HVAC Air Duct Velocity

install more supply ducts, dampers or diffusers, depending on cooling conditions/requirement

sir which option is better and economical,more supply duct or damper i think more supply duct will occupy more space

I/we can not tell you which is better, it depends on your EXACT system/arrangement.

There are numerous scenarios, for example:

You may have one supply duct into a room through the ceiling pointing straight down at the floor. The easiest cheapest solution is to install a diffuser.

I could go on, and on, with different scenarios/solutions - but you need to make all the considerations. Space, cost, ease of installation, effect on overall system, .......

thanks sir i will go for damper system because i dont have space requirement

A BIG note of caution for this approach:

You will effect the cooling of your room with this solution. Because you will be providing less air.

but sir if we dont have space , damper reduce the required air quantity what is the solution

Does this HVAC/room have a return air system?

Is the room that is being supplied with "high velocity air" sealed/pressurized well?

continuing on ... (this could go for awhile as I try to extract information/understand your situation)

How is the supply air temperature controlled? Is the air conditioner near full capacity (that is - is it fully loaded at all times, or does it cycle)?

How is your suppply fan driven? Belt or direct drive?

Where in the world are you?

Where is this room located? In the middle of a relatively stable temperature plant? Is it a small room by itself in the middle of the desert?

Dear Sir i m from India, and working in automobile testing equipment maker. my work is here to design the havc system for automotive industry. so i am working out the heat load , duct size, cooling tower capacity etc so i have to go for best economical solution for that sir i m now in office and just now busy with client i will send u all information as soon as possible we will be continue on this topic

sir may i know about you

dear sir yes this room actually is automovtive testing workshop like powertrain lab where testing equipment have to be placed. it has a return air duct system which is going to be mixed with fresh supply sir and be recycled to the lab. some air is going to be exhaust. this system has been designed at full load with VFD Control bcoz all time equippment wil not be in running mode so all pump, fan, compressor shall be variable speed type for energy efficency purpose

Hi Manis2k3,

I've read all contents above and got confused what is your problem now. As you mentioned, your system is integrated with Variable-speed devices, including pump, fan, compressor. Is the "fan" means blower or the fan of FCU or AHU? What's your arrangement? If the blower is variable speed, then what's the problem? Or if the blower is not VSD, then why not use one? Because you mentioned that the heat load of the system you designed is not steady, so it's the most proper appoach in my opinion without the consideration of your project bottom-line. I think this part of cost is necessary for your project.

Given a cooling capacity requirment of the area, you can confirm the air volume you need and calculate how many diffusers you need to keep the room at positive pressure(test room and lab needs) meanwhile to keep the velocity of each air-outlet within the standard speed. Too high velocity make people feel uncomfortable and make it more noise.

So you'd better use adequate diffusers with a variable section air-duct, while you may need some manual-regular air-gate-way set in the branch duct to help you test and adjust the head, volume and velocity. Add more diffusers is another proper way to keep your whole lab area with positive pressure and enough cooling capacity.

If you can reduce the air-volume (if it's extensive and air-duct can not be modified), velocity and pressure head will also reduced, it's the most low-cost way and you can be free about the space problem especially if you can not enlarge the air-duct section or diffuser. And the risk is the cooling capacity allowance is reduced, also the head is reduced it may involves your other diffuser or branch duct which conects the duct you changed volume, you must keep each diffuser has accectable air velocity and keep each room with positive pressure. So if the lab requiement is allowed, you may reduce the return air volume at first, then fresh air may also be reduced.

If the air volume can not be changed, then you can enlarge your air duct section. The weakness is you need space, moreover, the large air duct make your project cost increased. The larger section, the thicker materials you need, or it will make more noise and more vibration to the air duct. The thicker materials, the thicher money you pay. Also the pressure head will be reduced, you will meet the same problem mentioned at previous.

You should know the square of your air duct section and diffuser or in another word, outlet is solid. If you change your velocity, then it may involves the pressure-head, volume and cooling capacity of the area is changed. The best senenario is to use all variable speed devices, sensors and controllers then leave it to the control stragy-software.

Given the noted restrictions from the other posts, and necessity to maintain required comfort, I would recommend looking at changing the diffuser first. Review the performance data and installation requirements and select from that. Here's a link to EH Price's catalog for diffusers: http://www.price-hvac.com/catalog/C_all/SectionCIndex.aspx?pageRequest=CD

In one of your more recent responses, you indicated that this installation is a testing lab with multiple pieces of equipment that will not all be operating simultaneously and that you already have a VFD on supply air fan because of this diversity.

IF the high air velocity only occurs when the system is at FULL volume (100% of VFD speed) then you probably will not have a problem during "normal" operation because of the diversity. IF you are experiencing uncomfortable conditions during "normal" operation, then you should change the drive sheave on the fan based on 80% of current speed - 1.25 times the current drive ratio (if pitch diameter ratio is, say 8:1, increase ratio to 10:1) by changing the drive sheave (assuming that it is "reasonable" size and the reduced size will not cause excess belt curvature).

