What control system

If you tell me what controler is on the unit maybe i can send the program for it.
also should be in the spec book for the machine.
setup would depend on were you are for temp settings.

 

I will be controlling the systems via a JCI/DX-9100, I already have the units programmed and online but I can't even get answers on PROPER sequence from the Aaon reps, maybe they're just over protective of they're equipment. What are the optimal conditions to run or not run the heat wheel. It sounds as though the wheel runs below 50'F OAT and above 60'F. I'm looking for the most optimal way to control this beast. Any info would be appreciated.
Thanks, TechHead

 

The company I work for writes a sequence of ops specifically for each application. But you're right, there's a more or less basic sequence that we start with and modify to a greater or lesser extent according the customer's needs. It would be wrong for me to speculate on what the proper sequence would be for another company's product because of variations in design as well as the performance of the wheel. 50' and 60'F are common for "economizer mode" - i.e. wheel off but OA continuing to be supplied by the fans. However, if it's 55'F and raining does the unit have an enthalpy sensor or dry bulb only sensor? And how effective is that wheel in terms of its latent capacity? etc.

I can send you a 'generic' sequence of operation but i'll need to know the unit configuration - location of wheel, cooling coil, heating coil, and whether it's DX, Chilled Water, Electric Coil, Hot Water, etc.

 

Chill,
Thanks for the response. I have several Aaons. One example would be. 4 Stage DX, HT Whl(constant volume), Econ Dmprs 0-10vdc, Bypass Dmprs 0-10vdc, SF VFD, EA VFD, OA CFM. No Gas Ht or HW Heat. I'm sharing OA Temp globally through our JCI BACNET.
Thanks Again, Chris
chrisclarkhvac@yahoo.com

 

Id like a copy also. absrbrtek@yahoo.com

 

Energy Recovery Units-Sequence of Operation

Energy recovery units are being used widely in schools because of the population density and ASHRAE ventilation guidelines in classrooms. They are extremely viable particularly with AAON units using the Airexchange heat wheels. The 65-70% efficient wheels have fewer potential for frost formation than the so called "85%" efficient wheels. The higher efficiency wheels tend to form frost on the exhaust air side of the wheelat higher ambient temperatures than the lower efficiency wheels. Devices that are used for defrost control, i.e. VSD's to slow wheels, thermostats that stutter start and stop the heat wheels or radiant heaters to warm and defrost the higher efficiency wheel undermine the purpose of the wheel such that the annualized efficiency is probably no better than an energy recovery wheel that has a lower eficiency and never stops during operation. (Sorry for the wordiness!)
Now, getting to your issue, first determine two parameters:
1. Desired space temperature.
2. Minimum outside air temperature for which the space requires cooling.
The heat wheel should not be operating at any temperature between the mnimum outside air temperature for which the indoor space requires cooling because the warmer exhaust air stream would preheat the outside air and nullify it's cooling effect. This would result in unnecessary operation of compressor(s) to cool the space. The energy recovery wheel wheel should be kept off and the bypass damper opened to maintain the proper mixed air temperature (economizer cycle)until the outside air temperature rises to about 5 degrees above the space temperature. The relatively cooler exhaust air will begin providing some precooling of the outside air. The energy recovery wheel should run continuously (except for defrost operation) during the heating season and when the ambient temperature is at least 5 degrees above space temperature. This is based on personal experience and not cast in stone.
As far as humidity entry to the space with ventilation air, total enthalpy wheels incorporate a dessicant (or molecular sieve) that plays "catch and return" with outdoor humidity. The dessicant or sieve captures most of the moisture present in the outdoor air on the incoming side of the energy recovery wheel, and that humidity is purged to the outdoors by the exhaust air stream when the wheel rotates into the exhaust air stream. It is obvious that a total energy wheel would not be a good choice for a shower room.
A sensible energy recovery wheel or other sensible heat exchanger such as a plate heat exchanger, heat pipe, or runaround loop would be more suitable for an application where the exiting air is heavily moisture laden.

 

The field for discussion of the different wheels and their construction, desiccant composition, substrate materials, efficiencies (sensible and latent), propensities for cross-contamination, life-span, ARI performance ratings, etc. has been opened.

However, I will limit my remarks to two. I contend that 1) because ventilation is the primary goal in school applications, putting a preheat on the ERV to prevent frost is a necessary evil in order to maintain compliance with ASHRAE requirements. It's still better than bringing in raw, non-preconditioned outside air to the air-handler or RTU. If, in fact, there is a difference in frost points between high and low efficiency wheels, it cannot be significant when viewed relative to the amount of energy a wheel is saving (even with preheat energized) vs a conventional system (w/o a wheel or other energy recovery device). Further, the geographic location becomes important when talking about the gains from 15% higher efficiency for a large number of annual hours without preheat and the much smaller number of annual hours when preheat is actually utilized. 2) The options for different wheels (much less other types of heat exchangers) are many and, as with anything else, the buyer needs to do their due diligence and determine for themselves what the advantages are of using one over another for their specific application.

