In which line are condensate drains used?



1 CONDENSATE DRAIN 1.1 Condensate in pipes In pipes through which flows, condensate forms due to the cooling down of the heat loss. The length of the pipe, the pipe insulation and the overheating of the pipe are decisive for the amount of condensate. The condensate must be drained out of the pipe. At flow velocities in the line of m / s, even small amounts of condensate lead to material damage due to erosion. Condensate has a destructive effect from flow speeds above 3 m / s. The inside of the pipe is washed out and the wall thickness is reduced. In a short time, leaks can occur due to washouts. (see also chapter post-evaporation). The far bigger problems, however, are the blows in the lines. Condensate collects in the pipeline and cools down. It pours over it, which is much hotter. The bubbles condense suddenly. The sudden change in volume results in loud noises and pressure surges in the pipeline. Such impacts then lead to movements of the pipelines or to damage to devices such as heat exchangers or fittings. Steam traps are designed to prevent this build-up of condensate. Mounted in the right place on the pipeline, they also drain off large amounts of condensate. 1.2 Accumulation of condensate in pipelines If the pipeline is short and insulated and flows continuously through the pipeline, there is almost no condensate. If the pipeline is very long and in parts not insulated, the amount of condensate will increase due to the loss of heat. In long pipelines, several condensate drains should therefore be used, which are installed at the lowest points of the pipeline. Long lines are always laid with a slope so that the condensate can drain off quickly. G e fä lle D a m p f D a m p f H a n d - e n t w ä s s e u n g in s F r e e K o n d e n s a t - a b l e iter r S a m m e lle itu n g KARSTEN BERLIN PAGE 1 OF 11

2 It is problematic to restart a pipeline after the pipeline has cooled down. Until the pipeline has heated up to approximately the same temperature as it, very large amounts of condensate with rust particles accumulate. Manual drainage at the lowest point of the pipeline is certainly useful for long pipelines with a larger diameter. Table below: Information on the amount of condensate that can arise in a pipe. The pipeline is in operation and there is full flow through it. The condensate quantities can be used to design condensate drains. The condensate quantities were not calculated for all nominal sizes. When looking at the tables, however, you get a feeling of how large the amount of condensate is, e.g. with a nominal size of DN350. The quantities of condensate when starting up the line (from a cold state) are, however, significantly higher! (Tube ST35.8 / normal wall thickness) KARSTEN BERLIN PAGE 2 OF 11

3 1.3 Condensate in heat exchangers Condensate in steam-heated heat exchangers leads to a reduction in the heat exchanger surface (see also the section on heat exchangers). This makes the heat exchanger less effective. In heat exchangers, there may be a slight vacuum build-up due to condensation. This means that a negative pressure is created in the heat exchanger, which prevents the condensate from flowing out. The condensate backs up. Water / flow A backflow water / return water / flow water / return Max backflow Condensate drain Condensate drain If the condensate drain is approx. 2 m below the heat exchanger, the backflow is still present. The backflow is then in the pipeline above the condensate drain. The heat exchanger surface remains free. Condensate in heat exchangers also leads to damage due to shock. This is another reason why the condensate drain should always be installed as far away as possible from the heat exchanger. 1.4 Types of steam traps There are different types of steam traps. The float steam trap and the bimetal steam trap are probably the best known. KARSTEN BERLIN PAGE 3 OF 11

4 float condensate drain: a metal ball functions as a buoyancy device. This means that if condensate flows into the trap, the ball floats upwards. The condensate drain opens and condensate flows until the ball sinks again. The ball can also float loosely in the steam trap and, as a result of the movement, close or open the outlet opening. Float steam traps are available in every installation position and are largely maintenance-free. CLOSED OPEN Ball condensate Simplified representation of thermal steam traps: A bimetal package or a liquid in a capsule is mounted in such a way that a closing element is opened or closed due to thermal expansion. In other words, if there is cold condensate, the condensate drain opens. If it is hotter, the condensate drain is closed again. Accordingly, very small temperature differences between condensate and are used. Bimetal or capsule with liquid Simplified representation of condensate Bimetal condensate drains are available in every installation position and are largely maintenance-free. The bimetal condensate drain recognizes the condensate and the low temperature difference. Therefore, these condensate drains should be installed as far away as possible from the pipeline or the heat exchanger. KARSTEN BERLIN PAGE 4 OF 11

