Condensation and heat recovery in a refrigeration system
The liquefaction of refrigerant
When superheated steam is cooled in a heat exchanger, the following steps occur (Figure 01):
◆ steam deheater . This is normal gas cooling.
◆ Condensation on the wall. A certain subcooling of the steam is necessary. Liquefaction can only begin if suitable condensation nuclei are present. This can be a heat exchanger surface, a dust particle, etc. Note the similarity to boiling.
◆ Subcooling of the condensate.
Figure 01: Condensation temperature profile
A) A "normal" temperature profile. The temperature pinch should be no closer than a few degrees.
B) If the water flow rate decreases, the outlet temperature increases. But long before it reaches the vapor temperature, there is a temperature pinch at the point where the vapor begins to condense.
C) It is somewhat ironic that the lower the water inlet temperature, the higher the outlet temperature can be.
Liquefaction can be divided into the following types:
◆ Droplet condensation occurs when the liquid does not wet the surface. The condensate forms liquid droplets on the heat exchanger surface. When large enough, they coalesce into larger clusters that flow off the surface.
◆ Film condensation. When the liquid wets the surface, a continuously sinking liquid film is formed. Film flow can be either laminar or turbulent. The heat transfer coefficient for laminar film condensation decreases with increasing flow velocity. When turbulent flow is achieved, the heat transfer coefficient increases again.
Droplet condensation has much higher heat transfer coefficients than film condensation. Unfortunately, it is difficult to maintain stable droplet condensation, so it is difficult to design a liquefier using this liquefaction method. In certain cases - e.g. B. Liquefaction on Teflon - is it possible to achieve stable droplet condensation. It is conceivable that liquefaction of vapor on an oily surface not wetted by condensate could cause droplet condensation. An example of this could be the ammonia condenser of a refrigeration plant, where the ammonia delivers oil in the form of droplets.
The selection of the condenser type, the cooling medium, the temperature level, the control system, etc. depends very much on the purpose of the plant. Just a few examples are given below:
A) If the condenser is installed in an air conditioning or refrigeration system without heat recovery, the sole purpose of this condenser is to deposit the heat in a suitable sink. The condenser type then depends on this sink and must be as small as possible for the given data.
B) The correct condensing temperature, or better the pressure in an installation as in A), depends on the type of evaporator:
A direct expansion system requires a certain minimum pressure in order to provide the TEV with sufficient motive power.
The driving force in a flooded system is less critical and the condensing pressure could be adjusted to vary with the temperature of the cooling medium, also saving driving energy for the compressor.
C) Air conditioning can be used to recover heat from the condenser, usually the higher value heat from the superheated vapor. A special desuperheater - a water-cooled heat exchanger - is used for this. The control system could be set to maintain a minimum refrigerant temperature to ensure the required water temperature.
D) Condensation is the primary task in a heat pump. Condensation could be split into a desuperheater for heating tap water and a condenser for space heating. The control system would have to be adjusted to maintain a predetermined minimum temperature.
In a condenser, the refrigerant enters superheated, with temperatures ranging between approximately 60°C and 120°C. Temperatures higher than 120°C, although possible, will rarely occur as the oil in the compressor will then begin to degrade. The system is therefore designed in such a way that the discharge gas temperature remains below this value.
As in all heat exchangers, the temperature of the cold side can only approach the temperature of the warm side, but never quite reaches it. When the leaving water temperature increases due to a reduced water flow rate, the water temperature curve approaches the steam temperature curve. But not at the final temperature, but at the point where the vapor begins to condense.
The water outlet temperature is still far from the steam inlet temperature. Figure 01 shows this using the example of the desuperheating and liquefaction of 1 kg/s R22 vapor, which enters at 87 °C, condenses at 45 and exits supercooled to 40. 3.28 kg/s of water are heated to 25 - 40 °C.
The total power in Figure 01 is around 206 kW. It is tempting to feel this warmth e.g. B. for heating tap water. Unfortunately, only part of the total condenser capacity is available for heating water to temperatures close to the vapor inlet temperature. In order to make hot water suitable for use as tap water, it must be heated to around 80°C.