 

I'm currently designing a heat wheel control. The heat wheel is from AIRotor. The unit has no preheat but has a heating coil, cooling coil and humidifier. There is also a recirc damper and a bypass damper on the exhaust side of the heat wheel. The manufacture of the heat wheel is supplying only the heat wheel and it will controlled by VFD. The constant volume supply fan and VFD on the exhaust fan.

I have attached the sequence supplied the engineer

Outdoor Air/Economizer Control
Interlock outdoor and return air dampers to operate in sequence. Outdoor air damper is closed during unoccupied cycle and opens to specified minimum position during occupied hours. Monitor supply air temperature, return air temperature and humidity and outdoor air temperature and humidity. When outdoor enthalpy is less than return air enthalpy, modulate outdoor open and return air dampers closed in sequence with cooling control valves as required to maintain 55 deg F supply temperature. When outdoor air enthalpy is above return air enthalpy, outdoor air damper returns to minimum position setting.

Warm-up Control
Whenever outdoor temperature is below 55 deg F and system status changes from unoccupied to occupied mode, BAS initiates a warm-up sequence. Hold outdoor air damper closed and return air damper open and activate heating coil to raise discharge air temperature to 80 deg F until return air temperature rises to within 2 deg F of space setpoint.

Occupancy Control
Unit supply fanis on during occupied hours and runs intermittently during unoccupied hours.

Exhaust Fan Control
The exhaust fan runs during the occupied cycle whenever the outside air is more than 25% open and is off during occupied hours. Modulate the fan speed thru the VFD as required to maintain building pressure at 0.05 inch wc in the Board Room. Prove exhaust damper open before starting the exhaust fan. If supply fan is off, stop the exhaust fan and close the exhaust damper.

Energy Recovery Wheel Control
Activate the energy wheel whenever the unit is in the occupied mode. The energy recovery wheel bypass damper is closed except when economizer is operating. Bypass damper is open on a call for economizer operation. Modulate bypass damper closed as required to maintain 55 deg F. mixed air temp when CO2 detector is controlling outside air damper above minimum position.

Frost Control
Monitor the exhaust air temp and humidity. Modulate exhaust bypass damper open as required to prevent heat exchanger frosting.

Outside Air/Recirc Damper Control
The outside air damper works in sequence with the recirc damper. The outside air damper opens to minimum position when system is in the occupied mode. On a call for cooling with outdoor less than return air enthalpy, outside air damper is sequenced with recirc damper as required to maintain 55 deg F. supply air temperature. Outside air damper returns to minimum position when outside air enthalpy exceeds return air enthalpy. Outside air damper closes and recirc damper opens when unit is off. Monitor the return air CO2 and modulate the outside air damper open/recirc damper closed as required to maintain 700 ppm CO2 (adjustable).

I have only just now started to educate myself on ERU and from everything I have read this sequence appears to be not using the heat wheel to it full extend and lacking in some key operation areas.

The question really boils down to should the engineer revise this sequence because I can't see how this will work.

 

I'm currently designing a heat wheel control. The heat wheel is from AIRotor. The unit has no preheat but has a heating coil, cooling coil and humidifier. There is also a recirc damper and a bypass damper on the exhaust side of the heat wheel. The manufacture of the heat wheel is supplying only the heat wheel and it will controlled by VFD. The constant volume supply fan and VFD on the exhaust fan.

I have attached the sequence supplied the engineer

Outdoor Air/Economizer Control
Interlock outdoor and return air dampers to operate in sequence. Outdoor air damper is closed during unoccupied cycle and opens to specified minimum position during occupied hours. Monitor supply air temperature, return air temperature and humidity and outdoor air temperature and humidity. When outdoor enthalpy is less than return air enthalpy, modulate outdoor open and return air dampers closed in sequence with cooling control valves as required to maintain 55 deg F supply temperature. When outdoor air enthalpy is above return air enthalpy, outdoor air damper returns to minimum position setting.


Outside Air/Recirc Damper Control
The outside air damper works in sequence with the recirc damper. The outside air damper opens to minimum position when system is in the occupied mode. On a call for cooling with outdoor less than return air enthalpy, outside air damper is sequenced with recirc damper as required to maintain 55 deg F. supply air temperature. Outside air damper returns to minimum position when outside air enthalpy exceeds return air enthalpy. Outside air damper closes and recirc damper opens when unit is off. Monitor the return air CO2 and modulate the outside air damper open/recirc damper closed as required to maintain 700 ppm CO2 (adjustable).


