E1 · HVAC Equipment and Components

Foundation lesson · 18 minute study session

Separate tower range from approach.

Use the water temperatures to find duty and the wet-bulb temperature to judge the cooling boundary.

By the end of this lesson

Calculate range and approach using their correct temperature pairs.
Estimate tower heat rejection from water flow and temperature change.
Distinguish evaporation from total makeup water.

The tower rejects the condenser load.

In a conventional refrigeration system, the condenser rejects evaporator heat plus compressor energy entering that cycle boundary. Tower duty therefore generally exceeds the refrigeration load. A tower rating ton and a refrigeration ton can use different conventions; calculate in explicit heat-rate units first.

Two temperature differences answer two questions.

Range is the water cooling achieved through the tower. Approach is how close cold leaving water comes to entering-air wet-bulb temperature. A small approach can require more tower capability. Do not substitute dry-bulb temperature into the evaporative approach definition.

Water balance adds another boundary.

Evaporation transfers energy and concentrates dissolved solids. Makeup also replaces drift and blowdown. Estimating evaporated mass as duty divided by latent heat assumes all stated heat rejection is latent; an actual tower can also transfer sensible heat to air. A closed-circuit fluid cooler keeps the process fluid separate from the spray water.

A decision map for this lesson

Range and approach use different temperature pairs

The diagram uses the lesson’s water and entering wet-bulb temperatures. Approach is not the water temperature drop.

A realistic cooling-tower cutaway shows the top fan, water distribution, fill, side air inlets and bottom collection basin. — **Compare the correct temperature pairs**Connect the equipment to the calculation

Hot tower water at 100 degrees Fahrenheit and cold water at 88 define a 12-degree range. Cold water and entering wet bulb at 80 define an 8-degree approach. — **Compare the correct temperature pairs**Connect the equipment to the calculation

Choose the governing relationship and catch the wrong route
Condition or question	Governing decision	Error to avoid
Tower range	Use $R=T_{hot}-T_{cold}$ ; this is the water-side temperature change.	Subtracting entering wet bulb from hot-water temperature and calling it range.
Tower approach	Use $A=T_{cold}-T_{wb,in}$ .	Substituting entering dry bulb for the evaporative wet-bulb boundary.
Tower heat rejection	Use water flow and range on the stated thermal-factor basis.	Using approach as the water-side temperature change.
Makeup water	Balance evaporation, blowdown, drift and any other stated water transfers.	Treating an all-latent evaporation estimate as a complete tower performance rating.

On a narrow screen, scroll across the table to compare the decision and the error.

Relationships and their limits

Range and approach

R=T_{hot}-T_{cold};\qquad A=T_{cold}-T_{wb}

Temperatures refer to the stated tower inlet/outlet water and entering-air wet-bulb conditions.

Water-side heat rejection

\dot Q\approx500\dot V_{gpm}(T_{hot}-T_{cold})

Ordinary-water approximation, Btu/h, gpm and °F. Use supplied density and heat capacity when different.

Simplified evaporation

\dot m_{evap}\approx\dot Q/h_{fg}

Only with the stated all-latent assumption. It excludes drift, blowdown and sensible air heating.

Symbols and unit basis
Symbol	Meaning	Units or basis
$R$	Tower range	°F difference
$A$	Tower approach	°F difference
$T_{wb}$	Entering-air wet-bulb temperature	°F
$\dot V_{gpm}$	Water flow	gpm
$h_{fg}$	Supplied latent heat	Btu/lb
$T_{hot},\ T_{cold}$	Inlet and outlet tower-water temperatures	°F
$\dot Q$	Water-side heat rejection	Btu/h
$\dot m_{evap}$	Evaporation mass rate under supplied model	lb/h

One decision, fully worked

Apply the method.

A tower cools 300 gpm of water from 100°F to 88°F. Entering-air wet-bulb temperature is 80°F. Use the water factor 500 Btu/(h·gpm·°F). For an illustrative evaporation estimate only, assume all rejection is latent with $h_{fg}$ = 1,050 Btu/lb.

Use water inlet minus water outlet for range.
Use leaving-water minus wet-bulb for approach.
Calculate water duty; apply the explicitly simplified evaporation model separately.

R=100-88=12^{\circ}\mathrm F;\qquad A=88-80=8^{\circ}\mathrm F

\dot Q=500(300)(12)=1{,}800{,}000\ \mathrm{Btu/h}

\dot m_{evap}\approx1{,}800{,}000/1050=1714\ \mathrm{lb/h}

Decision

The tower has a 12°F range, an 8°F approach and 1.80 million Btu/h water-side duty. The simplified evaporation result is not a total makeup-water requirement.

Independent check

Cold water remains above entering wet-bulb in the stated operating condition, and reducing the water temperature by 12°F gives a positive heat-rejection rate.

Specialist extension

Separate evaporation from drift and blowdown

A tower range is hot-water minus cold-water temperature; approach is cold-water minus entering wet bulb. Neither is the evaporation rate. Under a supplied simple dissolved-solids balance with negligible drift and evaporation carrying no solids, cycles of concentration link evaporation and blowdown. Include drift or other losses when the problem states them. Tower heat duty includes the condenser rejection boundary, which differs from evaporator refrigeration capacity.

A new applied case

A tower evaporates 12 gpm. Drift is negligible and supplied cycles of concentration are 4. Use the stated balance B = E/(C − 1), with makeup M = E + B.

B=12/(4-1)=4\ \mathrm{gpm}

M=12+4=16\ \mathrm{gpm}

Decision. Blowdown is 4 gpm and makeup is 16 gpm under this model.

Cross-check. A makeup rate of only 12 gpm would not replace the stated blowdown.

Unit basis. E, B and M are gpm; concentration cycles C are dimensionless; the simplified balance is explicitly supplied.

NCEES PE Mechanical Reference Handbook v2.1; see the module reference path.

Open targeted practice in the study studio →

Transfer the method

HVAC Equipment and Components · HV-LC-E1

Try a new decision.

Select one

A cooling tower receives 200 gpm of water at 96°F and discharges it at 84°F. Entering-air wet-bulb temperature is 76°F. Use the water factor 500 Btu/(h·gpm·°F). The tower range, approach and water-side duty, respectively are most nearly:

A20°F; 8°F; 2.0 million Btu/hB8°F; 12°F; 0.8 million Btu/hC12°F; 20°F; 1.2 million Btu/hD12°F; 8°F; 1.2 million Btu/h

A lesson check supports practice, not a readiness score. Your response stays in this page session and clears on reload.

Catch the common errors

Using entering dry-bulb temperature for tower approach.
Equating condenser rejection with evaporator load while omitting compressor input.
Reporting evaporative loss as total makeup while ignoring drift and blowdown.

Handbook navigation drill

Find it, then explain it.

Search: Cooling Towers

Locate the tower range and approach definitions and compare the temperature pairs. Note the tower rating convention.

Check your navigation

Section 9.3.9 distinguishes range from approach. Keep any rating-ton convention separate from the 12,000 Btu/h refrigeration ton.

Reference path: §9.3.9, p. 524. These are printed handbook page numbers; section names and search terms remain the primary navigation method. See technical review status.

Theory sources

NCEES PE Mechanical Reference Handbook v2.1: §9.3.9, p. 524
Mechanical and Electrical Equipment for Buildings, 12th ed.: mechanical systems and energy principles. Supporting theory; not an examination reference.

Worked examples and lesson prose are original ReasonSpan^™ material. Obtain authorized publications separately; this preview does not reproduce the supplied books or handbook.