Our topic for this month is cooling water supply temperature and its impact on ejector vacuum system performance. Hot cooling water supply is a common problem during the warm summer months and often the cause of seasonal surging and vacuum instability.
A hot cooling water supply temperature can mean two things. First and most often, it means a cooling water supply temperature that is hotter than a system’s design. It can also mean a cooling water supply temperature that is hotter than what the system has previously seen during the cooler months of the year. A cooler than design water supply temperature will increase a condenser’s capacity, which can compensate for issues like fouling. An increase in cooling water supply temperature can cause that problem, previously being compensated for by the cooler water, to then impact the system’s vacuum. This will sometimes happen before a system reaches its design cooling water supply temperature. In this way, cooling water issues can be either a design issue or a relative issue depending on the condition of a system’s condensers. In both instances cooling water problems are most common during summertime operation.
Understanding how the warmer water impacts a condenser requires understanding that condensers are governed by a heat transfer relationship, as shown in the equation below.
Q = U*A*LMTD
Q = Amount of heat transferred (BTU/hr)
U = Overall Heat Transfer Rate (BTU/ hr ft2
A = Condenser Surface Area (ft2)
Log Mean Temperature Difference
All condensers are governed by the above equations. For an existing condenser, the surface area “A” will be constant. The overall heat transfer rate “U” will also remain relatively constant, decreasing if the unit becomes fouled. The condenser “LMTD” will change based on the cooling water conditions. Both changes in water temperature and flow will impact the LMTD. A change in the LMTD then directly impacts the “Q” value of the condenser.
An increase in cooling water supply temperature will also impact the condenser operating pressure. At the design operating pressure of a condenser, the water needs to be a certain temperature or cooler in order for the process vapors and any motive steam from the ejectors in the system to condense. As the cooling water supply temperature is increased, this relationship is also impacted. As the design cooling water supply temperature is exceeded, the operating pressure of the condenser will in turn degrade. In the case of a precondenser system, this will result in a higher pressure in the upstream process vessel. If the condenser is downstream of an ejector, this will result in a higher discharge pressure to the upstream ejector. That upstream ejector may struggle to overcome the higher back pressure, resulting in a poorer unstable pressure in the upstream equipment. In multiple stage ejector systems, the increase in cooling water supply temperature will first impact the condenser that is operating at the deepest vacuum, because it needs the coldest water.
This leads to the next question, “How to prevent hot cooling water supply temperature from impacting the performance of a vacuum system?”
Design for the Correct Cooling Water Supply Temperature
Although this does not help with a system already installed, on new installations it is very important when designing a new vacuum system that the vacuum system manufacturer is provided an accurate cooling water supply temperature. The specified cooling water supply temperature should be indicative of the hottest temperature that the system will see. If the temperature specified is cooler than what the system will see during summertime operation, then the system will not work properly when that temperature is exceeded. The temptation to specify a cooler water temperature to make the system smaller and less costly often results in a system that does not work year round.
Compensating with an Increased Cooling Water Flow
Adding additional cooling water flow will overcome an increase in the cooling water supply temperature by decreasing the cooling water outlet temperature and in turn impacting the LMTD calculation. This can often be accomplished with a booster pump or by manipulating the cooling water balance to direct more flow to the system. It should be stated that the ability to overcome a higher cooling water supply temperature with more flow is going to be limited by the condensing pressure temperature relationship. Condenser tube velocities also need to be considered so that the higher flows don’t damage the condenser tubes.
Installing a Mechanical Chiller on the Water Supply
Temperature and humidity will limit the effectiveness of cooling towers. Installing a mechanical chiller to provide additional cooling and drop the supply temperature is a very direct way to deal with a hot cooling water supply temperature. It eliminates the issues but at a fairly steep cost.
Adding External Cooling
A messy and somewhat unprecise way to compensate for hot cooling water supply is to add an external water spray to the outside of the condenser shell. The spray and evaporation off of the shell cools the shell allowing the internal vapors to condense against it. Effectively, this adds additional surface area.
Reduction in Process Loading
Cutting back on the process load will reduce the loading on the condensers and remove duty from the cooling water loop. This can help compensate for the hotter cooling water supply to some degree but will have an impact on production.
Cleaning the condensers on both the shell side and tube side helps to both maximize cooling water flow and the heat transfer rate. Cleaning the condensers helps the condenser capacity by ensuring its best operation and minimizing the impact of a hot cooling water supply temperature.
New Condensers / Redesign
In an installed system there are only so many things that can be done to compensate for hot cooling water supply. At some point condensers and systems need to be redesigned if the water supply temperature is going to be hotter than design for extended periods of time. The equipment manufacture should be consulted to see what changes are applicable for a specific system.