The question really boils down to should the engineer revise this sequence because I can't see how this will work.

Without writing a book here, the engineer should revise and improve this sequence significantly. Unless you are in a climate where heating is never needed - the economizer will cause major problems with space heating. There is no means to shut off the economizer cooling function when heat is needed in the space.

What controls the wheel VFD? If it is a constant speed wheel, why pay for a wheel VFD?

You mentioned a humidifier - that is usually an indication of cooler climates. What type of cooling coil? A chilled water cooling coil can freeze & burst without preheat to protect it. A DX coil can freeze and burst a nearby heating coil unless there is some distance between them and/or controls to prevent it.

OK, I will stop now. I said wasn't going to write a book...

 

Sorry didn't want you write a book but the information is helpful.

I agree that the engineer should imporve the sequence. Yes we are in a climate that needs heating. The engineer did not provide information on heating control. There is heating coil upstream of cooling coil (chilled water). There is a VFD on the heat wheel but the sequence makes no mention of modulating the wheel.

I currently writing in RFI to question the sequence. Since this is my first heat wheel control sequence I appreciate the help and would ask for any help that you can provide in a good control stragety of a heat wheel.

 

wish i had a choice to turn erv's off. My rtu's depend on fresh air through erv's. Our great design egganeer's do us hvac techs wonders

 

I spent a week last winter working on 20 LGA Lennox rooftop unit with RSI ERV units on them. The best temp rise that we ever got across the heat wheel was 10 deg. For all they cost what is the differance from a ten degree rise from 20 degrees to 30 degrees in air supplied to the building. I think that they are another example of more BLUE SKY sold to building owners. The best ventilation systems are ventialtion systems with make up air heat and cooling built into them, they are also the most expensive.
The best ERV units can do is "fire for effect". when it comes to effectivly ventilating a building.

 

ERU-rubobornot

Don't judge all ERU'a by this experience with Lennox. The ERU's wheel performance is a function of the design condition, i.e. design winter and summer outside air temperatures vs. exhaust air temperatures and relative flows of exhaust vs. outside air. 1:1 is pretty standard for exhaust to supply air ratios (unless the unit's total supply and exhaust capabilities are too great for the wheel,then there is a need to bypass air) If the exhaust to supply air ratio is greater than 1, the perfoemance is better; if less than 1 the performance is less. As the summer design temperature and winter design temperatuers for outside air move closer to the exhaust air temperatures (sumer OA temperature drops or winter OA temperature rises) the performance of the wheels drops off since the efficiency is reduced. If rubobonot checked temperatures on say, a 50 degree ambient temperature day (wheel heats outside air) then the rise in temperature would obviously be much lower. If the heat wheel were 100% efficient, then it would take the OA outdoor temperatures in summer and winter from outdoor design conditions to indoor exhaust air conditions. In real life this does not occur. The other factor to consider is the additional fan horsepower consumed by the supply and exhaust air streams going through the heat wheels as OAT approaches exhust air temperature (or economizer bypass temperature). The simple calculation is CFMXSP (through the wheel)/4000. Calculate this for both the supply and exhaust air streams. When is is less economical to run the ERU wheels vs. the benefit ofheat recovery. Just something else to chew on...

 

Absolutely, Rubobornot, please don't let your experience with that one unit sour you on energy recovery. There are a lot of companies in the industry and there is huge variation in the functionality of equipment. "RSI" could be one of two companies and there is a huge difference between them in terms of the construction and quality of the energy recovery wheel.

If the best temp rise across the wheel you ever saw was 10 degrees then that truly is pathetic performance. As CX states above, there are a lot of variables concerning airflow volumes and enthalpy differences in the exhaust (return)air and the outdoor (supply) air that affect performance. Given equal airflows and a reasonably efficient wheel, there would be no reason not to expect 30 degree or even better delta-ts on winter DESIGN days out of a quality energy recovery wheel. On the summer side a good enthalpy wheel can reduce a large portion of the latent load (less delta-t and more delta-h). Most of my equiment has a payback time frame in energy savings of less than 2 years.

 

I am working with some heat recovery units provided by AAON and am verifying operation based on this sequence. (see attached) The installed control system is controlling the operation, but I need to verify per this sequence. I am thinking that this attached sequence is great.

 

Anyone know how long a heat recovery wheel should last? Is there a time when the wheel starts to loose it's efficiency? I have 3 Aaon's with recovery wheels about 7 years old, client was asking how long he should get out of them.