Question about economizer control 2

[317069]

A standard 2 ton RTU with air side economizer serves internal zone which needs cooling all year.
During the winter, the unit is on cooling mode, ambient is about 0 F,
will the economizer cycle on and open 100% fresh air in this conditions?.
if yes what about the supply air temperature?

 

[sprinkler1000]

I believe the economizer damper will open a little and mix with return air, otherwise the supply temp will drop below acceptable values.
A 2 ton unit cooling only seems to be for a computer room.
Is that the case or you have the room occupied by people?

 

[317069]

economizer function as I know is to provide free cooling when the outside air temp allows. it will open his damper to 100% and close return damper to 0%, the unit will act like make up air unit, there is no mixing between return and fresh air in this case. this case also can be found with VAV systems where all zones are satisfied and RTU provide only ventilation cfm

 

[Xera]

Depends on how it is programmed. It should mix with the return air to temper it. Otherwise it will cycle on and off very quickly.

As Sprinkler1000 asked, what is the room used for?

Where are you in the world?

 

[MECHJAMIECTA]

As others have asked:

What kind of application is this? Is the space occupied? What are the temperature and humidity requirements of the space this unit is serving?

You make it sound like the normal operation is 100% return air and 0% outside air unless the OA temperature falls to a specified point at which case it is 100% OA and 0% RA all of a sudden. This kind of design does not seem very good for most applications unless it is a pretty primitive system possibly serving a mechanical room or something. Generally, a true "economizer" is inversely varying the amount of OA and RA to meet breathing/fresh air requirements (if occupied) while minimizing energy consumption.

Also, depending on the type of unit with these conditions you probably don't want large quantities of only 0°F air in your equipment. You'd want to find the right ratio of OA to RA to minimize energy usage from cooling, while not requiring substantial preheat (which would defeat the purpose) to prevent freezing equipment or fogging at the mixed air condition.

 

[MechEngNCPE]

If it is programmed incorrectly, then yes. It should modulate the OA and RA dampers to maintain a mixed air setpoint of 55 deg.

 

[cry22]

Rooftop units do not have the capability of using economizer at such low temperature (0F)- Economizer control for RTU's is full exhaust/100% OA, no mixing of RA/OA capability - Not modulation as there are no modulating dampers provided with RTU's
I understand that you are in Ontario, Canada form some of your posts.

Air-side economizer control for 2-ton RTU? Does not make sense, cannot be justified for an RTU this small, even large RTU, full modulation of RA/OA misture is not achievable without VFD's on Power exhaust fans, etc.

A 2-ton RTU is simply not conceived for such sophisticated control, it does not have the capability to do so. Forget it.
You should delete this thread altogether.

 

[317069]

Cry
you touched two points, one of them is the goal of my question.
- the first you said:
"Rooftop units do not have the capability of using economizer at such low temperature (0F)". why?
- the second you said" Economizer control for RTU's is full exhaust/100% OA"
this is my question, if economizer was 100%OA in winter(unit is in cooling mode)what about supply air temperature in this case.
- forget about small units and let us talk about large VAV units which most of the year are on cooling mode even in christmas, if you have 25 ton vav unit and all zones are satisfied then the RTU will provide only the ventilation requirments, in other word the air entering into the unit is only OA with no mixing with RA, what about supply temperature in this case too?
don't worry about 2 tons or 100 tons, it just a number to explain the question.

 

[simsd]

Okay,

So here's the deal.

Generally you are trying to maintain say 55 deg. F (adjustable) in the mixed air chamber (not supply) when you are in the economizer mode. Remember - mixed air - will become the supply air.

So if we look a standard economizer cycle, if the unit is calling for cooling, and the outside air is below 55 deg. F, then rather than using mechanical cooling, the unit will now look at using the outside air, right?

At this point, the unit does not simply throw the outside air dampers wide open to 100%. It looks at the mixed air temperature and compares it to it's setpoint of 55. Now what it will do, is it will start to slowly modulate (assuming you are using a modulating damper actuator) the outside and return air dampers in sequence with each other. So as the outside opens, the return closes.

So if the return is say 80 deg. and the outside air is 55 deg. then yes, it would probably go to 100% open. But, as the outside air starts to decrease, and the mixed air temperature also starts to decrease, the return air will start to open and the outside air damper will start to close in order to maintain this 55 in the mixed air chamber.

Does this make sense to you? It's all about the mixed air setpoint versus actual temperature.

Also, I too am originally from Ontario so there are other factors at play because of the cold conditions, but I am leaving those out of the equation for now.

