Economizer changeover: should you decide on dry-bulb or on enthalpy?

Situation

Say an office floor is served by one air handling unit (AHU) that normally mixes 20 % outdoor air (the ventilation minimum) with 80 % return air ahead of the cooling coil. In spring and autumn the outdoor air turns mild, and it is tempting to bring in more of it so the chiller runs less. That is an economizer. The question is when to open and close the dampers. The simplest rule is to open when the outdoor dry-bulb is below the return air, but humid air carries much more heat at the same dry-bulb, and the coil must dehumidify it as well. Below we calculate dry-bulb and enthalpy decisions and coil loads side by side for four outdoor conditions. The conditions are illustrative; a real decision uses the building's design conditions and the climate-zone criteria that apply to it.

Given conditions

Reading it on the chart

1. Plot the return point RA (26 °C, 50 %) and the coil leaving target SA (14 °C, 90 %).
2. Plot each outdoor point OA. On the line joining RA and OA, the mixed point M lies at the outdoor-air fraction measured from RA toward OA. Enthalpy and humidity ratio mix as mass-weighted averages.
3. The coil cools from M (or OA) toward SA; if the humidity ratio is already below SA's it is held while dry-bulb drops. Coil load is airflow times the enthalpy difference between entering and leaving air.
4. The enthalpy test asks whether OA enthalpy is below RA's; the dry-bulb test asks whether OA dry-bulb is below RA's. The days where the two disagree are the point of this example.

Step-by-step calculation

1) Supply and return air. Supply at 14 °C / 90 % has humidity ratio W_SA = 8.96 g/kg, enthalpy 36.72 kJ/kg and specific volume 0.8252 m³/kg. Converting 10,000 CMH to dry air mass flow gives ṁ = 10,000 / 3600 / 0.8252 = 3.3663 kg/s. Return air at 26 °C / 50 % has W = 10.50 g/kg and enthalpy 52.91 kJ/kg.

2) Outdoor states and decisions. Humidity ratio, enthalpy and the three decisions (on means increasing outdoor air is allowed):

3) Coil load. Once the entering state is known, the leaving air is 14 °C with the smaller of the entering humidity ratio and W_SA, and Q = ṁ(h_in − h_out). For the rainy day (24 °C / 90 %), the minimum-outdoor-air mix M is 25.60 °C with enthalpy 55.78 kJ/kg and the leaving enthalpy is 36.72, so Q = 3.3663 × (55.78 − 36.72) = 64.17 kW. Opening to 100 % outdoor air puts the outdoor state (enthalpy 67.27) straight on the coil: Q = 3.3663 × (67.27 − 36.72) = 102.82 kW, which is 38.65 kW more.

4) Modulating the ratio. When the outdoor air is colder than the supply target, like the cool day (10 °C), 100 % outdoor air would enter the coil at 10 °C and need heating up to 14 °C (13.66 kW of heat). Instead, mixing with return air until the mixed dry-bulb equals 14 °C needs neither cooling nor heating. That outdoor-air fraction is 75.2 %.

Interpreting the result

Rainy season 24 °C / 90 %. The dry-bulb 24 °C is below the return air's 26 °C and under the 24 °C limit, so both dry-bulb methods say on. But the outdoor enthalpy is 67.27 kJ/kg, 14.35 kJ/kg above return air (52.91). Its humidity ratio of 16.94 g/kg is nearly twice the supply target (8.96), so the coil must remove a lot of moisture on top of cooling. Opening to 100 % on a dry-bulb decision raises coil load from 64.2 kW to 102.8 kW, about 60 % more. On this day only the minimum outdoor air should be kept.

Dry spring day 18 °C / 40 %. All three decisions are on, and 100 % outdoor air really is cheapest (13.7 kW against 39.8 kW at minimum outdoor air, a saving of 26.1 kW). The outdoor humidity ratio of 5.11 g/kg is below the supply target, so there is no latent load and the coil does sensible cooling only. The leaving air is then 14 °C at 5.11 g/kg (about 52 % RH), much drier than the 90 % RH target. If space humidity tends to fall, check whether humidification is needed as well.