5 Rust particles from the pipeline are problematic for all condensate drains. Therefore see sketch below. Condensate drain Manual drainage into the open (1) into the open Collecting line The rust liquid is drained off via a manual drain when the system is restarted. The condensate drain then takes over the drainage. This is arranged in such a way that rust particles do not easily reach the condensate drain and are more likely to be deposited above the manual shut-off valve (1). 1.5 Design of a condensate drain Detailed descriptions of every condensate drain can be found in the manufacturers' catalogs. The pressure in the pipeline or the heat exchanger, the amount of condensate produced and the pressure downstream of the condensate drain are important for the design. EXAMPLE: A heat exchanger is to be heated at 5.5 t per hour. Technical data: Consumption: pressure: Pressure behind the condensate drain: 5.5 t / h 7.5 barg 1 barg (The heat exchanger drains into a pipe. The pipe is mounted on a tank. The pressure in the tank is 1barg. ) Calculation: Determination of the differential pressure: Determination of the condensate volume: 7.5 barg 1 barg = 6.5 barg 5.5 t / h Result: The condensate drain must be able to discharge 5.5 t / h of condensate at a differential pressure of 6.5 barg . Selection of the condensate drain using a diagram from the manufacturer's catalog: A type D condensate drain would have to be selected. (Diagram made up but similar) KARSTEN BERLIN PAGE 5 OF 11

6 Condensate throughput in kg / h Type A DN50 Type B DN50 Type C DN50 Type D DN50 Type E DN50 Type F DN50 6.5 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8 , 0 9.0 10 pressure difference in bar EXAMPLE: 45 t / h flow through a 50 m long, insulated pipe with a nominal width of DN200. A condensate drain is to be selected which can discharge the constant accumulation of condensate due to condensation due to heat losses in the pipeline. Technical data: amount: pressure: 45 t / h 7.5 barg temperature: 190 C pressure behind the condensate drain: 1 barg (the heat exchanger drains into a pipe. The pipe is mounted on a container. The pressure in the container is 1baru.) It is overheated by approx. 20 C. If it is overheated, the heat losses in the pipes are low. There are certainly calculation programs from the manufacturer of the condensate drain, which could now determine the amount of condensate in the pipeline based on the thickness of the insulation. The normal user can roughly estimate the amount of condensate at approx. 1 kg / h. (But that's a lot) If you take the manufacturer's diagram, no major (or even no) difference is found between 1 kg / h and 50 kg / h or even 100 kg / h throughput of condensate. KARSTEN BERLIN PAGE 6 OF 11

7 1.6 Condensate drain and re-evaporation The condensate drain can only drain properly if the pressure in the line is higher than the pressure behind the condensate drain. In general, it is better not to let the pipeline rise behind the condensate drain. But sometimes there is no other way. The condensate drain still drains if the pressure P1 is higher than the pressure P2. Pressure P1 manifold e.g. to tank pressure P2 condensate drain If the pipeline downstream of the condensate drain increases e.g. 2 meters, the pressure P1 should be at least 0.3 bar higher than the pressure P2. A steam trap works like a shut-off device. It separates one room with high pressure from the other room with low pressure. This means that the pressure on the condensate drain is reduced. After every pressure reduction of condensate there is re-evaporation. The important topic of the development of re-evaporation and the problems that may arise with it are dealt with in detail in the chapter on re-evaporation. How re-evaporation can be used again in the heating process is described in the chapter on re-evaporation and in the chapter on compressor. KARSTEN BERLIN PAGE 7 OF 11

