate you all educating me! I am reading online as well, but your answers are helping me focus my energy on the correct questions.