I am assuming that your current HVAC design is for a "standard" 18F (10C) differential temperature from coil outlet to room temperature- 73F (22.8C) room with 55F (12.8C) leaving air. The recommended sheave change will cut air volume by 80%, increasing the supply air deltaT from 18F (10C) to 22.5F (12.5C) with a drop in supply air temp from 55F (12.8C) to 50.5F (28C) which should not cause any problems at the coil or in the room. The reduced air volume will result in reduced air velocity by the same ratio- 1500 FPM will become 1200 FPM.

This change will be easy to implement, but- as an even easier change, program your VFD so that its maximum speed ration is 80% of the current peak. You will have the same end result without any material cost and very low labor costs. and be able to modify it easily in the future if necessary.

None of these changes will affect the total cooling supplied to the room because you will be getting less air at a higher cooling effect per CFM (CuMeter/hour).

By the way, the change to 80% of air volume will cut your fan energy by nearly 50% because fan energy is based on the cube of the ratio- 80% speed yields 80% airflow, 80% x 80% (64%) of original pressure drop in the ducts and 80% x 80% x 80% (51.2%) of original fan energy (it will actually be a little higher due to friction loads, but not much).

I'm afraid it's not such a simple question, my highness, EnergyGod (hehe...)

You see, if the factor of the heat exchanger (fin-coil or something else) is solid, the temperature differential △T may not increase to the demand value as theory calculation. So for some condition (such as dehumidifier) we need 3 way FCU, others need 2 way. Further more, if the heat exchanging is not effect enough, then it will decrease the temperature of return water (chiller) or decrease the compressor suction superheat and evaporator pressure. Then the chiller or the compressor may unload to save the energy while protect compressor. It all depends on the heat load indicated by return water or leaving water. The chiller will "think" the load is not so large and refuse to upload. As a result, the area, may remains a problem that cooling is not equalized to every square, the farther away from the FCU, the worse effect it gains.

So, if modify the sheave or we can say "make the FCU capacity smaller", it depends on whether the equipment has allowance to do it, it also where the risk embeded (You must care, when the system has operated for a long term, all the devices efficiency will decline inspite of the maintenance is given).

Another important question for lab or testing room or chamber is to keep the room positve presssure.

If you have calculated the air volume carefully, and now you determinded to reduce your FCU's capacity so as to reduce the velocity, then you must fill up the lack part of the equipment total capacity, that means add FCU(smaller ones) or diffuser.

In a word, devices more, devices smaller.

In item 10, the initiator states that ALL components of the HVAC system operate with either VFDs or variable output due to the fact that the system was designed to support ALL installed equipment operating. They also state that there is SOME amount of make-up air and that the room has an exhaust system. They did not specify positive or negative pressure.

They also did not specify whether the supply air temperature was constant or variable. If the supply air temperature currently is constant- it is still controlled and the control can be easily modified or converted so that room temperature controls supply air temperature, which will definitely drive supply air temperature lower with reduced airflow.

I have considerable experience with all of the equipment serving this space. If the cooling coil has adequate capacity to supply 100% airflow at some fixed temperature, it definitely will have the ability to supply 80% of the maximum airflow at a slightly reduced temperature. Lower supply air volume will still generate essentially the same Leaving Water Temperature even if the air volume is reduced because the amount of cooling required will not change, just the supply air temperature. The counterflow arrangement of the chilled water will still expose the return water to the warmest air. If you apply "Log Mean Delta T" to the original coil, you will see that the reduced airflow coil will still be able to provide adequate total cooling.

Because the room's thermal load will be whatever it is, the chiller loading will not change just because the supply air volume is slightly reduced.

My suggestion does NOT "make the FCU capacity smaller". It reduces the volume of air supplied by the FCU and, correspondingly, expands the thermal "value" of each CFM of the supply air inversely to the ratio of the airflow volume change.

Relative to your comment regarding lost efficiency of devices long term- Without proper maintenance, that happens regardless of whether any changes are made to the system components. However, reducing fan speed- directly via sheaves or indirectly with the VFD- will greatly expand the life expectancy of the FCU. Due to the fact that bearing loading is directly related to the square of the rotational speed, operating at 80% fan speed will increase the life of the fan bearings by over 60% (1 / (0.8 x0.8) or 1/0.64 or ~1.60 of original life, or 60% higher that original life.

Relative to your comment- "some condition (such as dehumidifier) we need 3 way FCU, others need 2 way"- dehumidification with an HVAC system requires cooling the supply air below the dew point of the return air- the choice or control valve style is not relevant because coil flow will be whatever is required to meet the temperature reduction requirement except that a 2-way valve will always lower pumping power unless the flow through the coil is 100% of design.

The airflow capacity has already been established as excessive relative to actual load vs. potential peak load. The OP indicated that the current "reduced" airflow is still a problem, so- without getting into absolute on-site specifics- I made a suggestion that will LIKELY satisfy space "comfort" requirements without compromising the ability to satisfy space conditions. I also provided for the potential of adjusting that "solution" if necessary in the future.

