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How HVAC Economizer Works ? Explanation about Air-Side and Water-Side Economizers Works ?


What is HVAC Economizer?

HVAC economizer is the mechanical equivalent of opening a window which is used to save the energy savings in your building. For most facility managers and building owners, saving energy is a necessity and upgrading your HVAC system is a great way to see a significant change in overall energy usage.Let talk about the benefits of an HVAC economizer, as well as how it works.


For an office building, an HVAC system typically draws 85% of the air from the conditioned space and 15% from outdoors all the time and mixes them, to dilute indoor odors and contaminants. When it's about 69 degrees or cooler outside and there's a cooling load in the building, dampers modulate in the economizer to increase the outdoor air to 100% to take advantage of that cooler air to help cool the building, and the compressors will run as needed to maintain temperature in the space, but less than they would without using that outside air.

All that air positively pressurizes the building, so smaller units are equipped with barometric relief, larger units equipped with power exhaust, to exhaust building air at the same rate as it's being brought in. The advantage of that, apart from "free" cooling, is the building gets flushed out with 100% outside air, always a good thing.


Air-Side Economizers :-
The air-side economizer on air-handling units was introduced in the previous chapters. It is the mixing arrangement that allows up to 100% outside air to be drawn in and relieved in order to take
advantage of cool outside air, providing “free cooling.” Nothing is free! The air-side economizer

equipment costs extra to purchase, there are more components to maintain, and, depending on the
climate, the hours when the economizer is actually saving cooling energy may be very limited. In
climates that are warm and humid, the number of hours when the outside air has a lower enthalpy
than the return air enthalpy may be very few. Thus, Standard 90.1 does not require air-side
economizers in most of Florida.


Advantages of the air-side economizer:-
1. A low air pressure drop.
2. Substantial mechanical-cooling energy savings.
3. Reduced water usage in cooling tower systems.


Disadvantages of the air-side economizer :-
1. Extra capital cost for the 100% intake and relief air equipment, which includes a return fan
on larger systems.
2. A higher ongoing electrical operating expense.
3. A potential requirement for additional humidification during winter operation.


Water-Side Economizers :-
The water-side economizer consists of a water-cooled coil, located in the air stream just before the mechanical-cooling coil. The coil can be supplied with water directly from the cooling tower or via a
plate heat exchanger. If the water is supplied directly from the tower, the water treatment and cleaning process must be of a high standard, to ensure that the valves and coil do not log up with
dirt.If a heat exchanger is used, there is the additional cost of the exchanger, and the heat transfer will be less efficient, since there has to be a temperature rise across the exchanger for it to work.

An example for packaged units is shown in Figure below . The three-port valve determines how much of the tower water flows through the economizer coil, and the two-port valve determines how much water bypasses the condenser to avoid the condenser being over cooled. The “head pressure” is the pressure in the refrigeration condenser.





Advantages of water-side economizers :-
1. Water-side economizers reduce compressor energy requirements by precooling the air.
2. Unlike air-side economizers, which need full sized intake and relief ducts for 100% outside air
entry or for
3. 100% exhaust, water-side economizers simply require space for two pipes.
4. Unlike the air-side economizer, the water-side economizer does not lower the humidity in
winter.
5. Saving on possible humidification costs.

Disadvantages of a water-side economizer :-
1. Higher resistance to airflow, therefore higher fan energy costs.
2. Increased tower operation with consequent reduction in life.
3. Increased water and chemicals cost.



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