8 1.7 General information on steam traps When designing a steam trap, the pressure upstream and downstream of the steam trap is important, regardless of the amount of condensate to be discharged. Similar to a control valve, the pressure downstream of the condensate drain must always be lower than that upstream of the condensate drain. Logical, otherwise the condensate would not drain away. As described in the chapter on re-evaporation, every time the pressure is reduced, re-evaporation occurs. The lower the pressure downstream of the condensate drain, the greater the re-evaporation. However, a large re-evaporation also means a high flow rate of the condensate, if the nominal pipe size is not increased accordingly. As explained in the chapter on re-evaporation, a condensate line with re-evaporation should not be designed as a water pipe, but as a pipe (see chapter on re-evaporation and piping). Note: If the condensate drain is installed as close as possible to a container or larger condensate collecting line, this reduces the costs for the pipeline construction. The pipeline behind the WT is laid out like a water pipe. 1.5t / h condensate (because 1.5t / h) = DN25 After the condensate drain, the pipeline is laid out like a line. For this purpose, the re-evaporation of 1.5t / h condensate at differential pressure P1 / P2 must be determined. (see chapter on re-evaporation) TIC condensate drain! min! 1.5t / h P1: 2.5barg 140 C heat exchanger WT condensate runs off, condensate container P2: 1barg of re-evaporation condensate in the condensate pipe KARSTEN BERLIN PAGE 8 OF 11

9 Depending on the size of the system and the condensate system, the planner of a system must now decide as follows: Do you have a lonely condensate drain with low levels of condensate hanging somewhere in the system and is it worth laying pipes to a container? Can different condensate drains in the system be combined into groups so that the post-evaporation can still be used for heating? (See also the chapter on cascade system) If you play it safe and lead the condensate collecting line of several condensate drains to a point in the - and condensate system at which the pressure is particularly low, in order to always allow perfect drainage or to use a smaller and therefore more cost-effective condensate drain to use? Disadvantage: The pipeline behind the condensate drain has to be designed larger in order to discharge the re-evaporation. Another disadvantage is that the amount produced during post-evaporation can only be used with difficulty or not at all in the heating system due to the low pressure. What to do with low energy Or are you braver and lead the condensate line to a point in the system where the pressure is still relatively high and the condensate drain can still drain under all operating conditions. Advantage: The nominal diameter of the pipeline can be smaller. The amount of post-evaporation can possibly be used to heat other consumers (see also the chapter on cascade system and compressor). P1 P2 P3 TIC TIC TIC Condensate in the pipe KARSTEN BERLIN PAGE 9 OF 11

10 What to do, good advice is expensive. In general, the condensate should not be discharged into the open. Depending on the size of the and condensate system, a commercial check must be carried out to determine whether appropriate modifications are technically sensible. If several consumers are drained into a pipeline with the help of condensate drains, this system of pipelines and condensate drains must be considered as a whole when designing them. The most unfavorable condition is assumed for the design. The most unfavorable condition is maximum heating pressure and maximum consumer demand. This results in the corresponding pipe dimensions. If the dimensions are too small, the consumers do not drain properly and the heat exchanger surface of the consumers is reduced as a result. 1.8 Condensate drain as level control on the tank A simple level control should be installed on a tank. A condensate drain can also be used for this. Atm. Condensate drain Condensate level The condensate drain is installed on the container at the exact height at which the level is to be set later. Large amounts of condensate can be drained away with several condensate drains mounted next to each other. The selection of the condensate drain (s) is again made using the manufacturer's diagram. The use of a condensate drain as a level control only works with containers in which no internal pressure arises due to the process. That is, only for containers that are open to the atmosphere! Otherwise, this level control is very reliable and inexpensive because it can be implemented without the need for measurement and control technology. KARSTEN BERLIN PAGE 10 OF 11

11 1.9 Condensate drain as ventilation The condensate drain can do even more. The pipelines and containers etc. of a and condensate system are to be replaced after standstills, e.g. filled with air after repairs. If condensation occurs in pipes and containers, a vacuum is created in the system parts due to the reduced volume. Through openings in the system, e.g. if Valves are replaced during maintenance work, or if the seals are damaged, air flows into the system. Air does not condense. That is why air is also referred to as a non-condensable gas (venting, see also the venting chapter). If air flows into the bimetal condensate drain, this air is significantly cooler than the condensate and also than the condensate. The bimetal opens and the air can flow out. With the float steam trap, the air does not create any buoyancy on the ball. For this reason, an additional bimetal is installed in the float steam trap, which keeps the ball open. The opening for venting is not closed until hotter condensate or flows in. KARSTEN BERLIN PAGE 11 OF 11