In the example above, the water is heated to 40 °C. What happens if we try to decrease the water flow to increase the outlet temperature? Figure 01 clearly shows that the outlet temperature is limited by the temperature pinch point. What is the highest outlet temperature then? The following relationship applies:
Δt cond / Δt total = Q cond / Q total
Δt cond = 42 - 25 = 20K (temperature increase in the condensation section)
q cond = 167KW (thermodynamic design)
Q total = 206KW (thermodynamic design)
Thus Δt total = 24.7 K and the outlet temperature is 49.7 °C, corresponding to the flow rate: 3.28 • 15 / 24.7 = 2.0 kg/s
Note that this is the limit for an infinitely large condenser. Even a lower flow rate would mean a higher temperature, which is impossible.
The nominal water flow rate of 3.28 kg/s results in a temperature of 37.5 °C at the pinch point, which is slightly below the practical limit for a heat exchanger.
In practice, reducing the water flow means that the condenser does not liquefy everything. So the pinch point has to move to the right to adapt to the water temperature. This applies regardless of the size of the heat exchanger. If a higher water temperature is needed, there are two ways to do it, independently or in combination:
◆ If a water flow rate of 3.28 kg/s is required, the condenser could be used to preheat the water. In winter, when the water temperature drops to zero, this could mean significant energy savings.
A separate desuperheater is installed (Figure 02). A BPHE is excellent for this application. Here a small stream of water is heated to 80 °C. The remaining latent heat is removed in either a water or air-cooled condenser.
In a PHE, the pressure drop is usually large enough to allow upward vapor flow. The inevitable liquefaction means that droplets (liquid refrigerant and oil) have to move upwards and a certain minimum pressure drop of 1 - 3 kPa/m flow path is necessary.
Figure 02: Desuperheater and condenser
One way to achieve higher temperatures, which z. B. when heating tap water is necessary, is to separate the heat dissipation and condensation tasks. The desuperheater heats a small amount of water to around 80°C in a PHE and the saturated vapor is liquefied in a separate condenser.
An air unit is used here, but a PHE could also be used depending on how (and if) the latent heat is used.
Desuperheaters and condensers of the same PHE model, either brazed, welded or cassette welded, can be integrated.
If a vapor containing an inert gas condenses in a condenser, the following phenomenon can be observed (Figure 03):
Image 03: Inerts
a) The near-wall vapor layer is increasingly enriched with gas as liquefaction progresses. This film of gas is like a barrier to the vapor. The vapor does not have direct access to the cold surface to condense. It must first diffuse through the inert gas layer.
b) The partial pressure of the vapor drops and the vapor-gas mixture must be cooled so that the saturation state is maintained and the liquefaction can continue.
c) The volume flow of the steam-gas mixture and the heat transfer coefficient decrease.
d) If the inert gas is not removed, the pressure will increase until the high pressure pressostat shuts down the compressor.
Even small amounts of non-condensable gases (inerts) can reduce the heat transfer coefficients and the MTD considerably, or worse: prevent the plant from operating altogether. Although non-condensable gases should not normally be present in a refrigeration circuit, they can be caused by the following circumstances, for example:
1) Decomposition of the refrigerant or the oil.
2) A defective vacuum pump.
3) The system was not properly evacuated before start-up.
4) An evaporator operating at negative pressure.
5) Pass-through installation, see example below.
A plant with a WPHE for propane liquefaction was often switched off when the high-pressure pressure switch tripped. Inerts were suspected. However, a test only showed 100% propane. A closer examination showed that it was not a real refrigeration system. The propane came from liquid gas tanks, in which some liquid is always evaporating. This propane vapor is compressed, liquefied and then returned to the tank farm. When the tank is emptied, the empty space above the propane is filled with an inert gas, usually nitrogen. The propane vapor to be compressed therefore contains nitrogen.
Furthermore, the analyzer was actually a butane device and the result was converted to propane via x% butane = y% propane. In this way, however, it is not possible to choose between 100% or e.g. B. 99.9% propane to distinguish. (A nitrogen test would have been better, the difference between 0% and 0.1% is large.)
Installing a ventilation system solved the problem.
Source: Alfa Laval