 

[317069]

Simsd
- Thank you for your help, but is it the standard economizer or you have to order additional special control system with it to perform this procedure?.
I mean do you have to order as follow:
" *** ton RTU with air side economizer" or
" *** ton RTU with air side economizer with *** control"
- also what about this procedure with VAV system as I mentioned before?

 

[cry22]

RTU and VAV is a tough animal.
Typically, you do not have dampers for full modulation with RTU's
I've seen systems with VFD's on Power exhaust fans to get some kind of economizer going, with little success.
A true economizer would need RA fan and air flow measuring at RA/OA, not much space to install air flow measuring devices in an RTU. The problem is that the power exhaust and OA/RA do not track the Total SA to the space. Space pressurization becomes a problem with power exhaust pulling too much air, your system can be in too much OA, and it becomes a heating mode as the RTU mixed air temp falls below fixed SA temp, which defeats the purpose of economizer.

A space pressure sensor can be installed to modulate the power exhaust VFD's. then where do you place the sensor?

In my opinion, Economizer without RA fans should not be considered, yes it can be done with relief fan directly from the space to outside instead of RF at unit.
Having power exhaust at the RTU for economizer is limited, power exhaust is OK for OA temperatures say between 50 and 65, but getting to 40 and below, power exhaust is not the way to go, I'd go with a relief fan connecting to the main RA duct with modulating dampers.

 

[317069]

"In my opinion, Economizer without RA fans should not be considered" what we have to consider in case the RTU does not have return fan?
- Ok let us go with RA in VAV system, still no avsware for the question: if all zones are satisfied, then RTU provide only ventilation requirment, it is 100% OA is this right or wrong?
if it is right what about SA temperature in winter situation with unit in cooling mode?, do we have to provide an extra control devices for this matter?.

- what the difference between exhaust an amount of air directly from space to outside or from return duct or through power exhasut fan, all is about cfm In, and cfm Out.

- the main thread didn't mean to go deep in control, it is all about that everywhere they say econonmizer provide free cooling (100% OA) and they only refer to milde days, but no body refer to winter situation.

 

[cry22]

three one seven zero six nine

in my neck of the wood, rooftop units are also known as "roof shakers". a.k.a as the lowest grade AC system you buck can buy. Let's just say it will cool the space, and that's about it. forget bells and whistles (such as economizers) with RTU's

RTU in deep freeze climate? it simply does not have the capability, It does not have the necessary full damper modulation and controls to achieve what you're trying to do.
without RF, you don't maintain proper space pressurization, your power exhaust suck too much air and you mixed air temp is affected as well. Instead of being in "free cooling", you will be in heating mode.
the difference between power exhaust and relief fan is that the power exhaust is not fully controllable while the relief fan is, it is a separate entity, VFD, space pressure sensor.

Not sure I understood your 2nd sentence.

 

[carlosgw]

This is a good topic.
The question is - Do economizers on smaller RTU's operate so that the temperature is limited (if it is 0 deg and the space is calling for cooling, the supply air would be in the ball park of 55 deg)?
I am talking about standard single zone gas fired 5 ton or less.
I am in a cold climate and would never spec an RTU without an economizer.

 

[317069]

- Cry
I agree with you, but the question is about economizer performance in specific conditions (unit is in cooling mode in the winter), no matter if we have Return fan or not, VAV or CV unit, large or small unit, economizer still an economizer with any kind of unit we use in an air system.
Simsd provide an idea of the control but didn't say if this is a standard situation or we need to order a special control with the unit.
you said unit will switch to heating mode, it is fine that mean unit will cycle quickly and cause damage with the time in addition to problems resulting of allowing air at 0F to flow into the ducts.

-Carlosgw
Could you please advise how the supply air would be in the ball park of 55 deg? if the ambient is 0 F

 

[Drazen]

I believe NCPR gave simple and straightforward answer to straightforward question.

In short, air economizers are not intended to work with 100% outside air, ever, when outside temp is 0F; they work to mix supply air to 55 normally, while priority control takes care that minimum outside air requirement is satisfied.

OA can be open to 100% only when outside temp. is in range 52-60 or so, that depends on fan power and supply duct gains on lower end and on OA enthalpy on higher end (it should be lower than recirculation air enthalpy, than 100% OA can be acceptable even if dry bulb. temp. is somewhat higher for OA than for RECA, logic could be seen in psychro diagram).

 

[msnyder95]

It is no problem making this work, the problem is cost. The amount of controls necessary to make a 2 ton RTU modulate to maintain a fixed supply air temp is literally going to cost as much as the unit itself would.

There are a number of third parties that sell economizers that can be added to a specific manufacturer's RTU. They have the controls to disable the compressor and modulate the dampers.