The condenser is a coil that rejects to the outside everything the cycle moved out of the cold space plus the work or heat that drove it, turning hot vapour back into liquid. It needs either free airflow around the tubes or a bath of cooling water, and the colder that sink the less work the whole machine needs.
Part of Refrigeration
Step two of the cycle: giving the hot vapour’s heat to air or water until it becomes liquid, and how to size, place and keep a condenser working.
The Condenser Must Reject More Heat Than The Cold Box Gave Up
The condenser has to carry away the heat absorbed in the evaporator and the work added by the compressor, as The Refrigeration Cycle shows. A condenser sized only for the box’s heat load will run too hot. Because the vapour must condense at a temperature above the air or water that cools it, a hot sink forces a higher pressure and a larger lift, as the arithmetic in Compression shows.
The carrier gives two arrangements. Air cooled: a coil of tubes in open air where air can flow around it freely, simple, no water, but only as cool as the air. Water cooled: the coil immersed in a tank or in a flow of cooling water, more effective, because water takes heat from a surface far better than still air does, but it needs a supply of cool water and the tank itself warms up unless it is refreshed.
What You Need
- Tube. The carrier’s absorption example uses 5 to 10 m of steel tube for the coil, and says more surface gives better cooling. For ammonia it must be steel; copper is attacked (Ammonia).
- Airflow or water. A shaded spot with natural draft, a fan if power is available, or a tank with a way to keep the water cool.
- Slope. Arrange the coil so condensed liquid drains away downhill and does not pool in the tubes.
Method: Placing And Plumbing A Condenser
Step 1: Put it where the coolest air or water is. Shade, the windward side, and clear of the machine’s own exhaust heat. Never in a closed cupboard, and never right behind the cold box that is itself warm.
Step 2: Feed the hot vapour in at the top and take liquid out at the bottom. Gravity helps condensed liquid leave. When it is right, the top of the coil is hot, the bottom is warm, and the liquid line is only just warm.
Step 3: Keep it clean. Dust, scale and algae insulate the tubes. A condenser that has to work harder to reject the same heat pushes the high-side pressure up.
Step 4: Watch for signs of going wrong. A very hot liquid line means condensation is incomplete; a rising pressure with a warm coil means the sink is too hot or the coil too small or dirty.
Worked Example: How Much Cooling Water A Bath Needs
Assume the condenser must reject 105 W (100 W taken from the box plus an assumed 5 W of work) and the water may warm by 5 K before being replaced.
- Heat per second: 105 W = 105 J/s.
- Water heating: the corpus gives 4,186 J/kg·K for water.
- Flow needed: 105 ÷ (4,186 × 5) = 0.0050 kg/s.
- Per hour: 0.0050 × 3,600 = 18 kg, which is about 18 litres.
A small bath therefore needs a steady exchange of water, or enough volume that its own warming stays small over a day. Over 24 hours the same load puts 105 × 86,400 = 9.07 MJ into the water, which would heat a 200 litre tank by 9.07 × 10⁶ ÷ (200 × 4,186) = about 10.8 K. The tank would end the day well above its starting temperature, and a hotter bath means a higher condensing pressure. The load figures are assumptions for the arithmetic, not a design.
Common Mistakes
- Sizing for the box’s heat only. The compressor’s work also has to leave.
- A condenser in a sealed box. It recirculates its own hot air.
- Letting water warm up in a static bath. The cooling sink must be refreshed or large.
- Air in the high side. Air does not condense, so it stays in the condenser and raises its pressure above what the refrigerant alone would reach. How to spot and remove it is a job for someone who knows the particular system.
Safety
Condensers run hot and at the highest pressure in the circuit
The upper coil can burn skin. It is also the part of the circuit most likely to burst if a blocked line or a dead cooling supply lets the pressure climb, which is why the relief device must be on the high side and must not be isolable. See Pressure Vessels. Do not pour cold water on a hot condenser in an attempt to rescue a failing machine: thermal shock cracks cast fittings.
What’s Next
- Learn next: Expansion — the restriction that follows the condenser
- Combine with: Heat Exchanger Design — the corpus’s area arithmetic for coils