11 replies to this topic

[sanaullah]

Dear members,

Can anybody please describe the difference between rating and sizing?
For example pump rating, pump sizing and Exchanger rating , and exchanger sizing

Regards

[fallah]

sanaullah,

It should be presented in another forum, IMO in student forum.

Fallah

[fallah]

sanaullah,

Would you please explain about the reason of your negative feedback on my post.

Fallah

Edited by fallah, 08 October 2012 - 06:21 AM.

[Steve Hall]

Sizing: Determine the required capacity for the equipment, generally used in the specifications

Rating: Determine the actual capacity for the equipment, generally used for existing equipment or for the equipment selected in response to the specifications.

For a relief valve, for example, the orifices are provided in discrete sizes. When you "size" the valve you actually are determining the minimum orifice size. Then, you obtain the valve with a larger standard orifice. You could "rate" the one that is purchased.

[T.S.MURALI]

Sizing involves design -say Boiler ,H.E, P.V etc

Rating is a designation of an equipment say H. E in KW,Motor in KW,Power plant in Mw etc.

[pravin164]

Sizing refers deisgning and making specifications (with diamnesions) of a new equipement for required process/duty/design intent.
While rating is determing the adequacy of an existing equipment for revised/alternate process conditions required after revamping or any process parameter change.

[srfish]

Every one above has described design. Rating for heat exchangers takes a given design and gives an evaluation. It primarily calculates the amount of excess or deficient surface. It alco calculates the pressure drops of both sides.

[T.S.MURALI]

Rating represents/denotes a specific parameter of duty condition :e.g say capacity of Heat ex changer in kcal/hr ; pressure drop

allowable in mmwc etc

A well designed shell and tube heat ex changer will handle its rated load under the conditions for which it was

specified.

Emergencies occur in some processes, and it may be advantageous to have some extra margin of heat

ex-changer capacity to take care of them.

Another example is say Refrigerator.-where you find say Rated current :--- amps; max current -- amps;

Rated input power ---kw ; max power input----kw ; Rated frequency ---mhz/ cycles per sec.

Based on the performance evaluation under duty conditions for which the equipment is designed, one can check the Rating and de

value/de rate if called for. ( De rating the boiler based on longevity ,performance etc).

You have to size the equipment based on Duty called for -e.g 20 MM kcal/hr output for an exchanger for specific conditions of flow and

press ,temp etc.

Designed equipment for the above rating can reveal the adequacy during appropriately conducted trials.

So Rating gives the limits of duty. Sizing is done based on the required specifications/duty conditions,

using codes and design practices.

Another e.g

Pump is rated as capacity/pres/temp/NPSH/total head. M3/HR ,Metres of water column etc

Sizing of pump- impeller,volute casing bearing, coupling Torque/speed ,m.o.c etc are based on the above duty conditions.

[latexman]

Sizing - the minimum size/flow required.

Rating - actual flow at specified conditions (10% OP).

[S.AHMAD]

1. Design is to determine the size (area) and configure the heat heatexchanger that meet a specified heat duty. Simply put, heat duty is known, area unknown.
2. Rating is to determine the heat duty for a specified size and configuration of an exchanger. Simply put, size and configuration is known, duty unknown.

[T.S.MURALI]

I wish to cite another example :Air conditioner Ratng in ton of cooling say 2 t,5t etc .1t of cooling=200 btu/mt.

Sizing is the design of the equipment -HTA ,fluid flow rate etc for the respective rating.

Also for transformer Rating may be 32 KVA etc. sizing is based on the design -no of turns /coils etc

[Ceng]

Computer programs for heat exchangers have normally two modes of calculation:
design: determining exchanger size and geometry to meet users limitations
rating:determining whether an exchanger which users specified has adequate surface area