1. Dry-type paint-fume filter:

At a certain shipyard paint shop, the shop dimensions are 27 × 40 × 12 m, with 10 air changes per hour and a ventilation rate of 140,000 m3/h. The shop uses only four paint-fume filters, each with an airflow of 35,000 m3/h. The filters are wall-embedded; a reduced outlet can connect to a fan to reduce duct clogging by paint mist. The filters occupy almost no internal floor area. The side facing the workshop is the intake. Each filter is made of 24 units of 24" × 24" filter elements. The filter media area per unit is 9 m2, filtration velocity is 1.0 m/s, and full-load resistance is < 300 Pa. Filter media use a two-layer combination and purification efficiency is > 90%.

Because dry-type paint-fume filters do not require pumps, nozzles, or water-related structural corrosion concerns, they can be made of ordinary steel and thus have low cost. Dry exhaust purification has lower resistance than wet (600–1000 Pa), requires no pump electricity, and operation costs are much lower.

Using the shipyard example: paint-fume concentration = 300 mg/m3, painting time = 3 h/day, purification efficiency = 90%, filter media capacity = 4.5 kg/m2. Calculate media consumption cost and electricity cost.

a) Replacement cycle

Filter media paint capacity (two layers): M = 9 × 4.5 × 2 = 81 kg/d.

Paint collected by each filter: W = 300 × 10−6 × 35,000 × 3 × 0.9 = 28.35 kg/d.

Media cleaning cycle: T = 3 days.

Assuming media used 20 times, filter media replacement cycle ≈ 2 months.

b) Operating cost

Media replacement cost per filter: C = 18 × 70 / 2 = 630 RMB/month (filter media price = 70 RMB/m2).

Fan power per filter = 5.5 kW. Electricity at 1 RMB/kWh: P = 5.5 × 3 × 30 × 1 = 495 RMB/month.

Total shop cost: (630 + 495) × 4 = 4,500 RMB/month.

2. Water-curtain (wet) paint-fume filter:

For each filter at 35,000 m3/h: pump power N = 4 kW, filter resistance P = 600 Pa, fan power N1 = 11 kW. For 3 h/day painting, electricity per filter: P = (11 + 4) × 3 × 30 × 1 = 1,350 RMB/month.

Assuming wastewater discharged every 3 days, each discharge 10 m3 per filter: total wastewater = 100 m3/month for the equipment. Water at 0.8 RMB/m3 → water cost = 80 RMB/month. Flocculant dosing cost S = 350 RMB/month. Total shop cost: (1,350 + 80 + 350) × 4 = 7,120 RMB/month.

The above costs do not include wastewater treatment operation costs or the cost of collecting dredged paint sludge. Construction of wastewater treatment facilities would require additional capital expense.

Key words

dry-type filter, water-curtain (wet) filter, paint-fume / spray paint mist, filtration efficiency, filter media, capacity, airflow / ventilation rate, pressure drop (resistance), operating cost, wastewater discharge, flocculant,shipyard paint shop

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