Cool day 10 °C / 60 %. 100 % outdoor air needs no coil but falls short of the supply temperature and needs 13.7 kW of heating. Mixing in 75.2 % outdoor air brings the mixed dry-bulb to 14 °C, so coil and heating are both zero. So economizing is not just on or off; it includes modulating the outdoor-air ratio. The saving over minimum outdoor air is 33.4 kW.

Hot dry day 28 °C / 30 %. The dry-bulb 28 °C is above return air's 26 °C, so both dry-bulb methods say off. Enthalpy is 46.15, 6.76 kJ/kg below return air, so only the enthalpy test says on. There is a gain, but 100 % outdoor air takes 48.0 kW against 49.9 kW at minimum outdoor air, a difference of just 1.92 kW. Enthalpy control identifies the gain more correctly, but where the gain is small, opening and closing the damper often buys almost nothing, so control logic usually has a deadband.

Which criterion to use

Dry-bulb criteria (a fixed limit, or comparison with return air) need one sensor and are cheap, and are usually enough where humidity is low. On humid days like the rainy-season case, though, there are days with a low dry-bulb and a high enthalpy. Enthalpy control needs both temperature and humidity sensors, and if the humidity sensor drifts the decision flips. Relative humidity sensors have error too, and at 24 °C a 5 percentage-point change in RH is about 2.4 kJ/kg of enthalpy, so when the decision is close, check the sensor first.

Several things are left out. Extra outdoor air must leave the building through relief dampers or a relief fan, and more outdoor air changes filter load and fan power. This calculation assumes the coil cannot humidify and holds the supply state fixed, while in a real building the supply state follows the space load.

Common mistakes

• Opening to 100 % on dry-bulb alone. The rainy 24 °C / 90 % day passes the dry-bulb test but adds 38.7 kW to the coil load.
• Closing the minimum outdoor air to economize. The outdoor-air fraction never goes below the ventilation requirement. Column A in the table keeps that minimum.
• Assuming 100 % is best whenever it is cool. At 10 °C heating becomes necessary; reduce the ratio so the mixed dry-bulb meets the supply target.
• Using a fixed dry-bulb limit regardless of climate. Limits differ by climate zone. The 24 °C here is illustrative.
• Not checking the humidity sensor. Enthalpy control follows a wrong humidity reading to a wrong decision.

Try this calculation on the site

Open the rainy-season outdoor airOpen the cool dayOpen the rainy day on the chart

The economizer tool links fill in return air 26 °C / 50 %, supply 14 °C / 90 %, 10,000 CMH, 20 % minimum outdoor air and a 24 °C limit, and show the three decisions and the coil load by outdoor-air ratio. Change the outdoor temperature and humidity to find where the decisions part. The chart link plots the rainy-day outdoor, return, mixed and supply points with the coil process. For the coil leaving state and reheat see Summer cooling coil selection and reheat, for plotting see How to read a psychrometric chart, and the cooling coil calculator has the calculator.

References

• ASHRAE Handbook Fundamentals 2017 (SI), chapter 1 Psychrometrics: humidity ratio, enthalpy, specific volume and saturation vapour pressure (Hyland-Wexler). The psychrometric numbers here were computed with this site's engine, which implements those equations at 101.325 kPa.
• ANSI/ASHRAE/IES Standard 90.1, section 6.5.1 Economizers: high-limit shutoff controls include fixed dry-bulb, differential dry-bulb, fixed enthalpy and differential enthalpy; which are allowed, and the limit values, depend on the climate zone and the edition. The fixed dry-bulb 24 °C here is the site tool's default, not a value from the standard's table. For design, check the edition your building follows.
• Coil load is the usual entering-minus-leaving enthalpy method. The coil is assumed unable to humidify, and the return state, supply state and airflow are example values.