How does steam in almost vacuum condense at 35 Celsius?

Question

I read about absorption chillers and found out that the water is evaporated/boiled off at the generator.

Water boils over at about 3.7 Celsius in the near-vacuum conditions in the generator. Now the boiling point is also the condensing point! But when this same water condenses at 35 Celsius in the condenser despite the pressure being the same near-vacuum (or is it?), it sends my brain to physics hell.

The only explanation my pea-brain can think of is that the steam itself creates pressure that sort of increases... I hate to say it... its own boiling (thus condensing) point. And so now it condenses at 35 rather than 3 Celsius. Does this even make sense to you?

Answer 1

Here is a quick walk-through of a typical ammonia-water chiller you may find useful.

1. An aqueous ammonia concentrated solution is evaporated using a heat source to generate a nearly dehydrated ammonia vapor. The vapor is at a decently high pressure.

2. The dehydrated ammonia vapor continues to the condenser where it is cooled and condensed. The pressure is slightly lowered.

3. The condensate then passes through an expansion valve lowering the temp and pressure of the fluid.

4. The ammonia liquid now enters the evaporator where it takes on heat from the substance being cooled (where the refrigeration takes place). After taking on this heat the ammonia liquid flashes and exits the evaporator as a vapor.

5. After exiting the evaporator as a vapor ammonia is pulled into the liquid phase by absorption. In the absorber unit an ammonia water mist is usually introduced. The ammonia vapor has a high affinity to this solution. The concentrated ammonia liquid is collected usually at the bottom of the unit and pumped to the generator. Where step 1 begins.

You will notice unlike the traditional refrigeration cycle we are using a pump to take the fluid from low to high pressure. There isn’t a compressor.

Now to answer your specific concern about how water is boiling and condensing at different temperatures:

• In the absorption chiller, the condenser is at a relatively high
  pressure. This is because its feed is coming from the generator whose purpose is to heat up the aqueous ammonia solution to achieve
  separation from the two compounds. The effluent hot dehydrated
  ammonia's saturated pressure will therefore be relatively high.

• In the evaporator section of the chiller (downstream and where we are chilling the external fluid) the entering ammonia liquid concentrate is at a lower temp (Te) and pressure. Why? Because it was just
  throttled through an expansion valve. Hence the saturated vapor temp of evap unit is lower than that of condenser unit.

To Answer your specific concern about the applicability of ammonia water chilling to LiBr.

• How does ammonia-water chiller apply to a different technology like water-LiBr chiller? Simply put the coolant role played by ammonia in the example is played by water in the LiBr chiller. The absorption role played by water in the example above is played by LiBr.

In the absorption chilling cycle pressures and temperatures will vary greatly depending on what unit you are talking about, but there isn’t any whacky thermo going on here. You described the generator being coldest and at low pressure in the problem statement, usually the generator is at the highest pressure and the hottest. I don’t know your set-up but you may have confused the generator with the evaporator unit!

Answer 2

The whole point of an absorption chiller is that it doesn't use condensation: it uses absorption.

The steam doesn't 'condense'. It is 'absorbed'.

It is, as it happens, absorbed into a liquid phase, but that liquid is not water, and the steam doesn't become liquid by condensation: it becomes liquid by absorption.

Although the words 'condenser' and 'condensation' are often used by well meaning people when describing an absorption chiller that works between gas and liquid phases, that can be confusing and misleading.