Refrigerated Dryer Sizing and Common Problems

Summer dew point won't come down. Nine times out of ten, it's a sizing problem.

The processing capacity on the nameplate is rated at standard conditions. Common standard conditions: inlet pressure 7 bar, inlet temperature 38°C, ambient temperature 38°C. On site, when do you ever get conditions that ideal? Compressor outlet temperature 45°C in summer is normal. Factory floor above 40°C is common. 6 bar systems are everywhere. Multiply the three correction factors together, processing capacity might only be 60% of rated.

Run an example. Compressor discharge 15 m³/min. Summer discharge temperature 45°C. Working pressure 6 bar. Machine room temperature 40°C. K1 is 0.82, K2 is 0.85, K3 is 0.92. Multiply them out, 0.641. 15 ÷ 0.641 = 23.4 m³/min. Need to pick a model rated 25 m³/min or above.

Diagnostics

Refrigeration System Pressures

If sizing is fine, check the refrigeration system pressures. High side normal range generally 1.2 to 1.8 MPa. Low side 0.3 to 0.5 MPa. Check the equipment manual for specifics. Low side too low, refrigerant leaked. Check weld joints, valve packing, compressor shaft seal. High side too high, heat dissipation problem. Time to clean the fins.

Both undersizing and refrigerant leak cause high dew point. How to tell: look at the low side. Undersized, low side is normal or high. Refrigerant leak, low side is low.

High inlet temperature and poor heat dissipation both cause high dew point too. Similar way to tell: high inlet temperature, low side runs high. Poor heat dissipation, high side runs high.

Evaporator temperature is normally 2°C to 10°C. The pressure dew point of the outlet air depends on this temperature.

Why not go lower? Below 0°C the condensate freezes. Ice layer builds up thicker and thicker. Heat exchange efficiency gets worse and worse. Eventually blocks the evaporator solid. Inside the dryer there's a hot gas bypass valve. When load is low, it routes some hot refrigerant to the evaporator inlet, keeping evaporator temperature above 0°C. This valve gets stuck or isn't adjusted right, evaporator ices up. Shows up as low side running low too.

Normal operation, the refrigeration compressor runs continuously. Cooling capacity is regulated by the hot gas bypass valve. Frequent start-stop means something's wrong.

Equipment oversized, this happens. Light load. Evaporator temperature quickly drops below the protection threshold. Shuts off. Temperature rises. Starts again. Winter when ambient is low also triggers this. Condenser dissipates heat too well. Can turn down the fan speed or block off part of the condenser.

Hot gas bypass valve fault also causes short cycling. Valve can't regulate properly, cooling capacity can't match the load. Evaporator temperature swings big.

Drainage

Condensate Management

Valves

Drain Systems

Drain pipe runs downhill. Can't have upward bends. Can't have traps. Can't be too long or too thin. Simple logic. Condensate flows down by gravity. Any high point in the middle and it won't get past.

Look once a day to see if the drain valve is working. You can hear the air-water mix being discharged. Nothing all day, either the valve is blocked, or it's winter with low load and there's just not much condensate.

No drainage, several possibilities: inlet screen clogged, float stuck, electronic valve coil burned out. Continuous air leak, several possibilities: gasket worn, float leaking so it can't rise, electronic valve spool stuck or spring broken. Continuous leak doesn't just waste compressed air. The airflow also re-entrains water from the separator.

Dryer goes after the compressor and receiver tank. Air coming out of the compressor is hot and pulsating. Let it settle in the receiver tank first, then process it.

Inlet temperature limit 45°C. Some high-temp models 50°C. Exceeds that, add an aftercooler in front.

Air-cooled condenser needs 0.5 meters of clear space in front. Three sides against walls in a corner, guaranteed high pressure alarm in summer.

Maintenance

Condenser Care

Inspection

Pressure Gauges

Annual

Professional Check

Clean condenser fins weekly. Compressed air or soft brush. Don't scrape with hard objects. Fins are thin, easy to bend over. If there are pressure gauges, glance at the high and low side while you're at it.

High side running high long-term is bad for the compressor. Heavy load, high temperature rise, shortened life. Lots of sites wait until the high pressure alarm goes off before thinking about cleaning the fins. By then the compressor has been running at high pressure for a long time already. Too much refrigerant charge also makes the high side run high. Have to bleed some off.

Measure outlet dew point once a month and write it down. Long-term tracking shows equipment condition changes. Check electrical connections too. Compressor and fan are both vibrating parts. Run long enough and terminals loosen. Loose terminals heat up.

Annually, have a professional check refrigerant charge and fan motor bearings. Worn bearings get noisy and run hot.