Tumble Dryer Evaporation Load and Makeup Air

A tumble dryer is an evaporator and its load is the pounds of water it has to boil off, so the number that sets the gas bill is not the weight of linen but the retained moisture after extraction. Evaporating water is expensive and mechanical extraction is cheap, and the ratio between them is roughly ten to one -- which makes the washer's extract speed the single most consequential number in a laundry's energy bill. Ten points of retained moisture on four thousand pounds of linen is four hundred pounds of water a day that either leaves in the extractor for pennies or leaves in the dryer for dollars: 45% down to 35% saves 685,714 BTU a day, $1,851 a year at $9 per MMBTU over 300 days, and the dryer does nothing differently. The airflow consequence is the one that causes building problems. Heat divided by 1.08 times the temperature rise gives the exhaust, and 4,000 lb a day at 45% over an eight hour shift is about 3,571 cfm continuous -- which is also 3,571 cfm of makeup air the room has to admit. At a 500 fpm louver face velocity that is 7.1 sq ft of free area, and after a 50% free-area fraction about 14 sq ft of gross louver. A laundry room with a 4 sq ft transfer grille is going to run negative, and it will pull the difference through the building: down water heater flues, under doors, past every combustion appliance in the mechanical room. That is a combustion safety problem as well as a performance one, because a dryer that cannot get air dries slowly and runs long, which costs more gas to remove the same water. The third consequence is lint, and it is a fire problem rather than an arithmetic one: exhaust ducting sized and routed for the calculated airflow still fails if it is not cleanable. The 1,200 BTU per pound figure covers latent heat plus the sensible heat of the water and typical exhaust losses and is a working approximation; a specific dryer's fuel consumption per pound of water from its manufacturer is better. This does not size the dryer, select the exhaust duct or evaluate its static pressure, or address lint accumulation and the cleaning access that fire safety requires -- dryer exhaust fires are a recognized hazard and duct design is governed by the mechanical code and NFPA rather than by an airflow number. It does not evaluate makeup air tempering, which in a cold climate is a substantial heating load of its own, or the combustion safety consequences of running the room negative, which must be checked separately. The dryer manufacturer's data, the adopted mechanical code, and NFPA govern.

water to remove = dry weight x retained moisture fraction; heat = water x about 1,200 BTU/lb / dryer efficiency; exhaust cfm = BTU/hr / (1.08 x temperature rise), where 1.08 = 60 min/hr x 0.075 lb/cu ft x 0.24 BTU/lb-F; makeup air = exhaust; louver free area = cfm / face velocity.

The evaporation load and airflow relations by name, with the adopted mechanical code and NFPA named for dryer exhaust duct sizing, routing, and lint cleaning access -- cited, not mirrored. The dryer manufacturer's fuel consumption per pound of water governs the energy figure.

Sensible and latent heat arithmetic; no code table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: dry_weight_lb_per_day, retained_moisture_fraction, improved_retained_moisture_fraction, dryer_efficiency, temp_rise_f, operating_hours_per_day, fuel_cost_per_mmbtu, days_per_year, louver_face_velocity_fpm, louver_free_area_fraction, water_lb_per_day, heat_btu_per_day, exhaust_cfm, louver_free_area_ft2, louver_gross_ft2, extraction_saving_annual

• 1,200 BTU per pound of water a working approximation; the dryer's own fuel per pound of water is betterstandard laundry practice
• Makeup air equals exhaust the room admits it deliberately or through the buildingmechanical code principle
• Lint is a fire problem, not an airflow one duct sizing does not make a duct cleanablethe adopted mechanical code and NFPA