high pressure system: are there issues with heating the water before it enters the pump?

[pa5tabear]

One of my projects is replacing a high pressure water system used to supply sanitation hoses. It consists of high pressure pumps (which put out water at ~600 psi), a water heater, and a control system to maintain the temperature and pressure.

I've talked to "experts" who have argued both sides- that the heating should occur after the pump, or that it should occur before the pump.

We are leaning toward using a direct contact gas-fired heater, and it's doubtful it could handle water at 600 psi, so this dictates that the water should be heater prior to pumping. The target water temperature is 160 °F, so I don't think it will pose any threat to the pump materials.

Has anyone heard of stuff going wrong in this sort of system configuration?

 

[BigInch]

My opinion is to heat before. It increases vapor pressure, so just make sure you have enough NPSH.

Independent events are seldomly independent.

 

[chicopee]

Is there going to be any storage tank for the hot water? Instead of a water heater, you may want to look at once-through steam/hot water generators which has no steam, water and mud drums. Once-through steam/water generators are high pressure units. Zurn makes such units; no I don't work or represent Zurn, although I did get involved in the field erection of a Zurn water tube boiler at a hospital in MA.

 

[pa5tabear]

We'd prefer no storage tank, but I haven't heard of large enough heating systems that are feasible for our site.

We need to heat the water from ~50 up to 160°F and often have flowrates as high as 120 gpm. This is around 7.8 million BTU/hr.

 

[ccfowler]

What sort of duty cycle is expected? Continuous flow "24/7" or some sort of on-off cycle? Will there be some significant variations in the flow rate during operation?

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[pa5tabear]

@ccfowler:

Not at all continuous. Frequent on-off and major variation in flow rate. The only repeating cycle is a weekly one: consistent high-volume demand on the weekends and random intermittent demand throughout the week.

It's for a sanitation cleaning system in a food plant.

 

[LittleInch]

Personally I can't see anything wrong in heating before the pump providing that these conditions are made clear on your data sheet or enquiry to the pump vendors. It might rule out some lower temperature seals but otherwise providing you get the NPSH correct, there should be no real issue.

You will though need to think about how to cope with any low or no flow conditions when you are in your intermittent mode as it won't take long for the water to reach boiling point and then turn to steam if the pressure in the pump falls to atmosphric conditions once you turn it off. You will need to protect the pump and asssociated pipework from contact with hands etc, and may need to include a temperature monitoring / cut out on the pump casing.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[ccfowler]

160 F is no problem of consequence for pump seals so long as it is properly anticipated in the pump selection.

I would prefer the water heating before the high pressure pump, but this assumes that the water heater is supplied with water at some moderate pressure.

My greater concerns would be addressing the range of flow rates and their duration. You may want to consider using multiple water heaters in parallel to deal with the flow variations. You may want to consider multiple water heaters paired with multiple pumps with controls to select the proper choice of water heater & pump pair(s) to be operated to serve the specific flow demand. Otherwise, controlling water temperature could get to be very messy. Some modest storage capacity with each water heater may be very helpful in handling the flow variations.

I would start structuring the design solution process around the intricate details of the most realistic possible modeling of the duty cycle(s) expected. Multiple pumps in parallel can work very nicely with pumps of different sizes so long as their characteristic curves are suitably compatible.

The most useful advice that I can offer is to be sure to fully disclose all of the details to both the pump and the water heater manufacturers. They can be most helpful, and responsible suppliers will want your system to work well just as much as you do.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[EdStainless]

I used to work with hot water systems built by a company out east (Heat Transfer Products

http://www.htproducts.com/modcon-vwh.html

). They make once through boiler systems and hot water heaters. I have seen them set up in cascade to make lots of hot water. The burner modulation is based on the water temp.
In our case we heated the water hotter than we needed and used a mixing valve to temper it. This gave us better response and more uniform temp when flows changed.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube