18 Aug Shell and Tube chiller evaporators in ammonia chillers

Who should read this blog post?

YOU must if you are one of the Ammonia chiller operators or HVAC consultants or HVAC engineer or industrial chiller manufacturers or heat exchanger manufacturers in India like us, you must know the challenges and their solutions in Shell and tube chiller evaporators used in Ammonia chillers.

Shell and tube chiller condensers and evaporators have been used since ages for the industrial chillers. The mathematical modelling and design have been optimised time and again to get the maximum efficiency and lowest cost of ownership. But this is only true for HFC and HCFC gases. For natural gases like R717( ammonia), CO2, ethylene based chillers, experimental data and in depth research analysis is not available in abundance.And most of the chiller supplier in India and elsewhere follow a rule of thumb instead of proper calculations for ammonia chiller manufacturing and operation.

So in this blog we will try to address the challenges faced in designing, constructing, assembling and operating shell and tube chiller evaporators in ammonia refrigeration. It might become too lengthy and boring at the same time so we will divide it in 2 parts.

As you already know Ammonia chiller is a tricky system to design, construct and operate versus R22 or R134a or R404a or any other HFC, HCFC gas based system.
Vast experience and in depth technical knowledge of the thermodynamic properties of Ammonia refrigerant are essential for Ammonia chiller manufacturers.

Background:

Ammonia is a natural gas available in the atmosphere and produced extensively throughout the world since decades for its use in production of majority of agrochem products.
This makes it an easily available,  abundant, environmentally safe and cost effective option as a refrigerant. Besides it is way more energy efficient (more than 20%) than HFCs and HCFCs in some cases.

And due to excellent thermo-physical properties of Ammonia, like immiscibility with refrigerant oil and high latent heat of evaporation, the most preferred configuration for industrial chiller evaporators is flooded type shell and tube chiller evaporators.
Here gas evaporates on the shell side and water/ brine circulates on the tube side for sensible cooling (without any phase change).
Optionally flooded type PHE ( plate heat exchanger) evaporators are also used but that configuration has more cons than pros due to the high gas side pressure drop and oil return issues at sub-zero conditions.
For low charge ammonia systems, Dry Expansion (DX) type shell and tube evaporator can also be discussed and thought of but to limit the content of this blog we shall consider only flooded type chiller evaporators as their operating efficiency is nearly 10% higher than DX evaporators though the required quantity of Ammonia charge is higher.

The general issues faced by ammonia chiller manufacturers are as described below

Design challenges in flooded type shell and tube chiller evaporators:

1. Heat transfer area with safety margin

Most critical challenge faced by Ammonia chiller manufacturers in India is the calculation of required heat transfer area for Ammonia chiller evaporators.

The overall heat transfer coefficient for shell and tube chiller evaporators is very difficult to calculate theoretically. (Experimental values range from 150 to 1500 in SI units for sub zero applications)This is because the characteristics of evaporating ammonia inside an evaporator shell and it’s heat transfer with the fluid flowing inside the tubes have not been studied in depth and published frequently.Moreover all the shell and tube heat exchanger manufacturer in Ahmedabad depend for design on refrigeration or HVAC consultants or ammonia system integrators.

With our immense experience we recommend that at least safety margin of 15% should be considered while providing the heat transfer area. This ensures guaranteed performance on site and trouble free operation even after fouling occurs.

2. MOC of shell and tube in heat exchanger

Another challenge for the industrial chiller manufacturer in India is the selection of material of construction for ammonia chillers shell and tube heat exchangers.

Ammonia is an alkaline and highly corrosive Gas which is not compatible with copper and it’s alloys.So the only practical and economically viable option for tube MOC is Carbon steel or Stainless steel 304. But again it is very important to note that if the tube side fluid is containing Free chlorine or higher chloride contents, SS might not be a compatible MOC as chlorine will induce slow pitting and eventually corrode the tubes from within.

Similarly as this is a high pressure vessel like many other vessels used in ammonia chillers is is advisable to use only boiler quality plates SA516 Gr. 70 for shell material which has 3 times tensile strength than the normally available IS2062 plate.

3. Reynold’s number in tubes, Water/ Brine flow rate and allowable pressure drop across tubes

Further more the requirement of maintaining turbulency across all tubes for better heat transfer, specially in Brine chillers, is generally neglected while designing shell and tube heat exchanger.

Ammonia chiller manufacturers should understand that by just providing enough heat transfer area, you cannot be sure of adequate performance. Aspects like less oil carry over to evaporator, maintaining turbulency inside tubes and employing surge tank for dynamic load conditions etc. should also be given equal importance while designing the heat exchanger.

In Brine chillers and process chillers, at lower temperatures the physical properties of the brine/ glycol like the viscosity, conductivity and specific heat degrade drastically adversely affecting the heat transfer rate. So tube side fluid flow rate must be optimized to get Reynold’s number in turbulent region inside the tubes (above 4000).
For instance flow rate of MeOH brine must be calculated with 2 deg C DT across the chiller evaporator for -35deg C brine chiller instead of 4degC as for water chillers.

Similarly, it is also important for refrigeration consultants and industrial chiller manufacturers in India to set practical limits for pressure drop across tubes in shell and tube chiller evaporators. Because setting DP limits (less than 1kgf/cm2) like water chiller restricts the tube side Reynold’s number into laminar region for brine chillers and glycol chillers.

4. Oil recovery unit

The compressor oil escaping the oil separator entrapped in the hot discharge gas finally lands in the high pressure receiver with liquid Ammonia and eventually sent to evaporator via liquid line. As we all know that ammonia is not miscible with refrigeration oil at lower temperatures. So it gets seperated in the flooded evaporator and settles at the bottom of the shell due to higher density.

So, unlike in all the HFCs or HCFCs , the oil recovery is comparatively simpler.By just providing a small recovery receiver attached to the ammonia chillers evaporator’s drain point the separated oil can be collected by gravity to be sent back to the compressor.

In the Next part of this blog post we shall discuss the challenges in constructing, assembling and operating shell and tube chiller evaporators.

How Advance Techno System excels in design:

Advance techno systems is one of the oldest and leading heat exchanger manufacturer in Ahmedabad, Gujarat with 34 years of experience in ammonia chiller manufacturing. Being one of the largest industrial chiller manufacturers in India, and biggest chiller manufacturer in Ahmedabad we manufacture our own shell and tube chiller condensers and evaporators ourselves at our works which comply to TEMA standards.

Advance Techno is the only plate type evaporative condenser manufacturers in India.We are the most automated heat exchanger manufacturer in Gujarat.

We have a trained team of designers for the optimization of shell and tube heat exchanger design as per the end user’s application requirements. Immense on-field knowledge and in depth theoretical analysis of the feedbacks enables use to deliver the most energy efficient and high performance chillers.
We guarantee our system performance on the basis of SEC ( specific energy consumption)  in kW/TR and prove it.

Advance Techno Systems uses the latest technology tools available in integration and execution to offer advance chiller at most affordable price points. Advance Techno System has developed and employs an auto oil recovery system in Ammonia chiller manufacturing to recirculate the used oil to the compressor without the need of draining the oil manually at regular intervals.

Our high product reliability and least LCC ( Life cycle cost) ensures a minimum of 12 years of uninterrupted life of our process chillers.
We are the authorized chiller system integrator of Daikin India Pvt. Ltd for their brine chillers inquiries.
Contact us for your chiller requirement on info@advanceahmedabad.com, or  advanceahmedabad@gmail.com.