By the way- the EnergyGod label was given by a former employer. I have earned it- I have 11 products in the patent process (including a 99% efficient CoGen system that cuts CO2 by 80% and NOx by 95%) that cut energy usage by up to 65% while still meeting all loads, have received an International First Place at the ASHRAE energy awards, typically identify 50% to 70% energy cost, usage and environmental release reductions for my clients regardless of the type of structure or process involved.

Thanks for your reply and congratrulate to your achievements, thanks for your contribution to the career of human being's evironment protection, my labled EnergyGod :-)

1. In item 10, the initiator states that ALL components of the HVAC system operate with either VFDs or variable output due to the fact that the system was designed to support ALL installed equipment operating WHILE the equipments will not run for all time. What does that mean? Mr.E.G.? Why they need VFD devices rather than shutting-down some of the fans, compressors by remote controller? Obviously, it means some testing equipments or lab devices will not run while others are running, because test bench or lab or test chamber is one procedure of a whole manufacturing process, sometimes manufacturing schedule is not full, so some of the lab systems run and generate thermal load. HOWEVER, all the clean rooms or labs or test chambers surely needs positive pressure to prevent the area from dusts or impurity. That's a comon sense. Whatever volume you supply to the room or exhaust from the room, positive is nessary, I think the ASARE also has the relative standards for your reference. So if you reduced the supplied volume, before it you have to reduced the volume exhausted, which means total recycling volume is reduced. AND it really equals that "capacity of the system is smaller then full-load one". Even for residential or normal office building, the positive pressure of a certain room is nessary, because if you are refrigerating a space then you surely have to prevent extra heat "leaks" to this space. Don't make a mistake that we used many expensive advanced Energy saving devices however while waste energy from other aspects.

2. According to the formula Q=cm△t, here c is the specific of substance such as air or water, for Forced convection heat transfer conditon, Q=cF△t, here c means the factor or the certain heat exchanger such as FCU, F means squares of the exchanger, and the △t means "log mean delta T" between air and coil surface, coil surface and chilled water. Of course, in Q=cm△t, if you reduced the "m" which means massflow, and enlarge the △t the Q is equalized, however, in Q=cF△t, c is the function value of the air & water flow velocity, if you are familar with Newton Cooling Formula, you should know, Q=αF(t_w-t_f), α=q/△t，and applying the Diffential equation of the α, then α_x=-λ/(t_w-t_f)(∂t/∂y), so the velocity is the key to the effect of the heat transfer. If you abadon the calculation and use a method of experiment, then Nu= f(Re, Pr), the higher flow speed, the higher Re value. Moreover, for doing Pro-active Maintenance for an equipment, or doing an improvement for it you will find, if there are two aspects which are affect the heat transfer, the better method is improve the weak aspect rather than making the strong side stronger. For FCU, air-side is weaker than water side. So, if the air velocity is reduced for a scale of 20%, of course, the heat transfer effect is decreased. SO it surely will reduce the cooling capacity because the △t of the Q=cm△t will not enlarge to the demand value. I can not tell you the exact value because it depends on the specific of the FCU.

3. There are a lot of methods for dehumidifier(wheel dehumidifier, coil dew point dehumidifers in AHU and etc,.), that's not the key. The key is labs always need a strict humidity and temperature condition because the lab devices emit the heat and humidity to the space. Usually we generate fresh dry air and mix it with the return air of the labs, then blow it to the labs, while some of the air has to exhaust, because we can not afford too large dehumidifiers regarding the cost(the RH of return air usually is lower than outside) while we also need the recycle the air of labs. Usually we need 4 times per hour, some situation we need 8 times or moreover. Reduce the volume may result in the difficulty of the humidity controlling. Because if the lab devices are emitting the humidity to the rooms you need to draw it from the room and make up more dry air.

4. The efficiency of all the equipments will decline surly whatever you give them maintenance or not. But as a result, the speed of "grow old" is different between those two. Of couse, the blower bearing will get benifit from the lower speed, but the motor, fin, copper coil, water pipe, filter, cooling tower, even chiller will still get efficiency decreased. Benchmarking method is one effect method for managing the equipment how "grow old" and give a metric for the level of the maintenance service.

5. In a word, for the project, if the cooling equipment capacity is excessive, then try to reduce the recycle volume but it's risky. The best way is to use more terminal air-side devices such as FCU of diffusers and make them smaller, the difficulty is the space (so maybe use the soft pipe if the pressure head is allowed).

6. Congratulate to EnergyGod for your achievement again. As a friendly guy, I wish you remember a proverb, the study is endless:-) I believe that ASHARE awards is great honor for persons in this field, I think I can learn a lot from you. And I have years-expierence of managing, maintenance，reconstruction of the facility for over 200 projects and 600 pcs chillers, the auxilary equipment is countless. I focus the result and operation effect more than others, you can discover some feeling in my comments, right? :-) In our career, safety and sustainability is the most important.

Is the HVAC unit servicing this room a stand alone system that serves just this room?

When you say human comfort; is it temperature, noise or air blowing on people problems?

if your AHU Motor is connected to VFD,then reduce the speed or you need Re-balancing of your system. simple as that if your duct system is supplying multiple air diffusers